[CORRECTED PATCH] git-fetch-pack: avoid unnecessary zero packing

If everything is up-to-date locally, we don't need to even ask for a
pack-file from the remote, or try to unpack it.

This is especially important for tags - since the pack-file common commit
logic is based purely on the commit history, it will never be able to find
a common tag, and will thus always end up re-fetching them.

Especially notably, if the tag points to a non-commit (eg a tagged tree),
the pack-file would be unnecessarily big, just because it cannot any most
recent common point between commits for pruning.

Short-circuiting the case where we already have that reference means that
we avoid a lot of these in the common case.

NOTE! This only matches remote ref names against the same local name,
which works well for tags, but is not as generic as it could be. If we
ever need to, we could match against _any_ local ref (if we have it, we
have it), but this "match against same name" is simpler and more
efficient, and covers the common case.

Renaming of refs is common for branch heads, but since those are always
commits, the pack-file generation can optimize that case.

In some cases we might still end up fetching pack-files unnecessarily, but
this at least avoids the re-fetching of tags over and over if you use a
regular

	git fetch --tags ...

which was the main reason behind the change.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

Ok, this should be the trivially fixed (famous last words) patch, which 
should correct the case where we don't have a local name for the remote 
ref at all.

If you already applied the previous patch, you just need to fix the

	if (read_ref(...) < 0)
		continue
	if (memcmp(..)) {

to be one case (a failing read_ref should do the exact same thing as a 
failed memcmp):

	if (read_ref(...) < 0 ||
	    memcmp(...) {

and everything should be ok.

This has gotten _some_ testing, but obviously not enough ;)

		Linus

diff --git a/fetch-pack.c b/fetch-pack.c
index 953c0cf..4597369 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -12,6 +12,7 @@ static const char *exec = "git-upload-pa
 static int find_common(int fd[2], unsigned char *result_sha1,
 		       struct ref *refs)
 {
+	int fetching;
 	static char line[1000];
 	int count = 0, flushes = 0, retval;
 	FILE *revs;
@@ -20,16 +21,19 @@ static int find_common(int fd[2], unsign
 	if (!revs)
 		die("unable to run 'git-rev-list'");
 
-	while (refs) {
+	fetching = 0;
+	for ( ; refs ; refs = refs->next) {
 		unsigned char *remote = refs->old_sha1;
-		if (verbose)
-			fprintf(stderr,
-				"want %s (%s)\n", sha1_to_hex(remote),
-				refs->name);
+		unsigned char *local = refs->new_sha1;
+
+		if (!memcmp(remote, local, 20))
+			continue;
 		packet_write(fd[1], "want %s\n", sha1_to_hex(remote));
-		refs = refs->next;
+		fetching++;
 	}
 	packet_flush(fd[1]);
+	if (!fetching)
+		return 1;
 	flushes = 1;
 	retval = -1;
 	while (fgets(line, sizeof(line), revs) != NULL) {
@@ -74,6 +78,35 @@ static int find_common(int fd[2], unsign
 	return retval;
 }
 
+static int everything_local(struct ref *refs)
+{
+	int retval;
+
+	for (retval = 1; refs ; refs = refs->next) {
+		const unsigned char *remote = refs->old_sha1;
+		unsigned char local[20];
+
+		if (read_ref(git_path("%s", refs->name), local) < 0 ||
+		    memcmp(remote, local, 20)) {
+			retval = 0;
+			if (!verbose)
+				continue;
+			fprintf(stderr,
+				"want %s (%s)\n", sha1_to_hex(remote),
+				refs->name);
+			continue;
+		}
+
+		memcpy(refs->new_sha1, local, 20);
+		if (!verbose)
+			continue;
+		fprintf(stderr,
+			"already have %s (%s)\n", sha1_to_hex(remote),
+			refs->name);
+	}
+	return retval;
+}
+
 static int fetch_pack(int fd[2], int nr_match, char **match)
 {
 	struct ref *ref;
@@ -86,6 +119,10 @@ static int fetch_pack(int fd[2], int nr_
 		packet_flush(fd[1]);
 		die("no matching remote head");
 	}
+	if (everything_local(ref)) {
+		packet_flush(fd[1]);
+		goto all_done;
+	}
 	if (find_common(fd, sha1, ref) < 0)
 		fprintf(stderr, "warning: no common commits\n");
 	pid = fork();
@@ -109,6 +146,7 @@ static int fetch_pack(int fd[2], int nr_
 		int code = WEXITSTATUS(status);
 		if (code)
 			die("git-unpack-objects died with error code %d", code);
+all_done:
 		while (ref) {
 			printf("%s %s\n",
 			       sha1_to_hex(ref->old_sha1), ref->name);
-
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