Fix protocol compatibility: Add feature negotiation for MSG_FLIST_COUNT to work with version <=3.4.1

- Add CF_SKIP_UNCHANGED compat flag (bit 9)
- Add 'U' flag to client_info to advertise support
- Only send MSG_FLIST_COUNT when both sides negotiated the feature
- Only process MSG_FLIST_COUNT when negotiated
- Prevents 'unexpected tag 11' error when mixing standard/custom rsync

This fixes the crash when custom rsync (receiver) communicates with
standard rsync (sender). The feature is now safely negotiated during
protocol setup.

Author: RyuX51

compat.c

@@ -82,6 +82,7 @@ int proper_seed_order = 0;
 int inplace_partial = 0;
 int do_negotiated_strings = 0;
 int xmit_id0_names = 0;
+int skip_unchanged_negotiated = 0;
 
 struct name_num_item *xattr_sum_nni;
 int xattr_sum_len = 0;
@@ -124,6 +125,7 @@ struct name_num_obj valid_compressions = {
 #define CF_INPLACE_PARTIAL_DIR (1<<6)
 #define CF_VARINT_FLIST_FLAGS (1<<7)
 #define CF_ID0_NAMES (1<<8)
+#define CF_SKIP_UNCHANGED (1<<9)
 
 static const char *client_info;
 
@@ -727,6 +729,8 @@ void setup_protocol(int f_out,int f_in)
 				compat_flags |= CF_INPLACE_PARTIAL_DIR;
 			if (strchr(client_info, 'u') != NULL)
 				compat_flags |= CF_ID0_NAMES;
+			if (strchr(client_info, 'U') != NULL)
+				compat_flags |= CF_SKIP_UNCHANGED;
 			if (strchr(client_info, 'v') != NULL) {
 				do_negotiated_strings = 1;
 				compat_flags |= CF_VARINT_FLIST_FLAGS;
@@ -748,6 +752,9 @@ void setup_protocol(int f_out,int f_in)
 		proper_seed_order = compat_flags & CF_CHKSUM_SEED_FIX ? 1 : 0;
 		xfer_flags_as_varint = compat_flags & CF_VARINT_FLIST_FLAGS ? 1 : 0;
 		xmit_id0_names = compat_flags & CF_ID0_NAMES ? 1 : 0;
+		if (compat_flags & CF_SKIP_UNCHANGED) {
+			skip_unchanged_negotiated = 1;
+		}
 		if (!xfer_flags_as_varint && preserve_crtimes) {
 			fprintf(stderr, "Both rsync versions must be at least 3.2.0 for --crtimes.\n");
 			exit_cleanup(RERR_PROTOCOL);

generator.c

@@ -34,6 +34,7 @@ extern int am_sender;
 extern int am_daemon;
 extern int inc_recurse;
 extern int no_i_r_skip_unchanged;
+extern int skip_unchanged_negotiated;
 extern int relative_paths;
 extern struct stats stats;
 extern int implied_dirs;
@@ -2299,7 +2300,7 @@ static void prescan_for_unchanged(const char *local_name, int f_out)
 			skipped_count, active_count, (double)stats.total_size / 1024 / 1024 / 1024);
 
 	/* Send skipped count and adjusted total_size to sender for accurate progress display */
-	if (f_out >= 0) {
+	if (f_out >= 0 && skip_unchanged_negotiated) {
 		char buf[12];
 		SIVAL(buf, 0, skipped_count);
 		SIVAL64(buf, 4, stats.total_size);

io.c

@@ -60,6 +60,7 @@ extern int daemon_connection;
 extern int protocol_version;
 extern int remove_source_files;
 extern int preserve_hard_links;
+extern int skip_unchanged_negotiated;
 extern BOOL extra_flist_sending_enabled;
 extern BOOL flush_ok_after_signal;
 extern struct stats stats;
@@ -1512,7 +1513,7 @@ static void read_a_msg(void)
 		iobuf.in_multiplexed = 1;
 		break;
 	case MSG_FLIST_COUNT:
-		if (msg_bytes != 12 || !am_sender)
+		if (msg_bytes != 12 || !am_sender || !skip_unchanged_negotiated)
 			goto invalid_msg;
 		val = raw_read_int();
 		stats.num_skipped_files = val;

options.c

@@ -3050,6 +3050,7 @@ int maybe_add_e_option(char *buf, int buf_len)
 		buf[x++] = 'I'; /* support inplace_partial behavior */
 		buf[x++] = 'v'; /* use varint for flist & compat flags; negotiate checksum */
 		buf[x++] = 'u'; /* include name of uid 0 & gid 0 in the id map */
+		buf[x++] = 'U'; /* support --no-i-r-skip-unchanged feature */
 
 		/* NOTE: Avoid using 'V' -- it was represented with the high bit of a write_byte() that became a write_varint(). */
 	}