send-pack: switch to using git-rev-list --stdin

When we are generating packs to update remote repositories we
want to supply as much information as possible about the revisions
that already exist to rev-list in order optimise the pack as much
as possible.  We need to pass two revisions for each branch we are
updating in the remote repository and one for each additional branch.
Where the remote repository has numerous branches we can run out
of command line space to pass them.

Utilise the git-rev-list --stdin mode to allow unlimited numbers
of revision constraints.  This allows us to move back to the much
simpler unordered revision selection code.

[jc: added some comments in the code to describe the pipe flow
 a bit.]

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

send-pack.c

@@ -38,53 +38,25 @@ static void exec_pack_objects(void)
 
 static void exec_rev_list(struct ref *refs)
 {
-	struct ref *ref;
-	static const char *args[1000];
-	int i = 0, j;
+	static const char *args[4];
+	int i = 0;
 
 	args[i++] = "rev-list";	/* 0 */
 	if (use_thin_pack)	/* 1 */
 		args[i++] = "--objects-edge";
 	else
 		args[i++] = "--objects";
 
-	/* First send the ones we care about most */
-	for (ref = refs; ref; ref = ref->next) {
-		if (900 < i)
-			die("git-rev-list environment overflow");
-		if (!is_zero_sha1(ref->new_sha1)) {
-			char *buf = xmalloc(100);
-			args[i++] = buf;
-			snprintf(buf, 50, "%s", sha1_to_hex(ref->new_sha1));
-			buf += 50;
-			if (!is_zero_sha1(ref->old_sha1) &&
-			    has_sha1_file(ref->old_sha1)) {
-				args[i++] = buf;
-				snprintf(buf, 50, "^%s",
-					 sha1_to_hex(ref->old_sha1));
-			}
-		}
-	}
+	args[i++] = "--stdin";
 
-	/* Then a handful of the remainder
-	 * NEEDSWORK: we would be better off if used the newer ones first.
-	 */
-	for (ref = refs, j = i + 16;
-	     i < 900 && i < j && ref;
-	     ref = ref->next) {
-		if (is_zero_sha1(ref->new_sha1) &&
-		    !is_zero_sha1(ref->old_sha1) &&
-		    has_sha1_file(ref->old_sha1)) {
-			char *buf = xmalloc(42);
-			args[i++] = buf;
-			snprintf(buf, 42, "^%s", sha1_to_hex(ref->old_sha1));
-		}
-	}
 	args[i] = NULL;
 	execv_git_cmd(args);
 	die("git-rev-list exec failed (%s)", strerror(errno));
 }
 
+/*
+ * Run "rev-list --stdin | pack-objects" pipe.
+ */
 static void rev_list(int fd, struct ref *refs)
 {
 	int pipe_fd[2];
@@ -94,6 +66,9 @@ static void rev_list(int fd, struct ref *refs)
 		die("rev-list setup: pipe failed");
 	pack_objects_pid = fork();
 	if (!pack_objects_pid) {
+		/* The child becomes pack-objects; reads from pipe
+		 * and writes to the original fd
+		 */
 		dup2(pipe_fd[0], 0);
 		dup2(fd, 1);
 		close(pipe_fd[0]);
@@ -104,20 +79,80 @@ static void rev_list(int fd, struct ref *refs)
 	}
 	if (pack_objects_pid < 0)
 		die("pack-objects fork failed");
+
+	/* We become rev-list --stdin; output goes to pipe. */
 	dup2(pipe_fd[1], 1);
 	close(pipe_fd[0]);
 	close(pipe_fd[1]);
 	close(fd);
 	exec_rev_list(refs);
 }
 
+/*
+ * Create "rev-list --stdin | pack-objects" pipe and feed
+ * the refs into the pipeline.
+ */
+static void rev_list_generate(int fd, struct ref *refs)
+{
+	int pipe_fd[2];
+	pid_t rev_list_generate_pid;
+
+	if (pipe(pipe_fd) < 0)
+		die("rev-list-generate setup: pipe failed");
+	rev_list_generate_pid = fork();
+	if (!rev_list_generate_pid) {
+		/* The child becomes the "rev-list | pack-objects"
+		 * pipeline.  It takes input from us, and its output
+		 * goes to fd.
+		 */
+		dup2(pipe_fd[0], 0);
+		dup2(fd, 1);
+		close(pipe_fd[0]);
+		close(pipe_fd[1]);
+		close(fd);
+		rev_list(fd, refs);
+		die("rev-list setup failed");
+	}
+	if (rev_list_generate_pid < 0)
+		die("rev-list-generate fork failed");
+
+	/* We feed the rev parameters to them.  We do not write into
+	 * fd nor read from the pipe.
+	 */
+	close(pipe_fd[0]);
+	close(fd);
+	while (refs) {
+		char buf[42];
+
+		if (!is_null_sha1(refs->old_sha1) &&
+		    has_sha1_file(refs->old_sha1)) {
+			memcpy(buf + 1, sha1_to_hex(refs->old_sha1), 40);
+			buf[0] = '^';
+			buf[41] = '\n';
+			write(pipe_fd[1], buf, 42);
+		}
+		if (!is_null_sha1(refs->new_sha1)) {
+			memcpy(buf, sha1_to_hex(refs->new_sha1), 40);
+			buf[40] = '\n';
+			write(pipe_fd[1], buf, 41);
+		}
+		refs = refs->next;
+	}
+	close(pipe_fd[1]);
+	// waitpid(rev_list_generate_pid);
+	exit(0);
+}
+
+/*
+ * Make a pack stream and spit it out into file descriptor fd
+ */
 static void pack_objects(int fd, struct ref *refs)
 {
 	pid_t rev_list_pid;
 
 	rev_list_pid = fork();
 	if (!rev_list_pid) {
-		rev_list(fd, refs);
+		rev_list_generate(fd, refs);
 		die("rev-list setup failed");
 	}
 	if (rev_list_pid < 0)