Send the uid/gid 0 name since not all systems use 0 for root.

compat.c

@@ -73,6 +73,7 @@ int want_xattr_optim = 0;
 int proper_seed_order = 0;
 int inplace_partial = 0;
 int do_negotiated_strings = 0;
+int xmit_id0_names = 0;
 
 /* These index values are for the file-list's extra-attribute array. */
 int pathname_ndx, depth_ndx, atimes_ndx, uid_ndx, gid_ndx, acls_ndx, xattrs_ndx, unsort_ndx;
@@ -109,6 +110,7 @@ struct name_num_obj valid_compressions = {
 #define CF_CHKSUM_SEED_FIX (1<<5)
 #define CF_INPLACE_PARTIAL_DIR (1<<6)
 #define CF_VARINT_FLIST_FLAGS (1<<7)
+#define CF_ID0_NAMES (1<<8)
 
 static const char *client_info;
 
@@ -694,6 +696,8 @@ void setup_protocol(int f_out,int f_in)
 				compat_flags |= CF_CHKSUM_SEED_FIX;
 			if (local_server || strchr(client_info, 'I') != NULL)
 				compat_flags |= CF_INPLACE_PARTIAL_DIR;
+			if (local_server || strchr(client_info, 'u') != NULL)
+				compat_flags |= CF_ID0_NAMES;
 			if (local_server || strchr(client_info, 'v') != NULL) {
 				do_negotiated_strings = 1;
 				compat_flags |= CF_VARINT_FLIST_FLAGS;
@@ -714,6 +718,7 @@ void setup_protocol(int f_out,int f_in)
 		want_xattr_optim = protocol_version >= 31 && !(compat_flags & CF_AVOID_XATTR_OPTIM);
 		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 (am_sender) {
 			receiver_symlink_times = am_server
 			    ? strchr(client_info, 'L') != NULL

flist.c

@@ -2412,7 +2412,7 @@ struct file_list *send_file_list(int f, int argc, char *argv[])
 	file_old_total += flist->used;
 
 	if (numeric_ids <= 0 && !inc_recurse)
-		send_id_list(f);
+		send_id_lists(f);
 
 	/* send the io_error flag */
 	if (protocol_version < 30)

options.c

@@ -2608,6 +2608,7 @@ void server_options(char **args, int *argc_p)
 		eFlags[x++] = 'C'; /* support checksum seed order fix */
 		eFlags[x++] = 'I'; /* support inplace_partial behavior */
 		eFlags[x++] = 'v'; /* use varint for flist & compat flags; negotiate checksum */
+		eFlags[x++] = 'u'; /* include name of uid 0 & gid 0 in the id map */
 		/* NOTE: Avoid using 'V' -- it was the high bit of a write_byte() that became write_varint(). */
 #undef eFlags
 	}

uidlist.c

@@ -33,6 +33,7 @@ extern int preserve_uid;
 extern int preserve_gid;
 extern int preserve_acls;
 extern int numeric_ids;
+extern int xmit_id0_names;
 extern gid_t our_gid;
 extern char *usermap;
 extern char *groupmap;
@@ -295,9 +296,6 @@ const char *add_uid(uid_t uid)
 	struct idlist *node;
 	union name_or_id noiu;
 
-	if (uid == 0)	/* don't map root */
-		return NULL;
-
 	for (list = uidlist; list; list = list->next) {
 		if (list->id == uid)
 			return NULL;
@@ -315,9 +313,6 @@ const char *add_gid(gid_t gid)
 	struct idlist *node;
 	union name_or_id noiu;
 
-	if (gid == 0)	/* don't map root */
-		return NULL;
-
 	for (list = gidlist; list; list = list->next) {
 		if (list->id == gid)
 			return NULL;
@@ -328,40 +323,43 @@ const char *add_gid(gid_t gid)
 	return node->u.name;
 }
 
-/* send a complete uid/gid mapping to the peer */
-void send_id_list(int f)
+static void send_one_name(int f, id_t id, const char *name)
+{
+	int len = strlen(name);
+	if (len > 255) /* Impossible? */
+		len = 255;
+
+	write_varint30(f, id);
+	write_byte(f, len);
+	write_buf(f, name, len);
+}
+
+static void send_one_list(int f, struct idlist *idlist, int usernames)
 {
 	struct idlist *list;
 
-	if (preserve_uid || preserve_acls) {
-		int len;
-		/* we send sequences of uid/byte-length/name */
-		for (list = uidlist; list; list = list->next) {
-			if (!list->u.name)
-				continue;
-			len = strlen(list->u.name);
-			write_varint30(f, list->id);
-			write_byte(f, len);
-			write_buf(f, list->u.name, len);
-		}
-
-		/* terminate the uid list with a 0 uid. We explicitly exclude
-		 * 0 from the list */
-		write_varint30(f, 0);
+	/* we send sequences of id/byte-len/name */
+	for (list = idlist; list; list = list->next) {
+		if (list->id && list->u.name)
+			send_one_name(f, list->id, list->u.name);
 	}
 
-	if (preserve_gid || preserve_acls) {
-		int len;
-		for (list = gidlist; list; list = list->next) {
-			if (!list->u.name)
-				continue;
-			len = strlen(list->u.name);
-			write_varint30(f, list->id);
-			write_byte(f, len);
-			write_buf(f, list->u.name, len);
-		}
+	/* Terminate the uid list with 0 (which was excluded above).
+	 * A modern rsync also sends the name of id 0. */
+	if (xmit_id0_names)
+		send_one_name(f, 0, usernames ? uid_to_user(0) : gid_to_group(0));
+	else
 		write_varint30(f, 0);
-	}
+}
+
+/* send a complete uid/gid mapping to the peer */
+void send_id_lists(int f)
+{
+	if (preserve_uid || preserve_acls)
+		send_one_list(f, uidlist, 1);
+
+	if (preserve_gid || preserve_acls)
+		send_one_list(f, gidlist, 0);
 }
 
 uid_t recv_user_name(int f, uid_t uid)
@@ -405,12 +403,16 @@ void recv_id_list(int f, struct file_list *flist)
 		/* read the uid list */
 		while ((id = read_varint30(f)) != 0)
 			recv_user_name(f, id);
+		if (xmit_id0_names)
+			recv_user_name(f, 0);
 	}
 
 	if ((preserve_gid || preserve_acls) && numeric_ids <= 0) {
 		/* read the gid list */
 		while ((id = read_varint30(f)) != 0)
 			recv_group_name(f, id, NULL);
+		if (xmit_id0_names)
+			recv_group_name(f, 0, NULL);
 	}
 
 	/* Now convert all the uids/gids from sender values to our values. */
@@ -502,8 +504,9 @@ void parse_name_map(char *map, BOOL usernames)
 		*--cp = '\0'; /* replace comma */
 	}
 
-	/* The 0 user/group doesn't get its name sent, so add it explicitly. */
-	recv_add_id(idlist_ptr, *idmap_ptr, 0, numeric_ids ? NULL : usernames ? uid_to_user(0) : gid_to_group(0));
+	/* If the sender isn't going to xmit the id0 name, we assume it's "root". */
+	if (!xmit_id0_names)
+		recv_add_id(idlist_ptr, *idmap_ptr, 0, numeric_ids ? NULL : "root");
 }
 
 #ifdef HAVE_GETGROUPLIST