prune: save reachable-from-recent objects with bitmaps

peff · gitster · commit 2ba582ba4c62 · 2021-04-29T10:38:25.000+09:00
We pass our prune expiration to mark_reachable_objects(), which will traverse not only the reachable objects, but consider any recent ones as tips for reachability; see d3038d2 (prune: keep objects reachable from recent objects, 2014-10-15) for details. However, this interacts badly with the bitmap code path added in fde67d6 (prune: use bitmaps for reachability traversal, 2019-02-13). If we hit the bitmap-optimized path, we return immediately to avoid the regular traversal, accidentally skipping the "also traverse recent" code. Instead, we should do an if-else for the bitmap versus regular traversal, and then follow up with the "recent" traversal in either case. This reuses the "rev_info" for a bitmap and then a regular traversal, but that should work OK (the bitmap code clears the pending array in the usual way, just like a regular traversal would). Note that I dropped the comment above the regular traversal here. It has little explanatory value, and makes the if-else logic much harder to read. Here are a few variants that I rejected: - it seems like both the reachability and recent traversals could be done in a single traversal. This was rejected by d3038d2 (prune: keep objects reachable from recent objects, 2014-10-15), though the balance may be different when using bitmaps. However, there's a subtle correctness issue, too: we use revs->ignore_missing_links for the recent traversal, but not the reachability one. - we could try using bitmaps for the recent traversal, too, which could possibly improve performance. But it would require some fixes in the bitmap code, which uses ignore_missing_links for its own purposes. Plus it would probably not help all that much in practice. We use the reachable tips to generate bitmaps, so those objects are likely not covered by bitmaps (unless they just became unreachable). And in general, we expect the set of unreachable objects to be much smaller anyway, so there's less to gain. The test in t5304 detects the bug and confirms the fix. I also beefed up the tests in t6501, which covers the mtime-checking code more thoroughly, to handle the bitmap case (in addition to just "loose" and "packed" cases). Interestingly, this test doesn't actually detect the bug, because it is running "git gc", and not "prune" directly. And "gc" will call "repack" first, which does not suffer the same bug. So the old-but-reachable-from-recent objects get scooped up into the new pack along with the actually-recent objects, which gives both a recent mtime. But it seemed prudent to get more coverage of the bitmap case for related code. Reported-by: David Emett <dave@sp4m.net> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/reachable.c b/reachable.c
@@ -227,17 +227,12 @@ void mark_reachable_objects(struct rev_info *revs, int mark_reflog,
 	if (bitmap_git) {
 		traverse_bitmap_commit_list(bitmap_git, revs, mark_object_seen);
 		free_bitmap_index(bitmap_git);
-		return;
+	} else {
+		if (prepare_revision_walk(revs))
+			die("revision walk setup failed");
+		traverse_commit_list(revs, mark_commit, mark_object, &cp);
 	}
 
-	/*
-	 * Set up the revision walk - this will move all commits
-	 * from the pending list to the commit walking list.
-	 */
-	if (prepare_revision_walk(revs))
-		die("revision walk setup failed");
-	traverse_commit_list(revs, mark_commit, mark_object, &cp);
-
 	if (mark_recent) {
 		revs->ignore_missing_links = 1;
 		if (add_unseen_recent_objects_to_traversal(revs, mark_recent))
diff --git a/t/t5304-prune.sh b/t/t5304-prune.sh
@@ -352,4 +352,20 @@ test_expect_success 'trivial prune with bitmaps enabled' '
 	test_must_fail git cat-file -e $blob
 '
 
+test_expect_success 'old reachable-from-recent retained with bitmaps' '
+	git repack -adb &&
+	to_drop=$(echo bitmap-from-recent-1 | git hash-object -w --stdin) &&
+	test-tool chmtime -86400 .git/objects/$(test_oid_to_path $to_drop) &&
+	to_save=$(echo bitmap-from-recent-2 | git hash-object -w --stdin) &&
+	test-tool chmtime -86400 .git/objects/$(test_oid_to_path $to_save) &&
+	tree=$(printf "100644 blob $to_save\tfile\n" | git mktree) &&
+	test-tool chmtime -86400 .git/objects/$(test_oid_to_path $tree) &&
+	commit=$(echo foo | git commit-tree $tree) &&
+	git prune --expire=12.hours.ago &&
+	git cat-file -e $commit &&
+	git cat-file -e $tree &&
+	git cat-file -e $to_save &&
+	test_must_fail git cat-file -e $to_drop
+'
+
 test_done
diff --git a/t/t6501-freshen-objects.sh b/t/t6501-freshen-objects.sh
@@ -43,15 +43,25 @@ commit () {
 }
 
 maybe_repack () {
-	if test -n "$repack"; then
+	case "$title" in
+	loose)
+		: skip repack
+		;;
+	repack)
 		git repack -ad
-	fi
+		;;
+	bitmap)
+		git repack -adb
+		;;
+	*)
+		echo >&2 "unknown test type in maybe_repack"
+		return 1
+		;;
+	esac
 }
 
-for repack in '' true; do
-	title=${repack:+repack}
-	title=${title:-loose}
-
+for title in loose repack bitmap
+do
 	test_expect_success "make repo completely empty ($title)" '
 		rm -rf .git &&
 		git init
