[PATCH] Cache stat_tracking_info() for faster status and branch -v

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

stat_tracking_info() is used to calculated how many commits ahead or
behind for a branch. Rev walking can be slow especially when the
branch is way behind its remote end. By caching the results, we won't
have to rev walk every time we need these information.
stat_tracking_info() cost can be greatly reduced this way.

This makes sure "git status" instant no matter how far behind HEAD
is, except the first time after HEAD changes. This also makes
"branch -v" usable (for me) as it's now also instant versus 3.5
seconds in non-cache case on my machine.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 I wanted guaranteed-fast status for another reason, but it turns out
 "branch -v" benefits even more. Recent commit walking is not
 efficiently optimized even with Shawn's pack bitmaps. This may be
 useful some people, I guess.

 remote.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/remote.c b/remote.c
index 04fd9ea..db825b8 100644
--- a/remote.c
+++ b/remote.c
@@ -1549,6 +1549,7 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 	struct rev_info revs;
 	const char *rev_argv[10], *base;
 	int rev_argc;
+	int fd;
 
 	/*
 	 * Nothing to report unless we are marked to build on top of
@@ -1579,6 +1580,33 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 	if (theirs == ours)
 		return 0;
 
+	if (!access(git_path("stat_tracking_cache/%s",
+			     branch->refname), R_OK)) {
+		struct strbuf sb = STRBUF_INIT;
+		unsigned char sha1_ours[20], sha1_theirs[20];
+		int n1, n2;
+		if ((fd = open(git_path("stat_tracking_cache/%s",
+					branch->refname),
+			       O_RDONLY)) != -1 &&
+		    strbuf_read(&sb, fd, 0) != -1 &&
+		    sb.len > (40 + 1) * 2 &&
+		    !get_sha1_hex(sb.buf, sha1_ours) &&
+		    sb.buf[40] == '\n' &&
+		    !get_sha1_hex(sb.buf + 41, sha1_theirs) &&
+		    sb.buf[81] == '\n' &&
+		    !hashcmp(sha1_ours, ours->object.sha1) &&
+		    !hashcmp(sha1_theirs, theirs->object.sha1) &&
+		    sscanf(sb.buf + 82, "%d\n%d\n", &n1, &n2) == 2) {
+			*num_ours = n1;
+			*num_theirs = n2;
+			close(fd);
+			strbuf_release(&sb);
+			return 1;
+		}
+		close(fd);
+		strbuf_release(&sb);
+	}
+
 	/* Run "rev-list --left-right ours...theirs" internally... */
 	rev_argc = 0;
 	rev_argv[rev_argc++] = NULL;
@@ -1608,6 +1636,20 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 			(*num_theirs)++;
 	}
 
+	if (!safe_create_leading_directories(git_path("stat_tracking_cache/%s",
+						      branch->refname)) &&
+	    (fd = open(git_path("stat_tracking_cache/%s",
+				branch->refname),
+		       O_CREAT | O_TRUNC | O_RDWR, 0644)) != -1) {
+		struct strbuf sb = STRBUF_INIT;
+		strbuf_addf(&sb, "%s\n", sha1_to_hex(ours->object.sha1));
+		strbuf_addf(&sb, "%s\n", sha1_to_hex(theirs->object.sha1));
+		strbuf_addf(&sb, "%d\n%d\n", *num_ours, *num_theirs);
+		write(fd, sb.buf, sb.len);
+		strbuf_release(&sb);
+		close(fd);
+	}
+
 	/* clear object flags smudged by the above traversal */
 	clear_commit_marks(ours, ALL_REV_FLAGS);
 	clear_commit_marks(theirs, ALL_REV_FLAGS);
-- 
1.8.0.rc2.21.g0695653