Racy GIT (part #2)

The previous round caught the most trivial case well, but broke
down once index file is updated again.  Smudge problematic
entries (they should be very few if any under normal interactive
workflow) before writing a new index file out.

Signed-off-by: Junio C Hamano <junkio@cox.net>

read-cache.c

@@ -87,7 +87,7 @@ static int ce_modified_check_fs(struct cache_entry *ce, struct stat *st)
 	return 0;
 }
 
-int ce_match_stat(struct cache_entry *ce, struct stat *st)
+static int ce_match_stat_basic(struct cache_entry *ce, struct stat *st)
 {
 	unsigned int changed = 0;
 
@@ -143,6 +143,13 @@ int ce_match_stat(struct cache_entry *ce, struct stat *st)
 	if (ce->ce_size != htonl(st->st_size))
 		changed |= DATA_CHANGED;
 
+	return changed;
+}
+
+int ce_match_stat(struct cache_entry *ce, struct stat *st)
+{
+	unsigned int changed = ce_match_stat_basic(ce, st);
+
 	/*
 	 * Within 1 second of this sequence:
 	 * 	echo xyzzy >file && git-update-index --add file
@@ -594,6 +601,26 @@ static int ce_flush(SHA_CTX *context, int fd)
 	return 0;
 }
 
+static void ce_smudge_racily_clean_entry(struct cache_entry *ce)
+{
+	/*
+	 * The only thing we care about in this function is to smudge the
+	 * falsely clean entry due to touch-update-touch race, so we leave
+	 * everything else as they are.  We are called for entries whose
+	 * ce_mtime match the index file mtime.
+	 */
+	struct stat st;
+
+	if (lstat(ce->name, &st) < 0)
+		return;
+	if (ce_match_stat_basic(ce, &st))
+		return;
+	if (ce_modified_check_fs(ce, &st)) {
+		/* This is "racily clean"; smudge it */
+		ce->ce_size = htonl(0);
+	}
+}
+
 int write_cache(int newfd, struct cache_entry **cache, int entries)
 {
 	SHA_CTX c;
@@ -616,6 +643,9 @@ int write_cache(int newfd, struct cache_entry **cache, int entries)
 		struct cache_entry *ce = cache[i];
 		if (!ce->ce_mode)
 			continue;
+		if (index_file_timestamp &&
+		    index_file_timestamp <= ntohl(ce->ce_mtime.sec))
+			ce_smudge_racily_clean_entry(ce);
 		if (ce_write(&c, newfd, ce, ce_size(ce)) < 0)
 			return -1;
 	}

t/t0010-racy-git.sh

@@ -7,18 +7,27 @@ test_description='racy GIT'
 # This test can give false success if your machine is sufficiently
 # slow or your trial happened to happen on second boundary.
 
-for trial in 0 1 2 3 4 5 6 7 8 9
+for trial in 0 1 2 3 4
 do
 	rm -f .git/index
 	echo frotz >infocom
-	echo xyzzy >activision
-	git update-index --add infocom activision
+	git update-index --add infocom
 	echo xyzzy >infocom
 
 	files=`git diff-files -p`
 	test_expect_success \
-	"Racy GIT trial #$trial" \
+	"Racy GIT trial #$trial part A" \
 	'test "" != "$files"'
+
+	sleep 1
+	echo xyzzy >cornerstone
+	git update-index --add cornerstone
+
+	files=`git diff-files -p`
+	test_expect_success \
+	"Racy GIT trial #$trial part B" \
+	'test "" != "$files"'
+
 done
 
 test_done