add valid_length to fix handling of !zs->filelen on 0 byte files

c/client.c

@@ -539,6 +539,19 @@ int main(int argc, char **argv) {
     strcpy(temp_file, filename);
     strcat(temp_file, ".part");
 
+    /* Bail out for 0 length files */
+    if (! zsync_filelen(zs)) {
+        FILE *f = fopen(filename, "w");
+        if (!f) {
+            perror("open");
+            fprintf(stderr, "Could not open %s for writing zero byte file\n", filename);
+            exit(1);
+        }
+        mtime = zsync_mtime(zs);
+        if (mtime != -1) set_mtime(filename, mtime);
+        return 0;
+    }
+
     {   /* STEP 2: read available local data and fill in what we know in the
          *target file */
         int i;

c/libzsync/zsync.c

@@ -42,6 +42,8 @@
 #include <ctype.h>
 #include <time.h>
 
+#include <errno.h>
+
 #include <arpa/inet.h>
 
 #ifdef WITH_DMALLOC
@@ -139,6 +141,7 @@ static char **append_ptrlist(int *n, char **p, char *a) {
 
 /* Constructor */
 struct zsync_state *zsync_begin(FILE * f) {
+  int valid_length=0;
     /* Defaults for the checksum bytes and sequential matches properties of the
      * rcksum_state. These are the defaults from versions of zsync before these
      * were variable. */
@@ -194,7 +197,11 @@ struct zsync_state *zsync_begin(FILE * f) {
                 }
             }
             else if (!strcmp(buf, "Length")) {
-                zs->filelen = atol(p);
+                errno = 0;
+                zs->filelen = strtol(p, 0, 10);
+                if (errno == 0) {
+                    valid_length = 1;
+                }
             }
             else if (!strcmp(buf, "Filename")) {
                 zs->filename = strdup(p);
@@ -303,7 +310,12 @@ struct zsync_state *zsync_begin(FILE * f) {
             return NULL;
         }
     }
-    if (!zs->filelen || !zs->blocksize) {
+    /* zero length file, skip blocksums */
+    if (valid_length && !zs->filelen) {
+        return zs;
+    }
+    /* cannot test !zs->filelen or we will fail on valid 0 length files */
+    if (!valid_length || !zs->blocksize) {
         fprintf(stderr, "Not a zsync file (looked for Blocksize and Length lines)\n");
         free(zs);
         return NULL;
@@ -391,6 +403,10 @@ char *zsync_filename(const struct zsync_state *zs) {
     return strdup(zs->gzhead && zs->zfilename ? zs->zfilename : zs->filename);
 }
 
+off_t zsync_filelen(const struct zsync_state *zs) {
+    return zs->filelen;
+}
+
 /* time_t = zsync_mtime(self)
  * Returns the mtime on the original copy of the target; for the client program
  * to set the mtime of the local file to match, if it so chooses.