This patch adds backward-compatibility support for the --xattrs option.
Since the main release has never had xattr support, the trunk doesn't
need this code.  If you want to make rsync 3.0.x communicate with an
older (patched) release, use this.

To use this patch, run these commands for a successful build:

    patch -p1 <patches/acls.diff
    patch -p1 <patches/xattrs.diff
    ./configure                         (optional if already run)
    make

--- old/compat.c
+++ new/compat.c
@@ -163,13 +163,6 @@ void setup_protocol(int f_out,int f_in)
 	if (protocol_version < 30) {
 		if (append_mode == 1)
 			append_mode = 2;
-		if (preserve_xattrs && !local_server) {
-			rprintf(FERROR,
-			    "--xattrs requires protocol 30 or higher"
-			    " (negotiated %d).\n",
-			    protocol_version);
-			exit_cleanup(RERR_PROTOCOL);
-		}
 	}
 
 	if (delete_mode && !(delete_before+delete_during+delete_after)) {
--- old/xattrs.c
+++ new/xattrs.c
@@ -21,6 +21,7 @@
 
 #include "rsync.h"
 #include "ifuncs.h"
+#include "io.h"
 #include "lib/sysxattrs.h"
 
 #ifdef SUPPORT_XATTRS
@@ -33,6 +34,7 @@ extern int read_only;
 extern int list_only;
 extern int preserve_xattrs;
 extern int checksum_seed;
+extern int protocol_version;
 
 #define RSYNC_XAL_INITIAL 5
 #define RSYNC_XAL_LIST_INITIAL 100
@@ -233,7 +235,7 @@ static int rsync_xal_get(const char *fna
 		if (!(ptr = get_xattr_data(fname, name, &datum_len, 0)))
 			return -1;
 
-		if (datum_len > MAX_FULL_DATUM) {
+		if (datum_len > MAX_FULL_DATUM && protocol_version >= 30) {
 			/* For large datums, we store a flag and a checksum. */
 			name_offset = 1 + MAX_DIGEST_LEN;
 			sum_init(checksum_seed);
@@ -297,7 +299,7 @@ static int find_matching_xattr(item_list
 			 || rxas1[j].datum_len != rxas2[j].datum_len
 			 || strcmp(rxas1[j].name, rxas2[j].name))
 				break;
-			if (rxas1[j].datum_len > MAX_FULL_DATUM) {
+			if (rxas1[j].datum_len > MAX_FULL_DATUM && protocol_version >= 30) {
 				if (memcmp(rxas1[j].datum + 1,
 					   rxas2[j].datum + 1,
 					   MAX_DIGEST_LEN) != 0)
@@ -334,34 +336,43 @@ int send_xattr(statx *sxp, int f)
 {
 	int ndx = find_matching_xattr(sxp->xattr);
 
-	/* Send 0 (-1 + 1) to indicate that literal xattr data follows. */
-	write_varint(f, ndx + 1);
+	if (protocol_version < 30) {
+		if (ndx < 0)
+			write_byte(f, 'X');
+		else {
+			write_byte(f, 'x');
+			write_int(f, ndx);
+		}
+	} else {
+		/* Send 0 (-1 + 1) to indicate that literal xattr data follows. */
+		write_varint(f, ndx + 1);
+	}
 
 	if (ndx < 0) {
 		rsync_xa *rxa;
 		int count = sxp->xattr->count;
-		write_varint(f, count);
+		write_varint30(f, count);
 		for (rxa = sxp->xattr->items; count--; rxa++) {
 #ifdef HAVE_LINUX_XATTRS
-			write_varint(f, rxa->name_len);
-			write_varint(f, rxa->datum_len);
+			write_varint30(f, rxa->name_len);
+			write_varint30(f, rxa->datum_len);
 			write_buf(f, rxa->name, rxa->name_len);
 #else
 			/* We strip the rsync prefix from disguised namespaces
 			 * and put everything else in the user namespace. */
 			if (HAS_PREFIX(rxa->name, RSYNC_PREFIX)
 			 && rxa->name[RPRE_LEN] != '%') {
-				write_varint(f, rxa->name_len - RPRE_LEN);
-				write_varint(f, rxa->datum_len);
+				write_varint30(f, rxa->name_len - RPRE_LEN);
+				write_varint30(f, rxa->datum_len);
 				write_buf(f, rxa->name + RPRE_LEN, rxa->name_len - RPRE_LEN);
 			} else {
-				write_varint(f, rxa->name_len + UPRE_LEN);
-				write_varint(f, rxa->datum_len);
+				write_varint30(f, rxa->name_len + UPRE_LEN);
+				write_varint30(f, rxa->datum_len);
 				write_buf(f, USER_PREFIX, UPRE_LEN);
 				write_buf(f, rxa->name, rxa->name_len);
 			}
 #endif
-			if (rxa->datum_len > MAX_FULL_DATUM)
+			if (rxa->datum_len > MAX_FULL_DATUM && protocol_version >= 30)
 				write_buf(f, rxa->datum + 1, MAX_DIGEST_LEN);
 			else
 				write_buf(f, rxa->datum, rxa->datum_len);
@@ -411,7 +422,7 @@ int xattr_diff(struct file_struct *file,
 		cmp = rec_cnt ? strcmp(snd_rxa->name, rec_rxa->name) : -1;
 		if (cmp > 0)
 			same = 0;
-		else if (snd_rxa->datum_len > MAX_FULL_DATUM) {
+		else if (snd_rxa->datum_len > MAX_FULL_DATUM && protocol_version >= 30) {
 			same = cmp == 0 && snd_rxa->datum_len == rec_rxa->datum_len
 			    && memcmp(snd_rxa->datum + 1, rec_rxa->datum + 1,
 				      MAX_DIGEST_LEN) == 0;
@@ -454,6 +465,9 @@ void send_xattr_request(const char *fnam
 	int j, cnt, prior_req = -1;
 	rsync_xa *rxa;
 
+	if (protocol_version < 30)
+		return;
+
 	lst += F_XATTR(file);
 	cnt = lst->count;
 	for (rxa = lst->items, j = 0; j < cnt; rxa++, j++) {
@@ -525,6 +539,9 @@ void recv_xattr_request(struct file_stru
 	rsync_xa *rxa;
 	int rel_pos, cnt;
 
+	if (protocol_version < 30)
+		return;
+
 	if (F_XATTR(file) < 0) {
 		rprintf(FERROR, "recv_xattr_request: internal data error!\n");
 		exit_cleanup(RERR_STREAMIO);
@@ -571,7 +588,22 @@ void receive_xattr(struct file_struct *f
 {
 	static item_list temp_xattr = EMPTY_ITEM_LIST;
 	int count;
-	int ndx = read_varint(f);
+	int ndx;
+
+	if (protocol_version >= 30)
+		ndx = read_varint(f);
+	else {
+		int tag = read_byte(f);
+		if (tag == 'x')
+			ndx = read_int(f) + 1;
+		else if (tag == 'X')
+			ndx = 0;
+		else {
+			rprintf(FERROR, "receive_xattr: unknown extended attribute"
+				" type tag (%02x) for %s\n", tag, f_name(file, NULL));
+			exit_cleanup(RERR_STREAMIO);
+		}
+	}
 
 	if (ndx < 0 || (size_t)ndx > rsync_xal_l.count) {
 		rprintf(FERROR, "receive_xattr: xa index %d out of"
@@ -584,7 +616,7 @@ void receive_xattr(struct file_struct *f
 		return;
 	}
 	
-	if ((count = read_varint(f)) != 0) {
+	if ((count = read_varint30(f)) != 0) {
 		(void)EXPAND_ITEM_LIST(&temp_xattr, rsync_xa, count);
 		temp_xattr.count = 0;
 	}
@@ -592,9 +624,10 @@ void receive_xattr(struct file_struct *f
 	while (count--) {
 		char *ptr, *name;
 		rsync_xa *rxa;
-		size_t name_len = read_varint(f);
-		size_t datum_len = read_varint(f);
-		size_t dget_len = datum_len > MAX_FULL_DATUM ? 1 + MAX_DIGEST_LEN : datum_len;
+		size_t name_len = read_varint30(f);
+		size_t datum_len = read_varint30(f);
+		size_t dget_len = datum_len > MAX_FULL_DATUM && protocol_version >= 30
+				? 1 + MAX_DIGEST_LEN : datum_len;
 		size_t extra_len = MIGHT_NEED_RPRE ? RPRE_LEN : 0;
 		if (dget_len + extra_len < dget_len)
 			out_of_memory("receive_xattr"); /* overflow */