git-upload-pack: Support sending multiple ACK messages

The current fetch/upload protocol works like this:

- client sends revs it wants to have via "want" messages
- client sends a flush message (message with len 0)
- client sends revs it has via "have" messages
- after one window (32 revs), a flush is sent
- after each subsequent window, a flush is sent, and an ACK/NAK is received.
        (NAK means that server does not have any of the transmitted revs;
         ACK sends also the sha1 of the rev server has)
 - when the first ACK is received, client sends "done", and does not expect
        any further messages

One special case, though:

- if no ACK is received (only NAK's), and client runs out of revs to send,
        "done" is sent, and server sends just one more "NAK"

A smarter scheme, which actually has a chance to detect more than one
common rev, would be to send more than just one ACK. This patch implements
the server side of the following extension to the protocol:

- client sends at least one "want" message with "multi_ack" appended, like

        "want 1234567890123456789012345678901234567890 multi_ack"

- if the server understands that extension, it will send ACK messages for all
        revs it has, not just the first one

- server appends "continue" to the ACK messages like

        "ACK 1234567890123456789012345678901234567890 continue"

        until it has MAX_HAS-1 revs. In this manner, client knows when to
        stop sending revs by checking for the substring "continue" (and
        further knows that server understands multi_ack)

In this manner, the protocol stays backwards compatible, since both client
must send "want ... multi_ack" and server must answer with "ACK ...
continue" to enable the extension.

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

diff --git a/upload-pack.c b/upload-pack.c
index ab1981c..c3abf7b 100644 (file)
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -10,7 +10,7 @@ static const char upload_pack_usage[] = "git-upload-pack [--strict] [--timeout=n
 #define THEY_HAVE (1U << 0)
 #define MAX_HAS 256
 #define MAX_NEEDS 256
-static int nr_has = 0, nr_needs = 0;
+static int nr_has = 0, nr_needs = 0, multi_ack = 0;
 static unsigned char has_sha1[MAX_HAS][20];
 static unsigned char needs_sha1[MAX_NEEDS][20];
 static unsigned int timeout = 0;
@@ -124,39 +124,28 @@ static int get_common_commits(void)
                reset_timeout();
 
                if (!len) {
-                       packet_write(1, "NAK\n");
+                       if (multi_ack || nr_has == 0)
+                               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))
+                               packet_write(1, "ACK %s%s\n",
+                                       sha1_to_hex(sha1),
+                                       multi_ack && nr_has < MAX_HAS ?
+                                       " continue" : "");
                        continue;
                }
                if (!strcmp(line, "done")) {
+                       if (nr_has > 0)
+                               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)
@@ -185,6 +174,10 @@ 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;
+
                needs++;
        }
 }