rerere: fix overeager gc

'rerere gc' prunes resolutions of conflicted merges that occurred long
time ago, and when doing so it takes the creation time of the
conflicted automerge results into account.  This can cause the loss of
frequently used conflict resolutions (e.g. long-living topic branches
are merged into a regularly rebuilt integration branch (think of git's
pu)) when they become old enough to exceed 'rerere gc's threshold.

To prevent the loss of valuable merge resolutions 'rerere' will (1)
update the timestamp of the recorded conflict resolution (i.e.
'postimage') each time when encountering and resolving the same merge
conflict, and (2) take this timestamp, i.e. the time of the last usage
into account when gc'ing.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rerere.c

@@ -19,6 +19,12 @@ static time_t rerere_created_at(const char *name)
 	return stat(rerere_path(name, "preimage"), &st) ? (time_t) 0 : st.st_mtime;
 }
 
+static time_t rerere_last_used_at(const char *name)
+{
+	struct stat st;
+	return stat(rerere_path(name, "postimage"), &st) ? (time_t) 0 : st.st_mtime;
+}
+
 static void unlink_rr_item(const char *name)
 {
 	unlink(rerere_path(name, "thisimage"));
@@ -53,11 +59,16 @@ static void garbage_collect(struct string_list *rr)
 	while ((e = readdir(dir))) {
 		if (is_dot_or_dotdot(e->d_name))
 			continue;
-		then = rerere_created_at(e->d_name);
-		if (!then)
-			continue;
-		cutoff = (has_rerere_resolution(e->d_name)
-			  ? cutoff_resolve : cutoff_noresolve);
+
+		then = rerere_last_used_at(e->d_name);
+		if (then) {
+			cutoff = cutoff_resolve;
+		} else {
+			then = rerere_created_at(e->d_name);
+			if (!then)
+				continue;
+			cutoff = cutoff_noresolve;
+		}
 		if (then < now - cutoff * 86400)
 			string_list_append(e->d_name, &to_remove);
 	}

rerere.c

@@ -378,7 +378,13 @@ static int merge(const char *name, const char *path)
 	}
 	ret = ll_merge(&result, path, &base, NULL, &cur, "", &other, "", 0);
 	if (!ret) {
-		FILE *f = fopen(path, "w");
+		FILE *f;
+
+		if (utime(rerere_path(name, "postimage"), NULL) < 0)
+			warning("failed utime() on %s: %s",
+					rerere_path(name, "postimage"),
+					strerror(errno));
+		f = fopen(path, "w");
 		if (!f)
 			return error("Could not open %s: %s", path,
 				     strerror(errno));

t/t4200-rerere.sh

@@ -132,6 +132,8 @@ test_expect_success 'commit succeeds' \
 
 test_expect_success 'recorded postimage' "test -f $rr/postimage"
 
+oldmtimepost=$(test-chmtime -v -60 $rr/postimage |cut -f 1)
+
 test_expect_success 'another conflicting merge' '
 	git checkout -b third master &&
 	git show second^:a1 | sed "s/To die: t/To die! T/" > a1 &&
@@ -144,6 +146,11 @@ test_expect_success 'rerere kicked in' "! grep ^=======$ a1"
 
 test_expect_success 'rerere prefers first change' 'test_cmp a1 expect'
 
+test_expect_success 'rerere updates postimage timestamp' '
+	newmtimepost=$(test-chmtime -v +0 $rr/postimage |cut -f 1) &&
+	test $oldmtimepost -lt $newmtimepost
+'
+
 rm $rr/postimage
 echo "$sha1	a1" | perl -pe 'y/\012/\000/' > .git/MERGE_RR
 
@@ -165,15 +172,16 @@ just_over_15_days_ago=$((-1-15*86400))
 almost_60_days_ago=$((60-60*86400))
 just_over_60_days_ago=$((-1-60*86400))
 
-test-chmtime =$almost_60_days_ago $rr/preimage
+test-chmtime =$just_over_60_days_ago $rr/preimage
+test-chmtime =$almost_60_days_ago $rr/postimage
 test-chmtime =$almost_15_days_ago $rr2/preimage
 
 test_expect_success 'garbage collection (part1)' 'git rerere gc'
 
-test_expect_success 'young records still live' \
+test_expect_success 'young or recently used records still live' \
 	"test -f $rr/preimage && test -f $rr2/preimage"
 
-test-chmtime =$just_over_60_days_ago $rr/preimage
+test-chmtime =$just_over_60_days_ago $rr/postimage
 test-chmtime =$just_over_15_days_ago $rr2/preimage
 
 test_expect_success 'garbage collection (part2)' 'git rerere gc'