git-upload-pack: Support the multi_ack protocol

This implements three things (trying very hard to be backwards
compatible):

It sends the "multi_ack" capability via the mechanism proposed by
Sergey Vlasov.

When the client sends "multi_ack" with at least one "want", multi_ack
is enabled.

When multi_ack is enabled, "continue" is appended to each "ACK" until
either the server can not store more refs, or "done" is received.

In contrast to the original protocol, as long as "continue" is sent,
flushes are answered by a "NAK" (not just until an "ACK" was sent),
and if "continue" was sent at least once, the last message is an
"ACK" without "continue".

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

upload-pack.c

@@ -12,7 +12,7 @@ static const char upload_pack_usage[] = "git-upload-pack [--strict] [--timeout=n
 #define WANTED (1U << 2)
 #define MAX_HAS 256
 #define MAX_NEEDS 256
-static int nr_has = 0, nr_needs = 0, nr_our_refs = 0;
+static int nr_has = 0, nr_needs = 0, multi_ack = 0, nr_our_refs = 0;
 static unsigned char has_sha1[MAX_HAS][20];
 static unsigned char needs_sha1[MAX_NEEDS][20];
 static unsigned int timeout = 0;
@@ -119,7 +119,7 @@ static int got_sha1(char *hex, unsigned char *sha1)
 static int get_common_commits(void)
 {
 	static char line[1000];
-	unsigned char sha1[20];
+	unsigned char sha1[20], last_sha1[20];
 	int len;
 
 	track_object_refs = 0;
@@ -130,39 +130,36 @@ static int get_common_commits(void)
 		reset_timeout();
 
 		if (!len) {
-			packet_write(1, "NAK\n");
+			if (nr_has == 0 || multi_ack)
+				packet_write(1, "NAK\n");
 			continue;
 		}
 		len = strip(line, len);
 		if (!strncmp(line, "have ", 5)) {
-			if (got_sha1(line+5, sha1)) {
-				packet_write(1, "ACK %s\n", sha1_to_hex(sha1));
-				break;
+			if (got_sha1(line+5, sha1) &&
+					(multi_ack || nr_has == 1)) {
+				if (nr_has >= MAX_HAS)
+					multi_ack = 0;
+				packet_write(1, "ACK %s%s\n",
+					sha1_to_hex(sha1),
+					multi_ack ?  " continue" : "");
+				if (multi_ack)
+					memcpy(last_sha1, sha1, 20);
 			}
 			continue;
 		}
 		if (!strcmp(line, "done")) {
+			if (nr_has > 0) {
+				if (multi_ack)
+					packet_write(1, "ACK %s\n",
+							sha1_to_hex(last_sha1));
+				return 0;
+			}
 			packet_write(1, "NAK\n");
 			return -1;
 		}
 		die("git-upload-pack: expected SHA1 list, got '%s'", line);
 	}
-
-	for (;;) {
-		len = packet_read_line(0, line, sizeof(line));
-		reset_timeout();
-		if (!len)
-			continue;
-		len = strip(line, len);
-		if (!strncmp(line, "have ", 5)) {
-			got_sha1(line+5, sha1);
-			continue;
-		}
-		if (!strcmp(line, "done"))
-			break;
-		die("git-upload-pack: expected SHA1 list, got '%s'", line);
-	}
-	return 0;
 }
 
 static int receive_needs(void)
@@ -192,6 +189,8 @@ static int receive_needs(void)
 		if (strncmp("want ", line, 5) || get_sha1_hex(line+5, sha1_buf))
 			die("git-upload-pack: protocol error, "
 			    "expected to get sha, not '%s'", line);
+		if (strstr(line+45, "multi_ack"))
+			multi_ack = 1;
 
 		/* We have sent all our refs already, and the other end
 		 * should have chosen out of them; otherwise they are
@@ -213,9 +212,11 @@ static int receive_needs(void)
 
 static int send_ref(const char *refname, const unsigned char *sha1)
 {
+	static char *capabilities = "\0multi_ack";
 	struct object *o = parse_object(sha1);
 
-	packet_write(1, "%s %s\n", sha1_to_hex(sha1), refname);
+	packet_write(1, "%s %s%s\n", sha1_to_hex(sha1), refname, capabilities);
+	capabilities = "";
 	if (!(o->flags & OUR_REF)) {
 		o->flags |= OUR_REF;
 		nr_our_refs++;