Add option --time-only

generator.c

@@ -68,6 +68,7 @@ extern int make_backups;
 extern int csum_length;
 extern int ignore_times;
 extern int size_only;
+extern int time_only;
 extern OFF_T max_size;
 extern OFF_T min_size;
 extern int io_error;
@@ -618,7 +619,7 @@ int quick_check_ok(enum filetype ftype, const char *fn, struct file_struct *file
 {
 	switch (ftype) {
 	  case FT_REG:
-		if (st->st_size != F_LENGTH(file))
+		if (!time_only && st->st_size != F_LENGTH(file))
 			return 0;
 
 		/* If always_checksum is set then we use the checksum instead

options.c

@@ -119,6 +119,7 @@ int safe_symlinks = 0;
 int copy_unsafe_links = 0;
 int munge_symlinks = 0;
 int size_only = 0;
+int time_only = 0;
 int daemon_bwlimit = 0;
 int bwlimit = 0;
 int fuzzy_basis = 0;
@@ -689,6 +690,7 @@ static struct poptOption long_options[] = {
   {"chmod",            0,  POPT_ARG_STRING, 0, OPT_CHMOD, 0, 0 },
   {"ignore-times",    'I', POPT_ARG_NONE,   &ignore_times, 0, 0, 0 },
   {"size-only",        0,  POPT_ARG_NONE,   &size_only, 0, 0, 0 },
+  {"time-only",        0,  POPT_ARG_NONE,   &time_only, 0, 0, 0 },
   {"one-file-system", 'x', POPT_ARG_NONE,   0, 'x', 0, 0 },
   {"no-one-file-system",0, POPT_ARG_VAL,    &one_file_system, 0, 0, 0 },
   {"no-x",             0,  POPT_ARG_VAL,    &one_file_system, 0, 0, 0 },
@@ -2835,6 +2837,16 @@ void server_options(char **args, int *argc_p)
 			args[ac++] = "--super";
 		if (size_only)
 			args[ac++] = "--size-only";
+		/* We don't send --time-only to server because: 
+		 * 1. sensible use of the --time-only option requires
+		 *    post-processing not done by rsync: compressing transferred 
+		 *    files and truncating the original while preserving the timestamp.
+		 * 2. older servers would croak.
+		 * It may be a good idea to throw an error with a better error
+		 * message...
+		 */
+		// if (time_only)
+		//   args[ac++] = "--time-only";
 		if (do_stats)
 			args[ac++] = "--stats";
 	} else {

rsync.1.md

@@ -504,6 +504,7 @@ has its own detailed description later in this manpage.
 --contimeout=SECONDS     set daemon connection timeout in seconds
 --ignore-times, -I       don't skip files that match size and time
 --size-only              skip files that match in size
+--time-only              skip files that match in time (PULL ONLY)
 --modify-window=NUM, -@  set the accuracy for mod-time comparisons
 --temp-dir=DIR, -T       create temporary files in directory DIR
 --fuzzy, -y              find similar file for basis if no dest file
@@ -776,6 +777,19 @@ expand it.
     after using another mirroring system which may not preserve timestamps
     exactly.
 
+0.  `--time-only`
+
+    This modifies rsync's "quick check" algorithm for finding files that need
+    to be transferred, changing it from the default of transferring files with
+    either a changed size or a changed last-modified time to just looking for
+    files that have a changed last-modified time, ignoring size changes.  This
+    is useful when remote files are uncompressed but a local copy should be
+    stored compressed, together with a zero-size stub to prevent re-transfers.
+    This option is only useful when pulling files, as it requires post-transfer
+    compression and truncation of files whilst preserving the original
+    modification time of the stub. Thus the option is not transmitted to the
+    remote side, also preventing the server from croaking on an unknown option.
+
 0.  `--modify-window=NUM`, `-@`
 
     When comparing two timestamps, rsync treats the timestamps as being equal
@@ -1031,7 +1045,8 @@ expand it.
     This forces rsync to skip any files which exist on the destination and have
     a modified time that is newer than the source file. (If an existing
     destination file has a modification time equal to the source file's, it
-    will be updated if the sizes are different.)
+    will be updated if the sizes are different - unless the --time-only flag
+    is set.)
 
     Note that this does not affect the copying of dirs, symlinks, or other
     special files.  Also, a difference of file format between the sender and