diff --git a/fuse_sideload.c b/fuse_sideload.c
index ab91def..4e11e01 100644
--- a/fuse_sideload.c
+++ b/fuse_sideload.c
@@ -53,6 +53,7 @@
 #include <string.h>
 #include <sys/inotify.h>
 #include <sys/mount.h>
+#include <sys/param.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/statfs.h>
@@ -117,15 +118,40 @@
 static int handle_init(void* data, struct fuse_data* fd, const struct fuse_in_header* hdr) {
     const struct fuse_init_in* req = data;
     struct fuse_init_out out;
+    size_t fuse_struct_size;
+
+
+    /* Kernel 2.6.16 is the first stable kernel with struct fuse_init_out
+     * defined (fuse version 7.6). The structure is the same from 7.6 through
+     * 7.22. Beginning with 7.23, the structure increased in size and added
+     * new parameters.
+     */
+    if (req->major != FUSE_KERNEL_VERSION || req->minor < 6) {
+        printf("Fuse kernel version mismatch: Kernel version %d.%d, Expected at least %d.6",
+               req->major, req->minor, FUSE_KERNEL_VERSION);
+        return -1;
+    }
+
+    out.minor = MIN(req->minor, FUSE_KERNEL_MINOR_VERSION);
+    fuse_struct_size = sizeof(out);
+#if defined(FUSE_COMPAT_22_INIT_OUT_SIZE)
+    /* FUSE_KERNEL_VERSION >= 23. */
+
+    /* If the kernel only works on minor revs older than or equal to 22,
+     * then use the older structure size since this code only uses the 7.22
+     * version of the structure. */
+    if (req->minor <= 22) {
+        fuse_struct_size = FUSE_COMPAT_22_INIT_OUT_SIZE;
+    }
+#endif
 
     out.major = FUSE_KERNEL_VERSION;
-    out.minor = FUSE_KERNEL_MINOR_VERSION;
     out.max_readahead = req->max_readahead;
     out.flags = 0;
     out.max_background = 32;
     out.congestion_threshold = 32;
     out.max_write = 4096;
-    fuse_reply(fd, hdr->unique, &out, sizeof(out));
+    fuse_reply(fd, hdr->unique, &out, fuse_struct_size);
 
     return NO_STATUS;
 }
diff --git a/updater/blockimg.c b/updater/blockimg.c
index 3026893..9c39634 100644
--- a/updater/blockimg.c
+++ b/updater/blockimg.c
@@ -16,6 +16,7 @@
 
 #include <ctype.h>
 #include <errno.h>
+#include <dirent.h>
 #include <fcntl.h>
 #include <inttypes.h>
 #include <pthread.h>
@@ -23,6 +24,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <sys/ioctl.h>
@@ -32,7 +34,7 @@
 #include "applypatch/applypatch.h"
 #include "edify/expr.h"
 #include "mincrypt/sha.h"
-#include "minzip/DirUtil.h"
+#include "minzip/Hash.h"
 #include "updater.h"
 
 #define BLOCKSIZE 4096
@@ -46,6 +48,10 @@
 #define BLKDISCARD _IO(0x12,119)
 #endif
 
+#define STASH_DIRECTORY_BASE "/cache/recovery"
+#define STASH_DIRECTORY_MODE 0700
+#define STASH_FILE_MODE 0600
+
 char* PrintSha1(const uint8_t* digest);
 
 typedef struct {
@@ -80,44 +86,77 @@
     return out;
 }
 
-static void readblock(int fd, uint8_t* data, size_t size) {
+static int range_overlaps(RangeSet* r1, RangeSet* r2) {
+    int i, j, r1_0, r1_1, r2_0, r2_1;
+
+    if (!r1 || !r2) {
+        return 0;
+    }
+
+    for (i = 0; i < r1->count; ++i) {
+        r1_0 = r1->pos[i * 2];
+        r1_1 = r1->pos[i * 2 + 1];
+
+        for (j = 0; j < r2->count; ++j) {
+            r2_0 = r2->pos[j * 2];
+            r2_1 = r2->pos[j * 2 + 1];
+
+            if (!(r2_0 > r1_1 || r1_0 > r2_1)) {
+                return 1;
+            }
+        }
+    }
+
+    return 0;
+}
+
+static int read_all(int fd, uint8_t* data, size_t size) {
     size_t so_far = 0;
     while (so_far < size) {
         ssize_t r = read(fd, data+so_far, size-so_far);
         if (r < 0 && errno != EINTR) {
             fprintf(stderr, "read failed: %s\n", strerror(errno));
-            return;
+            return -1;
         } else {
             so_far += r;
         }
     }
+    return 0;
 }
 
-static void writeblock(int fd, const uint8_t* data, size_t size) {
+static int write_all(int fd, const uint8_t* data, size_t size) {
     size_t written = 0;
     while (written < size) {
         ssize_t w = write(fd, data+written, size-written);
         if (w < 0 && errno != EINTR) {
             fprintf(stderr, "write failed: %s\n", strerror(errno));
-            return;
+            return -1;
         } else {
             written += w;
         }
     }
+
+    if (fsync(fd) == -1) {
+        fprintf(stderr, "fsync failed: %s\n", strerror(errno));
+        return -1;
+    }
+
+    return 0;
 }
 
-static void check_lseek(int fd, off64_t offset, int whence) {
+static int check_lseek(int fd, off64_t offset, int whence) {
     while (true) {
         off64_t ret = lseek64(fd, offset, whence);
         if (ret < 0) {
             if (errno != EINTR) {
                 fprintf(stderr, "lseek64 failed: %s\n", strerror(errno));
-                exit(1);
+                return -1;
             }
         } else {
             break;
         }
     }
+    return 0;
 }
 
 static void allocate(size_t size, uint8_t** buffer, size_t* buffer_alloc) {
@@ -146,14 +185,21 @@
 
     if (rss->p_remain <= 0) {
         fprintf(stderr, "range sink write overrun");
-        exit(1);
+        return 0;
     }
 
     ssize_t written = 0;
     while (size > 0) {
         size_t write_now = size;
-        if (rss->p_remain < write_now) write_now = rss->p_remain;
-        writeblock(rss->fd, data, write_now);
+
+        if (rss->p_remain < write_now) {
+            write_now = rss->p_remain;
+        }
+
+        if (write_all(rss->fd, data, write_now) == -1) {
+            break;
+        }
+
         data += write_now;
         size -= write_now;
 
@@ -164,12 +210,17 @@
             // move to the next block
             ++rss->p_block;
             if (rss->p_block < rss->tgt->count) {
-                rss->p_remain = (rss->tgt->pos[rss->p_block*2+1] - rss->tgt->pos[rss->p_block*2]) * BLOCKSIZE;
-                check_lseek(rss->fd, (off64_t)rss->tgt->pos[rss->p_block*2] * BLOCKSIZE, SEEK_SET);
+                rss->p_remain = (rss->tgt->pos[rss->p_block * 2 + 1] -
+                                 rss->tgt->pos[rss->p_block * 2]) * BLOCKSIZE;
+
+                if (check_lseek(rss->fd, (off64_t)rss->tgt->pos[rss->p_block*2] * BLOCKSIZE,
+                        SEEK_SET) == -1) {
+                    break;
+                }
             } else {
                 // we can't write any more; return how many bytes have
                 // been written so far.
-                return written;
+                break;
             }
         }
     }
@@ -245,6 +296,58 @@
     return NULL;
 }
 
+static int ReadBlocks(RangeSet* src, uint8_t* buffer, int fd) {
+    int i;
+    size_t p = 0;
+    size_t size;
+
+    if (!src || !buffer) {
+        return -1;
+    }
+
+    for (i = 0; i < src->count; ++i) {
+        if (check_lseek(fd, (off64_t) src->pos[i * 2] * BLOCKSIZE, SEEK_SET) == -1) {
+            return -1;
+        }
+
+        size = (src->pos[i * 2 + 1] - src->pos[i * 2]) * BLOCKSIZE;
+
+        if (read_all(fd, buffer + p, size) == -1) {
+            return -1;
+        }
+
+        p += size;
+    }
+
+    return 0;
+}
+
+static int WriteBlocks(RangeSet* tgt, uint8_t* buffer, int fd) {
+    int i;
+    size_t p = 0;
+    size_t size;
+
+    if (!tgt || !buffer) {
+        return -1;
+    }
+
+    for (i = 0; i < tgt->count; ++i) {
+        if (check_lseek(fd, (off64_t) tgt->pos[i * 2] * BLOCKSIZE, SEEK_SET) == -1) {
+            return -1;
+        }
+
+        size = (tgt->pos[i * 2 + 1] - tgt->pos[i * 2]) * BLOCKSIZE;
+
+        if (write_all(fd, buffer + p, size) == -1) {
+            return -1;
+        }
+
+        p += size;
+    }
+
+    return 0;
+}
+
 // Do a source/target load for move/bsdiff/imgdiff in version 1.
 // 'wordsave' is the save_ptr of a strtok_r()-in-progress.  We expect
 // to parse the remainder of the string as:
@@ -255,30 +358,489 @@
 // it to make it larger if necessary.  The target ranges are returned
 // in *tgt, if tgt is non-NULL.
 
-static void LoadSrcTgtVersion1(char* wordsave, RangeSet** tgt, int* src_blocks,
+static int LoadSrcTgtVersion1(char** wordsave, RangeSet** tgt, int* src_blocks,
                                uint8_t** buffer, size_t* buffer_alloc, int fd) {
     char* word;
+    int rc;
 
-    word = strtok_r(NULL, " ", &wordsave);
+    word = strtok_r(NULL, " ", wordsave);
     RangeSet* src = parse_range(word);
 
     if (tgt != NULL) {
-        word = strtok_r(NULL, " ", &wordsave);
+        word = strtok_r(NULL, " ", wordsave);
         *tgt = parse_range(word);
     }
 
     allocate(src->size * BLOCKSIZE, buffer, buffer_alloc);
-    size_t p = 0;
-    int i;
-    for (i = 0; i < src->count; ++i) {
-        check_lseek(fd, (off64_t)src->pos[i*2] * BLOCKSIZE, SEEK_SET);
-        size_t sz = (src->pos[i*2+1] - src->pos[i*2]) * BLOCKSIZE;
-        readblock(fd, *buffer+p, sz);
-        p += sz;
+    rc = ReadBlocks(src, *buffer, fd);
+    *src_blocks = src->size;
+
+    free(src);
+    return rc;
+}
+
+static int VerifyBlocks(const char *expected, const uint8_t *buffer,
+                        size_t blocks, int printerror) {
+    char* hexdigest = NULL;
+    int rc = -1;
+    uint8_t digest[SHA_DIGEST_SIZE];
+
+    if (!expected || !buffer) {
+        return rc;
     }
 
-    *src_blocks = src->size;
-    free(src);
+    SHA_hash(buffer, blocks * BLOCKSIZE, digest);
+    hexdigest = PrintSha1(digest);
+
+    if (hexdigest != NULL) {
+        rc = strcmp(expected, hexdigest);
+
+        if (rc != 0 && printerror) {
+            fprintf(stderr, "failed to verify blocks (expected %s, read %s)\n",
+                expected, hexdigest);
+        }
+
+        free(hexdigest);
+    }
+
+    return rc;
+}
+
+static char* GetStashFileName(const char* base, const char* id, const char* postfix) {
+    char* fn;
+    int len;
+    int res;
+
+    if (base == NULL) {
+        return NULL;
+    }
+
+    if (id == NULL) {
+        id = "";
+    }
+
+    if (postfix == NULL) {
+        postfix = "";
+    }
+
+    len = strlen(STASH_DIRECTORY_BASE) + 1 + strlen(base) + 1 + strlen(id) + strlen(postfix) + 1;
+    fn = malloc(len);
+
+    if (fn == NULL) {
+        fprintf(stderr, "failed to malloc %d bytes for fn\n", len);
+        return NULL;
+    }
+
+    res = snprintf(fn, len, STASH_DIRECTORY_BASE "/%s/%s%s", base, id, postfix);
+
+    if (res < 0 || res >= len) {
+        fprintf(stderr, "failed to format file name (return value %d)\n", res);
+        free(fn);
+        return NULL;
+    }
+
+    return fn;
+}
+
+typedef void (*StashCallback)(const char*, void*);
+
+// Does a best effort enumeration of stash files. Ignores possible non-file
+// items in the stash directory and continues despite of errors. Calls the
+// 'callback' function for each file and passes 'data' to the function as a
+// parameter.
+
+static void EnumerateStash(const char* dirname, StashCallback callback, void* data) {
+    char* fn;
+    DIR* directory;
+    int len;
+    int res;
+    struct dirent* item;
+
+    if (dirname == NULL || callback == NULL) {
+        return;
+    }
+
+    directory = opendir(dirname);
+
+    if (directory == NULL) {
+        if (errno != ENOENT) {
+            fprintf(stderr, "failed to opendir %s (errno %d)\n", dirname, errno);
+        }
+        return;
+    }
+
+    while ((item = readdir(directory)) != NULL) {
+        if (item->d_type != DT_REG) {
+            continue;
+        }
+
+        len = strlen(dirname) + 1 + strlen(item->d_name) + 1;
+        fn = malloc(len);
+
+        if (fn == NULL) {
+            fprintf(stderr, "failed to malloc %d bytes for fn\n", len);
+            continue;
+        }
+
+        res = snprintf(fn, len, "%s/%s", dirname, item->d_name);
+
+        if (res < 0 || res >= len) {
+            fprintf(stderr, "failed to format file name (return value %d)\n", res);
+            free(fn);
+            continue;
+        }
+
+        callback(fn, data);
+        free(fn);
+    }
+
+    if (closedir(directory) == -1) {
+        fprintf(stderr, "failed to closedir %s (errno %d)\n", dirname, errno);
+    }
+}
+
+static void UpdateFileSize(const char* fn, void* data) {
+    int* size = (int*) data;
+    struct stat st;
+
+    if (!fn || !data) {
+        return;
+    }
+
+    if (stat(fn, &st) == -1) {
+        fprintf(stderr, "failed to stat %s (errno %d)\n", fn, errno);
+        return;
+    }
+
+    *size += st.st_size;
+}
+
+// Deletes the stash directory and all files in it. Assumes that it only
+// contains files. There is nothing we can do about unlikely, but possible
+// errors, so they are merely logged.
+
+static void DeleteFile(const char* fn, void* data) {
+    if (fn) {
+        fprintf(stderr, "deleting %s\n", fn);
+
+        if (unlink(fn) == -1 && errno != ENOENT) {
+            fprintf(stderr, "failed to unlink %s (errno %d)\n", fn, errno);
+        }
+    }
+}
+
+static void DeletePartial(const char* fn, void* data) {
+    if (fn && strstr(fn, ".partial") != NULL) {
+        DeleteFile(fn, data);
+    }
+}
+
+static void DeleteStash(const char* base) {
+    char* dirname;
+
+    if (base == NULL) {
+        return;
+    }
+
+    dirname = GetStashFileName(base, NULL, NULL);
+
+    if (dirname == NULL) {
+        return;
+    }
+
+    fprintf(stderr, "deleting stash %s\n", base);
+    EnumerateStash(dirname, DeleteFile, NULL);
+
+    if (rmdir(dirname) == -1) {
+        if (errno != ENOENT && errno != ENOTDIR) {
+            fprintf(stderr, "failed to rmdir %s (errno %d)\n", dirname, errno);
+        }
+    }
+
+    free(dirname);
+}
+
+static int LoadStash(const char* base, const char* id, int verify, int* blocks, uint8_t** buffer,
+        size_t* buffer_alloc, int printnoent) {
+    char *fn = NULL;
+    int blockcount = 0;
+    int fd = -1;
+    int rc = -1;
+    int res;
+    struct stat st;
+
+    if (!base || !id || !buffer || !buffer_alloc) {
+        goto lsout;
+    }
+
+    if (!blocks) {
+        blocks = &blockcount;
+    }
+
+    fn = GetStashFileName(base, id, NULL);
+
+    if (fn == NULL) {
+        goto lsout;
+    }
+
+    res = stat(fn, &st);
+
+    if (res == -1) {
+        if (errno != ENOENT || printnoent) {
+            fprintf(stderr, "failed to stat %s (errno %d)\n", fn, errno);
+        }
+        goto lsout;
+    }
+
+    fprintf(stderr, " loading %s\n", fn);
+
+    if ((st.st_size % BLOCKSIZE) != 0) {
+        fprintf(stderr, "%s size %zd not multiple of block size %d", fn, st.st_size, BLOCKSIZE);
+        goto lsout;
+    }
+
+    fd = TEMP_FAILURE_RETRY(open(fn, O_RDONLY));
+
+    if (fd == -1) {
+        fprintf(stderr, "failed to open %s (errno %d)\n", fn, errno);
+        goto lsout;
+    }
+
+    allocate(st.st_size, buffer, buffer_alloc);
+
+    if (read_all(fd, *buffer, st.st_size) == -1) {
+        goto lsout;
+    }
+
+    *blocks = st.st_size / BLOCKSIZE;
+
+    if (verify && VerifyBlocks(id, *buffer, *blocks, 1) != 0) {
+        fprintf(stderr, "unexpected contents in %s\n", fn);
+        DeleteFile(fn, NULL);
+        goto lsout;
+    }
+
+    rc = 0;
+
+lsout:
+    if (fd != -1) {
+        TEMP_FAILURE_RETRY(close(fd));
+    }
+
+    if (fn) {
+        free(fn);
+    }
+
+    return rc;
+}
+
+static int WriteStash(const char* base, const char* id, int blocks, uint8_t* buffer,
+        int checkspace) {
+    char *fn = NULL;
+    char *cn = NULL;
+    int fd = -1;
+    int rc = -1;
+    int res;
+
+    if (base == NULL || buffer == NULL) {
+        goto wsout;
+    }
+
+    if (checkspace && CacheSizeCheck(blocks * BLOCKSIZE) != 0) {
+        fprintf(stderr, "not enough space to write stash\n");
+        goto wsout;
+    }
+
+    fn = GetStashFileName(base, id, ".partial");
+    cn = GetStashFileName(base, id, NULL);
+
+    if (fn == NULL || cn == NULL) {
+        goto wsout;
+    }
+
+    fprintf(stderr, " writing %d blocks to %s\n", blocks, cn);
+
+    fd = TEMP_FAILURE_RETRY(open(fn, O_WRONLY | O_CREAT | O_TRUNC | O_SYNC, STASH_FILE_MODE));
+
+    if (fd == -1) {
+        fprintf(stderr, "failed to create %s (errno %d)\n", fn, errno);
+        goto wsout;
+    }
+
+    if (write_all(fd, buffer, blocks * BLOCKSIZE) == -1) {
+        goto wsout;
+    }
+
+    if (fsync(fd) == -1) {
+        fprintf(stderr, "failed to fsync %s (errno %d)\n", fn, errno);
+        goto wsout;
+    }
+
+    if (rename(fn, cn) == -1) {
+        fprintf(stderr, "failed to rename %s to %s (errno %d)\n", fn, cn, errno);
+        goto wsout;
+    }
+
+    rc = 0;
+
+wsout:
+    if (fd != -1) {
+        TEMP_FAILURE_RETRY(close(fd));
+    }
+
+    if (fn) {
+        free(fn);
+    }
+
+    if (cn) {
+        free(cn);
+    }
+
+    return rc;
+}
+
+// Creates a directory for storing stash files and checks if the /cache partition
+// hash enough space for the expected amount of blocks we need to store. Returns
+// >0 if we created the directory, zero if it existed already, and <0 of failure.
+
+static int CreateStash(State* state, int maxblocks, const char* blockdev, char** base) {
+    char* dirname = NULL;
+    const uint8_t* digest;
+    int rc = -1;
+    int res;
+    int size = 0;
+    SHA_CTX ctx;
+    struct stat st;
+
+    if (blockdev == NULL || base == NULL) {
+        goto csout;
+    }
+
+    // Stash directory should be different for each partition to avoid conflicts
+    // when updating multiple partitions at the same time, so we use the hash of
+    // the block device name as the base directory
+    SHA_init(&ctx);
+    SHA_update(&ctx, blockdev, strlen(blockdev));
+    digest = SHA_final(&ctx);
+    *base = PrintSha1(digest);
+
+    if (*base == NULL) {
+        goto csout;
+    }
+
+    dirname = GetStashFileName(*base, NULL, NULL);
+
+    if (dirname == NULL) {
+        goto csout;
+    }
+
+    res = stat(dirname, &st);
+
+    if (res == -1 && errno != ENOENT) {
+        ErrorAbort(state, "failed to stat %s (errno %d)\n", dirname, errno);
+        goto csout;
+    } else if (res != 0) {
+        fprintf(stderr, "creating stash %s\n", dirname);
+        res = mkdir(dirname, STASH_DIRECTORY_MODE);
+
+        if (res != 0) {
+            ErrorAbort(state, "failed to create %s (errno %d)\n", dirname, errno);
+            goto csout;
+        }
+
+        if (CacheSizeCheck(maxblocks * BLOCKSIZE) != 0) {
+            ErrorAbort(state, "not enough space for stash\n");
+            goto csout;
+        }
+
+        rc = 1; // Created directory
+        goto csout;
+    }
+
+    fprintf(stderr, "using existing stash %s\n", dirname);
+
+    // If the directory already exists, calculate the space already allocated to
+    // stash files and check if there's enough for all required blocks. Delete any
+    // partially completed stash files first.
+
+    EnumerateStash(dirname, DeletePartial, NULL);
+    EnumerateStash(dirname, UpdateFileSize, &size);
+
+    size = (maxblocks * BLOCKSIZE) - size;
+
+    if (size > 0 && CacheSizeCheck(size) != 0) {
+        ErrorAbort(state, "not enough space for stash (%d more needed)\n", size);
+        goto csout;
+    }
+
+    rc = 0; // Using existing directory
+
+csout:
+    if (dirname) {
+        free(dirname);
+    }
+
+    return rc;
+}
+
+static int SaveStash(const char* base, char** wordsave, uint8_t** buffer, size_t* buffer_alloc,
+                      int fd, int usehash, int* isunresumable) {
+    char *id = NULL;
+    int res = -1;
+    int blocks = 0;
+
+    if (!wordsave || !buffer || !buffer_alloc || !isunresumable) {
+        return -1;
+    }
+
+    id = strtok_r(NULL, " ", wordsave);
+
+    if (id == NULL) {
+        fprintf(stderr, "missing id field in stash command\n");
+        return -1;
+    }
+
+    if (usehash && LoadStash(base, id, 1, &blocks, buffer, buffer_alloc, 0) == 0) {
+        // Stash file already exists and has expected contents. Do not
+        // read from source again, as the source may have been already
+        // overwritten during a previous attempt.
+        return 0;
+    }
+
+    if (LoadSrcTgtVersion1(wordsave, NULL, &blocks, buffer, buffer_alloc, fd) == -1) {
+        return -1;
+    }
+
+    if (usehash && VerifyBlocks(id, *buffer, blocks, 1) != 0) {
+        // Source blocks have unexpected contents. If we actually need this
+        // data later, this is an unrecoverable error. However, the command
+        // that uses the data may have already completed previously, so the
+        // possible failure will occur during source block verification.
+        fprintf(stderr, "failed to load source blocks for stash %s\n", id);
+        return 0;
+    }
+
+    fprintf(stderr, "stashing %d blocks to %s\n", blocks, id);
+    return WriteStash(base, id, blocks, *buffer, 0);
+}
+
+static int FreeStash(const char* base, const char* id) {
+    char *fn = NULL;
+
+    if (base == NULL || id == NULL) {
+        return -1;
+    }
+
+    fn = GetStashFileName(base, id, NULL);
+
+    if (fn == NULL) {
+        return -1;
+    }
+
+    DeleteFile(fn, NULL);
+    free(fn);
+
+    return 0;
 }
 
 static void MoveRange(uint8_t* dest, RangeSet* locs, const uint8_t* source) {
@@ -312,64 +874,613 @@
 // On return, buffer is filled with the loaded source data (rearranged
 // and combined with stashed data as necessary).  buffer may be
 // reallocated if needed to accommodate the source data.  *tgt is the
-// target RangeSet.  Any stashes required are taken from stash_table
-// and free()'d after being used.
+// target RangeSet.  Any stashes required are loaded using LoadStash.
 
-static void LoadSrcTgtVersion2(char* wordsave, RangeSet** tgt, int* src_blocks,
+static int LoadSrcTgtVersion2(char** wordsave, RangeSet** tgt, int* src_blocks,
                                uint8_t** buffer, size_t* buffer_alloc, int fd,
-                               uint8_t** stash_table) {
+                               const char* stashbase, int* overlap) {
     char* word;
+    char* colonsave;
+    char* colon;
+    int id;
+    int res;
+    RangeSet* locs;
+    size_t stashalloc = 0;
+    uint8_t* stash = NULL;
 
     if (tgt != NULL) {
-        word = strtok_r(NULL, " ", &wordsave);
+        word = strtok_r(NULL, " ", wordsave);
         *tgt = parse_range(word);
     }
 
-    word = strtok_r(NULL, " ", &wordsave);
+    word = strtok_r(NULL, " ", wordsave);
     *src_blocks = strtol(word, NULL, 0);
 
     allocate(*src_blocks * BLOCKSIZE, buffer, buffer_alloc);
 
-    word = strtok_r(NULL, " ", &wordsave);
+    word = strtok_r(NULL, " ", wordsave);
     if (word[0] == '-' && word[1] == '\0') {
         // no source ranges, only stashes
     } else {
         RangeSet* src = parse_range(word);
+        res = ReadBlocks(src, *buffer, fd);
 
-        size_t p = 0;
-        int i;
-        for (i = 0; i < src->count; ++i) {
-            check_lseek(fd, (off64_t)src->pos[i*2] * BLOCKSIZE, SEEK_SET);
-            size_t sz = (src->pos[i*2+1] - src->pos[i*2]) * BLOCKSIZE;
-            readblock(fd, *buffer+p, sz);
-            p += sz;
+        if (overlap && tgt) {
+            *overlap = range_overlaps(src, *tgt);
         }
+
         free(src);
 
-        word = strtok_r(NULL, " ", &wordsave);
-        if (word == NULL) {
-            // no stashes, only source range
-            return;
+        if (res == -1) {
+            return -1;
         }
 
-        RangeSet* locs = parse_range(word);
+        word = strtok_r(NULL, " ", wordsave);
+        if (word == NULL) {
+            // no stashes, only source range
+            return 0;
+        }
+
+        locs = parse_range(word);
         MoveRange(*buffer, locs, *buffer);
+        free(locs);
     }
 
-    while ((word = strtok_r(NULL, " ", &wordsave)) != NULL) {
+    while ((word = strtok_r(NULL, " ", wordsave)) != NULL) {
         // Each word is a an index into the stash table, a colon, and
         // then a rangeset describing where in the source block that
         // stashed data should go.
-        char* colonsave = NULL;
-        char* colon = strtok_r(word, ":", &colonsave);
-        int stash_id = strtol(colon, NULL, 0);
+        colonsave = NULL;
+        colon = strtok_r(word, ":", &colonsave);
+
+        res = LoadStash(stashbase, colon, 0, NULL, &stash, &stashalloc, 1);
+
+        if (res == -1) {
+            // These source blocks will fail verification if used later, but we
+            // will let the caller decide if this is a fatal failure
+            fprintf(stderr, "failed to load stash %s\n", colon);
+            continue;
+        }
+
         colon = strtok_r(NULL, ":", &colonsave);
-        RangeSet* locs = parse_range(colon);
-        MoveRange(*buffer, locs, stash_table[stash_id]);
-        free(stash_table[stash_id]);
-        stash_table[stash_id] = NULL;
+        locs = parse_range(colon);
+
+        MoveRange(*buffer, locs, stash);
         free(locs);
     }
+
+    if (stash) {
+        free(stash);
+    }
+
+    return 0;
+}
+
+// Parameters for transfer list command functions
+typedef struct {
+    char* cmdname;
+    char* cpos;
+    char* freestash;
+    char* stashbase;
+    int canwrite;
+    int createdstash;
+    int fd;
+    int foundwrites;
+    int isunresumable;
+    int version;
+    int written;
+    NewThreadInfo nti;
+    pthread_t thread;
+    size_t bufsize;
+    uint8_t* buffer;
+    uint8_t* patch_start;
+} CommandParameters;
+
+// Do a source/target load for move/bsdiff/imgdiff in version 3.
+//
+// Parameters are the same as for LoadSrcTgtVersion2, except for 'onehash', which
+// tells the function whether to expect separate source and targe block hashes, or
+// if they are both the same and only one hash should be expected, and
+// 'isunresumable', which receives a non-zero value if block verification fails in
+// a way that the update cannot be resumed anymore.
+//
+// If the function is unable to load the necessary blocks or their contents don't
+// match the hashes, the return value is -1 and the command should be aborted.
+//
+// If the return value is 1, the command has already been completed according to
+// the contents of the target blocks, and should not be performed again.
+//
+// If the return value is 0, source blocks have expected content and the command
+// can be performed.
+
+static int LoadSrcTgtVersion3(CommandParameters* params, RangeSet** tgt, int* src_blocks,
+                              int onehash, int* overlap) {
+    char* srchash = NULL;
+    char* tgthash = NULL;
+    int overlap_blocks = 0;
+    int rc = -1;
+    uint8_t* tgtbuffer = NULL;
+
+    if (!params|| !tgt || !src_blocks || !overlap) {
+        goto v3out;
+    }
+
+    srchash = strtok_r(NULL, " ", &params->cpos);
+
+    if (srchash == NULL) {
+        fprintf(stderr, "missing source hash\n");
+        goto v3out;
+    }
+
+    if (onehash) {
+        tgthash = srchash;
+    } else {
+        tgthash = strtok_r(NULL, " ", &params->cpos);
+
+        if (tgthash == NULL) {
+            fprintf(stderr, "missing target hash\n");
+            goto v3out;
+        }
+    }
+
+    if (LoadSrcTgtVersion2(&params->cpos, tgt, src_blocks, &params->buffer, &params->bufsize,
+            params->fd, params->stashbase, overlap) == -1) {
+        goto v3out;
+    }
+
+    tgtbuffer = (uint8_t*) malloc((*tgt)->size * BLOCKSIZE);
+
+    if (tgtbuffer == NULL) {
+        fprintf(stderr, "failed to allocate %d bytes\n", (*tgt)->size * BLOCKSIZE);
+        goto v3out;
+    }
+
+    if (ReadBlocks(*tgt, tgtbuffer, params->fd) == -1) {
+        goto v3out;
+    }
+
+    if (VerifyBlocks(tgthash, tgtbuffer, (*tgt)->size, 0) == 0) {
+        // Target blocks already have expected content, command should be skipped
+        rc = 1;
+        goto v3out;
+    }
+
+    if (VerifyBlocks(srchash, params->buffer, *src_blocks, 1) == 0) {
+        // If source and target blocks overlap, stash the source blocks so we can
+        // resume from possible write errors
+        if (*overlap) {
+            fprintf(stderr, "stashing %d overlapping blocks to %s\n", *src_blocks,
+                srchash);
+
+            if (WriteStash(params->stashbase, srchash, *src_blocks, params->buffer, 1) != 0) {
+                fprintf(stderr, "failed to stash overlapping source blocks\n");
+                goto v3out;
+            }
+
+            // Can be deleted when the write has completed
+            params->freestash = srchash;
+        }
+
+        // Source blocks have expected content, command can proceed
+        rc = 0;
+        goto v3out;
+    }
+
+    if (*overlap && LoadStash(params->stashbase, srchash, 1, NULL, &params->buffer,
+                        &params->bufsize, 1) == 0) {
+        // Overlapping source blocks were previously stashed, command can proceed
+        if (params->canwrite) {
+            // We didn't create the stash, so delete after write only if we will
+            // actually perform the write
+            params->freestash = srchash;
+        }
+        rc = 0;
+        goto v3out;
+    }
+
+    // Valid source data not available, update cannot be resumed
+    fprintf(stderr, "partition has unexpected contents\n");
+    params->isunresumable = 1;
+
+v3out:
+    if (tgtbuffer) {
+        free(tgtbuffer);
+    }
+
+    return rc;
+}
+
+static int PerformCommandMove(CommandParameters* params) {
+    int blocks = 0;
+    int overlap = 0;
+    int rc = -1;
+    int status = 0;
+    RangeSet* tgt = NULL;
+
+    if (!params) {
+        goto pcmout;
+    }
+
+    if (params->version == 1) {
+        status = LoadSrcTgtVersion1(&params->cpos, &tgt, &blocks, &params->buffer,
+                    &params->bufsize, params->fd);
+    } else if (params->version == 2) {
+        status = LoadSrcTgtVersion2(&params->cpos, &tgt, &blocks, &params->buffer,
+                    &params->bufsize, params->fd, params->stashbase, NULL);
+    } else if (params->version >= 3) {
+        status = LoadSrcTgtVersion3(params, &tgt, &blocks, 1, &overlap);
+    }
+
+    if (status == -1) {
+        fprintf(stderr, "failed to read blocks for move\n");
+        goto pcmout;
+    }
+
+    if (status == 0) {
+        params->foundwrites = 1;
+    } else if (params->foundwrites) {
+        fprintf(stderr, "warning: commands executed out of order [%s]\n", params->cmdname);
+    }
+
+    if (params->canwrite) {
+        if (status == 0) {
+            fprintf(stderr, "  moving %d blocks\n", blocks);
+
+            if (WriteBlocks(tgt, params->buffer, params->fd) == -1) {
+                goto pcmout;
+            }
+        } else {
+            fprintf(stderr, "skipping %d already moved blocks\n", blocks);
+        }
+
+    }
+
+    if (params->freestash) {
+        FreeStash(params->stashbase, params->freestash);
+        params->freestash = NULL;
+    }
+
+    params->written += tgt->size;
+    rc = 0;
+
+pcmout:
+    if (tgt) {
+        free(tgt);
+    }
+
+    return rc;
+}
+
+static int PerformCommandStash(CommandParameters* params) {
+    if (!params) {
+        return -1;
+    }
+
+    return SaveStash(params->stashbase, &params->cpos, &params->buffer, &params->bufsize,
+                params->fd, (params->version >= 3), &params->isunresumable);
+}
+
+static int PerformCommandFree(CommandParameters* params) {
+    if (!params) {
+        return -1;
+    }
+
+    if (params->createdstash || params->canwrite) {
+        return FreeStash(params->stashbase, params->cpos);
+    }
+
+    return 0;
+}
+
+static int PerformCommandZero(CommandParameters* params) {
+    char* range = NULL;
+    int i;
+    int j;
+    int rc = -1;
+    RangeSet* tgt = NULL;
+
+    if (!params) {
+        goto pczout;
+    }
+
+    range = strtok_r(NULL, " ", &params->cpos);
+
+    if (range == NULL) {
+        fprintf(stderr, "missing target blocks for zero\n");
+        goto pczout;
+    }
+
+    tgt = parse_range(range);
+
+    fprintf(stderr, "  zeroing %d blocks\n", tgt->size);
+
+    allocate(BLOCKSIZE, &params->buffer, &params->bufsize);
+    memset(params->buffer, 0, BLOCKSIZE);
+
+    if (params->canwrite) {
+        for (i = 0; i < tgt->count; ++i) {
+            if (check_lseek(params->fd, (off64_t) tgt->pos[i * 2] * BLOCKSIZE, SEEK_SET) == -1) {
+                goto pczout;
+            }
+
+            for (j = tgt->pos[i * 2]; j < tgt->pos[i * 2 + 1]; ++j) {
+                if (write_all(params->fd, params->buffer, BLOCKSIZE) == -1) {
+                    goto pczout;
+                }
+            }
+        }
+    }
+
+    if (params->cmdname[0] == 'z') {
+        // Update only for the zero command, as the erase command will call
+        // this if DEBUG_ERASE is defined.
+        params->written += tgt->size;
+    }
+
+    rc = 0;
+
+pczout:
+    if (tgt) {
+        free(tgt);
+    }
+
+    return rc;
+}
+
+static int PerformCommandNew(CommandParameters* params) {
+    char* range = NULL;
+    int rc = -1;
+    RangeSet* tgt = NULL;
+    RangeSinkState rss;
+
+    if (!params) {
+        goto pcnout;
+    }
+
+    range = strtok_r(NULL, " ", &params->cpos);
+
+    if (range == NULL) {
+        goto pcnout;
+    }
+
+    tgt = parse_range(range);
+
+    if (params->canwrite) {
+        fprintf(stderr, " writing %d blocks of new data\n", tgt->size);
+
+        rss.fd = params->fd;
+        rss.tgt = tgt;
+        rss.p_block = 0;
+        rss.p_remain = (tgt->pos[1] - tgt->pos[0]) * BLOCKSIZE;
+
+        if (check_lseek(params->fd, (off64_t) tgt->pos[0] * BLOCKSIZE, SEEK_SET) == -1) {
+            goto pcnout;
+        }
+
+        pthread_mutex_lock(&params->nti.mu);
+        params->nti.rss = &rss;
+        pthread_cond_broadcast(&params->nti.cv);
+
+        while (params->nti.rss) {
+            pthread_cond_wait(&params->nti.cv, &params->nti.mu);
+        }
+
+        pthread_mutex_unlock(&params->nti.mu);
+    }
+
+    params->written += tgt->size;
+    rc = 0;
+
+pcnout:
+    if (tgt) {
+        free(tgt);
+    }
+
+    return rc;
+}
+
+static int PerformCommandDiff(CommandParameters* params) {
+    char* logparams = NULL;
+    char* value = NULL;
+    int blocks = 0;
+    int overlap = 0;
+    int rc = -1;
+    int status = 0;
+    RangeSet* tgt = NULL;
+    RangeSinkState rss;
+    size_t len = 0;
+    size_t offset = 0;
+    Value patch_value;
+
+    if (!params) {
+        goto pcdout;
+    }
+
+    logparams = strdup(params->cpos);
+    value = strtok_r(NULL, " ", &params->cpos);
+
+    if (value == NULL) {
+        fprintf(stderr, "missing patch offset for %s\n", params->cmdname);
+        goto pcdout;
+    }
+
+    offset = strtoul(value, NULL, 0);
+
+    value = strtok_r(NULL, " ", &params->cpos);
+
+    if (value == NULL) {
+        fprintf(stderr, "missing patch length for %s\n", params->cmdname);
+        goto pcdout;
+    }
+
+    len = strtoul(value, NULL, 0);
+
+    if (params->version == 1) {
+        status = LoadSrcTgtVersion1(&params->cpos, &tgt, &blocks, &params->buffer,
+                    &params->bufsize, params->fd);
+    } else if (params->version == 2) {
+        status = LoadSrcTgtVersion2(&params->cpos, &tgt, &blocks, &params->buffer,
+                    &params->bufsize, params->fd, params->stashbase, NULL);
+    } else if (params->version >= 3) {
+        status = LoadSrcTgtVersion3(params, &tgt, &blocks, 0, &overlap);
+    }
+
+    if (status == -1) {
+        fprintf(stderr, "failed to read blocks for diff\n");
+        goto pcdout;
+    }
+
+    if (status == 0) {
+        params->foundwrites = 1;
+    } else if (params->foundwrites) {
+        fprintf(stderr, "warning: commands executed out of order [%s]\n", params->cmdname);
+    }
+
+    if (params->canwrite) {
+        if (status == 0) {
+            fprintf(stderr, "patching %d blocks to %d\n", blocks, tgt->size);
+
+            patch_value.type = VAL_BLOB;
+            patch_value.size = len;
+            patch_value.data = (char*) (params->patch_start + offset);
+
+            rss.fd = params->fd;
+            rss.tgt = tgt;
+            rss.p_block = 0;
+            rss.p_remain = (tgt->pos[1] - tgt->pos[0]) * BLOCKSIZE;
+
+            if (check_lseek(params->fd, (off64_t) tgt->pos[0] * BLOCKSIZE, SEEK_SET) == -1) {
+                goto pcdout;
+            }
+
+            if (params->cmdname[0] == 'i') {      // imgdiff
+                ApplyImagePatch(params->buffer, blocks * BLOCKSIZE, &patch_value,
+                    &RangeSinkWrite, &rss, NULL, NULL);
+            } else {
+                ApplyBSDiffPatch(params->buffer, blocks * BLOCKSIZE, &patch_value,
+                    0, &RangeSinkWrite, &rss, NULL);
+            }
+
+            // We expect the output of the patcher to fill the tgt ranges exactly.
+            if (rss.p_block != tgt->count || rss.p_remain != 0) {
+                fprintf(stderr, "range sink underrun?\n");
+            }
+        } else {
+            fprintf(stderr, "skipping %d blocks already patched to %d [%s]\n",
+                blocks, tgt->size, logparams);
+        }
+    }
+
+    if (params->freestash) {
+        FreeStash(params->stashbase, params->freestash);
+        params->freestash = NULL;
+    }
+
+    params->written += tgt->size;
+    rc = 0;
+
+pcdout:
+    if (logparams) {
+        free(logparams);
+    }
+
+    if (tgt) {
+        free(tgt);
+    }
+
+    return rc;
+}
+
+static int PerformCommandErase(CommandParameters* params) {
+    char* range = NULL;
+    int i;
+    int rc = -1;
+    RangeSet* tgt = NULL;
+    struct stat st;
+    uint64_t blocks[2];
+
+    if (DEBUG_ERASE) {
+        return PerformCommandZero(params);
+    }
+
+    if (!params) {
+        goto pceout;
+    }
+
+    if (fstat(params->fd, &st) == -1) {
+        fprintf(stderr, "failed to fstat device to erase (errno %d)\n", errno);
+        goto pceout;
+    }
+
+    if (!S_ISBLK(st.st_mode)) {
+        fprintf(stderr, "not a block device; skipping erase\n");
+        rc = 0;
+        goto pceout;
+    }
+
+    range = strtok_r(NULL, " ", &params->cpos);
+
+    if (range == NULL) {
+        fprintf(stderr, "missing target blocks for zero\n");
+        goto pceout;
+    }
+
+    tgt = parse_range(range);
+
+    if (params->canwrite) {
+        fprintf(stderr, " erasing %d blocks\n", tgt->size);
+
+        for (i = 0; i < tgt->count; ++i) {
+            // offset in bytes
+            blocks[0] = tgt->pos[i * 2] * (uint64_t) BLOCKSIZE;
+            // length in bytes
+            blocks[1] = (tgt->pos[i * 2 + 1] - tgt->pos[i * 2]) * (uint64_t) BLOCKSIZE;
+
+            if (ioctl(params->fd, BLKDISCARD, &blocks) == -1) {
+                fprintf(stderr, "failed to blkdiscard (errno %d)\n", errno);
+                // Continue anyway, nothing we can do
+            }
+        }
+    }
+
+    rc = 0;
+
+pceout:
+    if (tgt) {
+        free(tgt);
+    }
+
+    return rc;
+}
+
+// Definitions for transfer list command functions
+typedef int (*CommandFunction)(CommandParameters*);
+
+typedef struct {
+    const char* name;
+    CommandFunction f;
+} Command;
+
+// CompareCommands and CompareCommandNames are for the hash table
+
+static int CompareCommands(const void* c1, const void* c2) {
+    return strcmp(((const Command*) c1)->name, ((const Command*) c2)->name);
+}
+
+static int CompareCommandNames(const void* c1, const void* c2) {
+    return strcmp(((const Command*) c1)->name, (const char*) c2);
+}
+
+// HashString is used to hash command names for the hash table
+
+static unsigned int HashString(const char *s) {
+    unsigned int hash = 0;
+    if (s) {
+        while (*s) {
+            hash = hash * 33 + *s++;
+        }
+    }
+    return hash;
 }
 
 // args:
@@ -378,379 +1489,371 @@
 //    - new data stream (filename within package.zip)
 //    - patch stream (filename within package.zip, must be uncompressed)
 
-Value* BlockImageUpdateFn(const char* name, State* state, int argc, Expr* argv[]) {
-    Value* blockdev_filename;
-    Value* transfer_list_value;
+static Value* PerformBlockImageUpdate(const char* name, State* state, int argc, Expr* argv[],
+            const Command* commands, int cmdcount, int dryrun) {
+
+    char* line = NULL;
+    char* linesave = NULL;
+    char* logcmd = NULL;
     char* transfer_list = NULL;
-    Value* new_data_fn;
-    Value* patch_data_fn;
-    bool success = false;
+    CommandParameters params;
+    const Command* cmd = NULL;
+    const ZipEntry* new_entry = NULL;
+    const ZipEntry* patch_entry = NULL;
+    FILE* cmd_pipe = NULL;
+    HashTable* cmdht = NULL;
+    int i;
+    int res;
+    int rc = -1;
+    int stash_max_blocks = 0;
+    int total_blocks = 0;
+    pthread_attr_t attr;
+    unsigned int cmdhash;
+    UpdaterInfo* ui = NULL;
+    Value* blockdev_filename = NULL;
+    Value* new_data_fn = NULL;
+    Value* patch_data_fn = NULL;
+    Value* transfer_list_value = NULL;
+    ZipArchive* za = NULL;
+
+    memset(&params, 0, sizeof(params));
+    params.canwrite = !dryrun;
+
+    fprintf(stderr, "performing %s\n", dryrun ? "verification" : "update");
 
     if (ReadValueArgs(state, argv, 4, &blockdev_filename, &transfer_list_value,
-                      &new_data_fn, &patch_data_fn) < 0) {
-        return NULL;
+            &new_data_fn, &patch_data_fn) < 0) {
+        goto pbiudone;
     }
 
     if (blockdev_filename->type != VAL_STRING) {
         ErrorAbort(state, "blockdev_filename argument to %s must be string", name);
-        goto done;
+        goto pbiudone;
     }
     if (transfer_list_value->type != VAL_BLOB) {
         ErrorAbort(state, "transfer_list argument to %s must be blob", name);
-        goto done;
+        goto pbiudone;
     }
     if (new_data_fn->type != VAL_STRING) {
         ErrorAbort(state, "new_data_fn argument to %s must be string", name);
-        goto done;
+        goto pbiudone;
     }
     if (patch_data_fn->type != VAL_STRING) {
         ErrorAbort(state, "patch_data_fn argument to %s must be string", name);
-        goto done;
+        goto pbiudone;
     }
 
-    UpdaterInfo* ui = (UpdaterInfo*)(state->cookie);
-    FILE* cmd_pipe = ui->cmd_pipe;
+    ui = (UpdaterInfo*) state->cookie;
 
-    ZipArchive* za = ((UpdaterInfo*)(state->cookie))->package_zip;
+    if (ui == NULL) {
+        goto pbiudone;
+    }
 
-    const ZipEntry* patch_entry = mzFindZipEntry(za, patch_data_fn->data);
+    cmd_pipe = ui->cmd_pipe;
+    za = ui->package_zip;
+
+    if (cmd_pipe == NULL || za == NULL) {
+        goto pbiudone;
+    }
+
+    patch_entry = mzFindZipEntry(za, patch_data_fn->data);
+
     if (patch_entry == NULL) {
-        ErrorAbort(state, "%s(): no file \"%s\" in package", name, patch_data_fn->data);
-        goto done;
+        fprintf(stderr, "%s(): no file \"%s\" in package", name, patch_data_fn->data);
+        goto pbiudone;
     }
 
-    uint8_t* patch_start = ((UpdaterInfo*)(state->cookie))->package_zip_addr +
-        mzGetZipEntryOffset(patch_entry);
+    params.patch_start = ui->package_zip_addr + mzGetZipEntryOffset(patch_entry);
+    new_entry = mzFindZipEntry(za, new_data_fn->data);
 
-    const ZipEntry* new_entry = mzFindZipEntry(za, new_data_fn->data);
     if (new_entry == NULL) {
-        ErrorAbort(state, "%s(): no file \"%s\" in package", name, new_data_fn->data);
-        goto done;
+        fprintf(stderr, "%s(): no file \"%s\" in package", name, new_data_fn->data);
+        goto pbiudone;
     }
 
-    // The transfer list is a text file containing commands to
-    // transfer data from one place to another on the target
-    // partition.  We parse it and execute the commands in order:
-    //
-    //    zero [rangeset]
-    //      - fill the indicated blocks with zeros
-    //
-    //    new [rangeset]
-    //      - fill the blocks with data read from the new_data file
-    //
-    //    erase [rangeset]
-    //      - mark the given blocks as empty
-    //
-    //    move <...>
-    //    bsdiff <patchstart> <patchlen> <...>
-    //    imgdiff <patchstart> <patchlen> <...>
-    //      - read the source blocks, apply a patch (or not in the
-    //        case of move), write result to target blocks.  bsdiff or
-    //        imgdiff specifies the type of patch; move means no patch
-    //        at all.
-    //
-    //        The format of <...> differs between versions 1 and 2;
-    //        see the LoadSrcTgtVersion{1,2}() functions for a
-    //        description of what's expected.
-    //
-    //    stash <stash_id> <src_range>
-    //      - (version 2 only) load the given source range and stash
-    //        the data in the given slot of the stash table.
-    //
-    // The creator of the transfer list will guarantee that no block
-    // is read (ie, used as the source for a patch or move) after it
-    // has been written.
-    //
-    // In version 2, the creator will guarantee that a given stash is
-    // loaded (with a stash command) before it's used in a
-    // move/bsdiff/imgdiff command.
-    //
-    // Within one command the source and target ranges may overlap so
-    // in general we need to read the entire source into memory before
-    // writing anything to the target blocks.
-    //
-    // All the patch data is concatenated into one patch_data file in
-    // the update package.  It must be stored uncompressed because we
-    // memory-map it in directly from the archive.  (Since patches are
-    // already compressed, we lose very little by not compressing
-    // their concatenation.)
+    params.fd = TEMP_FAILURE_RETRY(open(blockdev_filename->data, O_RDWR));
 
-    pthread_t new_data_thread;
-    NewThreadInfo nti;
-    nti.za = za;
-    nti.entry = new_entry;
-    nti.rss = NULL;
-    pthread_mutex_init(&nti.mu, NULL);
-    pthread_cond_init(&nti.cv, NULL);
-
-    pthread_attr_t attr;
-    pthread_attr_init(&attr);
-    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
-    pthread_create(&new_data_thread, &attr, unzip_new_data, &nti);
-
-    int i, j;
-
-    char* linesave;
-    char* wordsave;
-
-    int fd = open(blockdev_filename->data, O_RDWR);
-    if (fd < 0) {
-        ErrorAbort(state, "failed to open %s: %s", blockdev_filename->data, strerror(errno));
-        goto done;
+    if (params.fd == -1) {
+        fprintf(stderr, "failed to open %s: %s", blockdev_filename->data, strerror(errno));
+        goto pbiudone;
     }
 
-    char* line;
-    char* word;
+    if (params.canwrite) {
+        params.nti.za = za;
+        params.nti.entry = new_entry;
 
-    // The data in transfer_list_value is not necessarily
-    // null-terminated, so we need to copy it to a new buffer and add
-    // the null that strtok_r will need.
-    transfer_list = malloc(transfer_list_value->size+1);
+        pthread_mutex_init(&params.nti.mu, NULL);
+        pthread_cond_init(&params.nti.cv, NULL);
+        pthread_attr_init(&attr);
+        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
+
+        if (pthread_create(&params.thread, &attr, unzip_new_data, &params.nti) != 0) {
+            fprintf(stderr, "failed to create a thread (errno %d)\n", errno);
+            goto pbiudone;
+        }
+    }
+
+    // The data in transfer_list_value is not necessarily null-terminated, so we need
+    // to copy it to a new buffer and add the null that strtok_r will need.
+    transfer_list = malloc(transfer_list_value->size + 1);
+
     if (transfer_list == NULL) {
         fprintf(stderr, "failed to allocate %zd bytes for transfer list\n",
-                transfer_list_value->size+1);
-        exit(1);
+            transfer_list_value->size + 1);
+        goto pbiudone;
     }
+
     memcpy(transfer_list, transfer_list_value->data, transfer_list_value->size);
     transfer_list[transfer_list_value->size] = '\0';
 
+    // First line in transfer list is the version number
     line = strtok_r(transfer_list, "\n", &linesave);
+    params.version = strtol(line, NULL, 0);
 
-    int version;
-    // first line in transfer list is the version number; currently
-    // there's only version 1.
-    if (strcmp(line, "1") == 0) {
-        version = 1;
-    } else if (strcmp(line, "2") == 0) {
-        version = 2;
-    } else {
-        ErrorAbort(state, "unexpected transfer list version [%s]\n", line);
-        goto done;
+    if (params.version < 1 || params.version > 3) {
+        fprintf(stderr, "unexpected transfer list version [%s]\n", line);
+        goto pbiudone;
     }
-    printf("blockimg version is %d\n", version);
 
-    // second line in transfer list is the total number of blocks we
-    // expect to write.
+    fprintf(stderr, "blockimg version is %d\n", params.version);
+
+    // Second line in transfer list is the total number of blocks we expect to write
     line = strtok_r(NULL, "\n", &linesave);
-    int total_blocks = strtol(line, NULL, 0);
-    // shouldn't happen, but avoid divide by zero.
-    if (total_blocks == 0) ++total_blocks;
-    int blocks_so_far = 0;
+    total_blocks = strtol(line, NULL, 0);
 
-    uint8_t** stash_table = NULL;
-    if (version >= 2) {
-        // Next line is how many stash entries are needed simultaneously.
+    if (total_blocks < 0) {
+        ErrorAbort(state, "unexpected block count [%s]\n", line);
+        goto pbiudone;
+    } else if (total_blocks == 0) {
+        rc = 0;
+        goto pbiudone;
+    }
+
+    if (params.version >= 2) {
+        // Third line is how many stash entries are needed simultaneously
         line = strtok_r(NULL, "\n", &linesave);
-        int stash_entries = strtol(line, NULL, 0);
+        fprintf(stderr, "maximum stash entries %s\n", line);
 
-        stash_table = (uint8_t**) calloc(stash_entries, sizeof(uint8_t*));
-        if (stash_table == NULL) {
-            fprintf(stderr, "failed to allocate %d-entry stash table\n", stash_entries);
-            exit(1);
+        // Fourth line is the maximum number of blocks that will be stashed simultaneously
+        line = strtok_r(NULL, "\n", &linesave);
+        stash_max_blocks = strtol(line, NULL, 0);
+
+        if (stash_max_blocks < 0) {
+            ErrorAbort(state, "unexpected maximum stash blocks [%s]\n", line);
+            goto pbiudone;
         }
 
-        // Next line is the maximum number of blocks that will be
-        // stashed simultaneously.  This could be used to verify that
-        // enough memory or scratch disk space is available.
-        line = strtok_r(NULL, "\n", &linesave);
-        int stash_max_blocks = strtol(line, NULL, 0);
+        if (stash_max_blocks >= 0) {
+            res = CreateStash(state, stash_max_blocks, blockdev_filename->data,
+                    &params.stashbase);
+
+            if (res == -1) {
+                goto pbiudone;
+            }
+
+            params.createdstash = res;
+        }
     }
 
-    uint8_t* buffer = NULL;
-    size_t buffer_alloc = 0;
+    // Build a hash table of the available commands
+    cmdht = mzHashTableCreate(cmdcount, NULL);
 
-    // third and subsequent lines are all individual transfer commands.
+    for (i = 0; i < cmdcount; ++i) {
+        cmdhash = HashString(commands[i].name);
+        mzHashTableLookup(cmdht, cmdhash, (void*) &commands[i], CompareCommands, true);
+    }
+
+    // Subsequent lines are all individual transfer commands
     for (line = strtok_r(NULL, "\n", &linesave); line;
          line = strtok_r(NULL, "\n", &linesave)) {
 
-        char* style;
-        style = strtok_r(line, " ", &wordsave);
+        logcmd = strdup(line);
+        params.cmdname = strtok_r(line, " ", &params.cpos);
 
-        if (strcmp("move", style) == 0) {
-            RangeSet* tgt;
-            int src_blocks;
-            if (version == 1) {
-                LoadSrcTgtVersion1(wordsave, &tgt, &src_blocks,
-                                   &buffer, &buffer_alloc, fd);
-            } else if (version == 2) {
-                LoadSrcTgtVersion2(wordsave, &tgt, &src_blocks,
-                                   &buffer, &buffer_alloc, fd, stash_table);
-            }
+        if (params.cmdname == NULL) {
+            fprintf(stderr, "missing command [%s]\n", line);
+            goto pbiudone;
+        }
 
-            printf("  moving %d blocks\n", src_blocks);
+        cmdhash = HashString(params.cmdname);
+        cmd = (const Command*) mzHashTableLookup(cmdht, cmdhash, params.cmdname,
+                                    CompareCommandNames, false);
 
-            size_t p = 0;
-            for (i = 0; i < tgt->count; ++i) {
-                check_lseek(fd, (off64_t)tgt->pos[i*2] * BLOCKSIZE, SEEK_SET);
-                size_t sz = (tgt->pos[i*2+1] - tgt->pos[i*2]) * BLOCKSIZE;
-                writeblock(fd, buffer+p, sz);
-                p += sz;
-            }
+        if (cmd == NULL) {
+            fprintf(stderr, "unexpected command [%s]\n", params.cmdname);
+            goto pbiudone;
+        }
 
-            blocks_so_far += tgt->size;
-            fprintf(cmd_pipe, "set_progress %.4f\n", (double)blocks_so_far / total_blocks);
+        if (cmd->f != NULL && cmd->f(&params) == -1) {
+            fprintf(stderr, "failed to execute command [%s]\n",
+                logcmd ? logcmd : params.cmdname);
+            goto pbiudone;
+        }
+
+        if (logcmd) {
+            free(logcmd);
+            logcmd = NULL;
+        }
+
+        if (params.canwrite) {
+            fprintf(cmd_pipe, "set_progress %.4f\n", (double) params.written / total_blocks);
             fflush(cmd_pipe);
-
-            free(tgt);
-
-        } else if (strcmp("stash", style) == 0) {
-            word = strtok_r(NULL, " ", &wordsave);
-            int stash_id = strtol(word, NULL, 0);
-            int src_blocks;
-            size_t stash_alloc = 0;
-
-            // Even though the "stash" style only appears in version
-            // 2, the version 1 source loader happens to do exactly
-            // what we want to read data into the stash_table.
-            LoadSrcTgtVersion1(wordsave, NULL, &src_blocks,
-                               stash_table + stash_id, &stash_alloc, fd);
-
-        } else if (strcmp("zero", style) == 0 ||
-                   (DEBUG_ERASE && strcmp("erase", style) == 0)) {
-            word = strtok_r(NULL, " ", &wordsave);
-            RangeSet* tgt = parse_range(word);
-
-            printf("  zeroing %d blocks\n", tgt->size);
-
-            allocate(BLOCKSIZE, &buffer, &buffer_alloc);
-            memset(buffer, 0, BLOCKSIZE);
-            for (i = 0; i < tgt->count; ++i) {
-                check_lseek(fd, (off64_t)tgt->pos[i*2] * BLOCKSIZE, SEEK_SET);
-                for (j = tgt->pos[i*2]; j < tgt->pos[i*2+1]; ++j) {
-                    writeblock(fd, buffer, BLOCKSIZE);
-                }
-            }
-
-            if (style[0] == 'z') {   // "zero" but not "erase"
-                blocks_so_far += tgt->size;
-                fprintf(cmd_pipe, "set_progress %.4f\n", (double)blocks_so_far / total_blocks);
-                fflush(cmd_pipe);
-            }
-
-            free(tgt);
-        } else if (strcmp("new", style) == 0) {
-
-            word = strtok_r(NULL, " ", &wordsave);
-            RangeSet* tgt = parse_range(word);
-
-            printf("  writing %d blocks of new data\n", tgt->size);
-
-            RangeSinkState rss;
-            rss.fd = fd;
-            rss.tgt = tgt;
-            rss.p_block = 0;
-            rss.p_remain = (tgt->pos[1] - tgt->pos[0]) * BLOCKSIZE;
-            check_lseek(fd, (off64_t)tgt->pos[0] * BLOCKSIZE, SEEK_SET);
-
-            pthread_mutex_lock(&nti.mu);
-            nti.rss = &rss;
-            pthread_cond_broadcast(&nti.cv);
-            while (nti.rss) {
-                pthread_cond_wait(&nti.cv, &nti.mu);
-            }
-            pthread_mutex_unlock(&nti.mu);
-
-            blocks_so_far += tgt->size;
-            fprintf(cmd_pipe, "set_progress %.4f\n", (double)blocks_so_far / total_blocks);
-            fflush(cmd_pipe);
-
-            free(tgt);
-
-        } else if (strcmp("bsdiff", style) == 0 ||
-                   strcmp("imgdiff", style) == 0) {
-            word = strtok_r(NULL, " ", &wordsave);
-            size_t patch_offset = strtoul(word, NULL, 0);
-            word = strtok_r(NULL, " ", &wordsave);
-            size_t patch_len = strtoul(word, NULL, 0);
-
-            RangeSet* tgt;
-            int src_blocks;
-            if (version == 1) {
-                LoadSrcTgtVersion1(wordsave, &tgt, &src_blocks,
-                                   &buffer, &buffer_alloc, fd);
-            } else if (version == 2) {
-                LoadSrcTgtVersion2(wordsave, &tgt, &src_blocks,
-                                   &buffer, &buffer_alloc, fd, stash_table);
-            }
-
-            printf("  patching %d blocks to %d\n", src_blocks, tgt->size);
-
-            Value patch_value;
-            patch_value.type = VAL_BLOB;
-            patch_value.size = patch_len;
-            patch_value.data = (char*)(patch_start + patch_offset);
-
-            RangeSinkState rss;
-            rss.fd = fd;
-            rss.tgt = tgt;
-            rss.p_block = 0;
-            rss.p_remain = (tgt->pos[1] - tgt->pos[0]) * BLOCKSIZE;
-            check_lseek(fd, (off64_t)tgt->pos[0] * BLOCKSIZE, SEEK_SET);
-
-            if (style[0] == 'i') {      // imgdiff
-                ApplyImagePatch(buffer, src_blocks * BLOCKSIZE,
-                                &patch_value,
-                                &RangeSinkWrite, &rss, NULL, NULL);
-            } else {
-                ApplyBSDiffPatch(buffer, src_blocks * BLOCKSIZE,
-                                 &patch_value, 0,
-                                 &RangeSinkWrite, &rss, NULL);
-            }
-
-            // We expect the output of the patcher to fill the tgt ranges exactly.
-            if (rss.p_block != tgt->count || rss.p_remain != 0) {
-                fprintf(stderr, "range sink underrun?\n");
-            }
-
-            blocks_so_far += tgt->size;
-            fprintf(cmd_pipe, "set_progress %.4f\n", (double)blocks_so_far / total_blocks);
-            fflush(cmd_pipe);
-
-            free(tgt);
-        } else if (!DEBUG_ERASE && strcmp("erase", style) == 0) {
-            struct stat st;
-            if (fstat(fd, &st) == 0 && S_ISBLK(st.st_mode)) {
-                word = strtok_r(NULL, " ", &wordsave);
-                RangeSet* tgt = parse_range(word);
-
-                printf("  erasing %d blocks\n", tgt->size);
-
-                for (i = 0; i < tgt->count; ++i) {
-                    uint64_t range[2];
-                    // offset in bytes
-                    range[0] = tgt->pos[i*2] * (uint64_t)BLOCKSIZE;
-                    // len in bytes
-                    range[1] = (tgt->pos[i*2+1] - tgt->pos[i*2]) * (uint64_t)BLOCKSIZE;
-
-                    if (ioctl(fd, BLKDISCARD, &range) < 0) {
-                        printf("    blkdiscard failed: %s\n", strerror(errno));
-                    }
-                }
-
-                free(tgt);
-            } else {
-                printf("  ignoring erase (not block device)\n");
-            }
-        } else {
-            fprintf(stderr, "unknown transfer style \"%s\"\n", style);
-            exit(1);
         }
     }
 
-    pthread_join(new_data_thread, NULL);
-    success = true;
+    if (params.canwrite) {
+        pthread_join(params.thread, NULL);
 
-    free(buffer);
-    printf("wrote %d blocks; expected %d\n", blocks_so_far, total_blocks);
-    printf("max alloc needed was %zu\n", buffer_alloc);
+        fprintf(stderr, "wrote %d blocks; expected %d\n", params.written, total_blocks);
+        fprintf(stderr, "max alloc needed was %zu\n", params.bufsize);
 
-  done:
-    free(transfer_list);
-    FreeValue(blockdev_filename);
-    FreeValue(transfer_list_value);
-    FreeValue(new_data_fn);
-    FreeValue(patch_data_fn);
-    return StringValue(success ? strdup("t") : strdup(""));
+        // Delete stash only after successfully completing the update, as it
+        // may contain blocks needed to complete the update later.
+        DeleteStash(params.stashbase);
+    } else {
+        fprintf(stderr, "verified partition contents; update may be resumed\n");
+    }
+
+    rc = 0;
+
+pbiudone:
+    if (params.fd != -1) {
+        if (fsync(params.fd) == -1) {
+            fprintf(stderr, "failed to fsync device (errno %d)\n", errno);
+        }
+        TEMP_FAILURE_RETRY(close(params.fd));
+    }
+
+    if (logcmd) {
+        free(logcmd);
+    }
+
+    if (cmdht) {
+        mzHashTableFree(cmdht);
+    }
+
+    if (params.buffer) {
+        free(params.buffer);
+    }
+
+    if (transfer_list) {
+        free(transfer_list);
+    }
+
+    if (blockdev_filename) {
+        FreeValue(blockdev_filename);
+    }
+
+    if (transfer_list_value) {
+        FreeValue(transfer_list_value);
+    }
+
+    if (new_data_fn) {
+        FreeValue(new_data_fn);
+    }
+
+    if (patch_data_fn) {
+        FreeValue(patch_data_fn);
+    }
+
+    // Only delete the stash if the update cannot be resumed, or it's
+    // a verification run and we created the stash.
+    if (params.isunresumable || (!params.canwrite && params.createdstash)) {
+        DeleteStash(params.stashbase);
+    }
+
+    if (params.stashbase) {
+        free(params.stashbase);
+    }
+
+    return StringValue(rc == 0 ? strdup("t") : strdup(""));
+}
+
+// The transfer list is a text file containing commands to
+// transfer data from one place to another on the target
+// partition.  We parse it and execute the commands in order:
+//
+//    zero [rangeset]
+//      - fill the indicated blocks with zeros
+//
+//    new [rangeset]
+//      - fill the blocks with data read from the new_data file
+//
+//    erase [rangeset]
+//      - mark the given blocks as empty
+//
+//    move <...>
+//    bsdiff <patchstart> <patchlen> <...>
+//    imgdiff <patchstart> <patchlen> <...>
+//      - read the source blocks, apply a patch (or not in the
+//        case of move), write result to target blocks.  bsdiff or
+//        imgdiff specifies the type of patch; move means no patch
+//        at all.
+//
+//        The format of <...> differs between versions 1 and 2;
+//        see the LoadSrcTgtVersion{1,2}() functions for a
+//        description of what's expected.
+//
+//    stash <stash_id> <src_range>
+//      - (version 2+ only) load the given source range and stash
+//        the data in the given slot of the stash table.
+//
+// The creator of the transfer list will guarantee that no block
+// is read (ie, used as the source for a patch or move) after it
+// has been written.
+//
+// In version 2, the creator will guarantee that a given stash is
+// loaded (with a stash command) before it's used in a
+// move/bsdiff/imgdiff command.
+//
+// Within one command the source and target ranges may overlap so
+// in general we need to read the entire source into memory before
+// writing anything to the target blocks.
+//
+// All the patch data is concatenated into one patch_data file in
+// the update package.  It must be stored uncompressed because we
+// memory-map it in directly from the archive.  (Since patches are
+// already compressed, we lose very little by not compressing
+// their concatenation.)
+//
+// In version 3, commands that read data from the partition (i.e.
+// move/bsdiff/imgdiff/stash) have one or more additional hashes
+// before the range parameters, which are used to check if the
+// command has already been completed and verify the integrity of
+// the source data.
+
+Value* BlockImageVerifyFn(const char* name, State* state, int argc, Expr* argv[]) {
+    // Commands which are not tested are set to NULL to skip them completely
+    const Command commands[] = {
+        { "bsdiff",     PerformCommandDiff  },
+        { "erase",      NULL                },
+        { "free",       PerformCommandFree  },
+        { "imgdiff",    PerformCommandDiff  },
+        { "move",       PerformCommandMove  },
+        { "new",        NULL                },
+        { "stash",      PerformCommandStash },
+        { "zero",       NULL                }
+    };
+
+    // Perform a dry run without writing to test if an update can proceed
+    return PerformBlockImageUpdate(name, state, argc, argv, commands,
+                sizeof(commands) / sizeof(commands[0]), 1);
+}
+
+Value* BlockImageUpdateFn(const char* name, State* state, int argc, Expr* argv[]) {
+    const Command commands[] = {
+        { "bsdiff",     PerformCommandDiff  },
+        { "erase",      PerformCommandErase },
+        { "free",       PerformCommandFree  },
+        { "imgdiff",    PerformCommandDiff  },
+        { "move",       PerformCommandMove  },
+        { "new",        PerformCommandNew   },
+        { "stash",      PerformCommandStash },
+        { "zero",       PerformCommandZero  }
+    };
+
+    return PerformBlockImageUpdate(name, state, argc, argv, commands,
+                sizeof(commands) / sizeof(commands[0]), 0);
 }
 
 Value* RangeSha1Fn(const char* name, State* state, int argc, Expr* argv[]) {
@@ -784,9 +1887,19 @@
 
     int i, j;
     for (i = 0; i < rs->count; ++i) {
-        check_lseek(fd, (off64_t)rs->pos[i*2] * BLOCKSIZE, SEEK_SET);
+        if (check_lseek(fd, (off64_t)rs->pos[i*2] * BLOCKSIZE, SEEK_SET) == -1) {
+            ErrorAbort(state, "failed to seek %s: %s", blockdev_filename->data,
+                strerror(errno));
+            goto done;
+        }
+
         for (j = rs->pos[i*2]; j < rs->pos[i*2+1]; ++j) {
-            readblock(fd, buffer, BLOCKSIZE);
+            if (read_all(fd, buffer, BLOCKSIZE) == -1) {
+                ErrorAbort(state, "failed to read %s: %s", blockdev_filename->data,
+                    strerror(errno));
+                goto done;
+            }
+
             SHA_update(&ctx, buffer, BLOCKSIZE);
         }
     }
@@ -804,6 +1917,7 @@
 }
 
 void RegisterBlockImageFunctions() {
+    RegisterFunction("block_image_verify", BlockImageVerifyFn);
     RegisterFunction("block_image_update", BlockImageUpdateFn);
     RegisterFunction("range_sha1", RangeSha1Fn);
 }