builtin/repack.c: avoid making cruft packs preferred

When doing a `--geometric` repack, we make sure that the preferred pack
(if writing a MIDX) is the largest pack that we *didn't* repack. That
has the effect of keeping the preferred pack in sync with the pack
containing a majority of the repository's reachable objects.

But if the repository happens to double in size, we'll repack
everything. Here we don't specify any `--preferred-pack`, and instead
let the MIDX code choose.

In the past, that worked fine, since there would only be one pack to
choose from: the one we just wrote. But it's no longer necessarily the
case that there is one pack to choose from. It's possible that the
repository also has a cruft pack, too.

If the cruft pack happens to come earlier in lexical order (and has an
earlier mtime than any non-cruft pack), we'll pick that pack as
preferred. This makes it impossible to reuse chunks of the reachable
pack verbatim from pack-objects, so is sub-optimal.

Luckily, this is a somewhat rare circumstance to be in, since we would
have to repack the entire repository during a `--geometric` repack, and
the cruft pack would have to sort ahead of the pack we just created.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/repack.c b/builtin/repack.c
index 04770b15fe7ee21727fdda173bbabdb5f5760d73..a1a893d9524fd175bfe0793f5e1e0e0aea3a3947 100644 (file)
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -355,6 +355,18 @@ static struct generated_pack_data *populate_pack_exts(const char *name)
        return data;
 }
 
+static int has_pack_ext(const struct generated_pack_data *data,
+                       const char *ext)
+{
+       int i;
+       for (i = 0; i < ARRAY_SIZE(exts); i++) {
+               if (strcmp(exts[i].name, ext))
+                       continue;
+               return !!data->tempfiles[i];
+       }
+       BUG("unknown pack extension: '%s'", ext);
+}
+
 static void repack_promisor_objects(const struct pack_objects_args *args,
                                    struct string_list *names)
 {
@@ -772,6 +784,7 @@ static void midx_included_packs(struct string_list *include,
 
 static int write_midx_included_packs(struct string_list *include,
                                     struct pack_geometry *geometry,
+                                    struct string_list *names,
                                     const char *refs_snapshot,
                                     int show_progress, int write_bitmaps)
 {
@@ -801,6 +814,38 @@ static int write_midx_included_packs(struct string_list *include,
        if (preferred)
                strvec_pushf(&cmd.args, "--preferred-pack=%s",
                             pack_basename(preferred));
+       else if (names->nr) {
+               /* The largest pack was repacked, meaning that either
+                * one or two packs exist depending on whether the
+                * repository has a cruft pack or not.
+                *
+                * Select the non-cruft one as preferred to encourage
+                * pack-reuse among packs containing reachable objects
+                * over unreachable ones.
+                *
+                * (Note we could write multiple packs here if
+                * `--max-pack-size` was given, but any one of them
+                * will suffice, so pick the first one.)
+                */
+               for_each_string_list_item(item, names) {
+                       struct generated_pack_data *data = item->util;
+                       if (has_pack_ext(data, ".mtimes"))
+                               continue;
+
+                       strvec_pushf(&cmd.args, "--preferred-pack=pack-%s.pack",
+                                    item->string);
+                       break;
+               }
+       } else {
+               /*
+                * No packs were kept, and no packs were written. The
+                * only thing remaining are .keep packs (unless
+                * --pack-kept-objects was given).
+                *
+                * Set the `--preferred-pack` arbitrarily here.
+                */
+               ;
+       }
 
        if (refs_snapshot)
                strvec_pushf(&cmd.args, "--refs-snapshot=%s", refs_snapshot);
@@ -1360,7 +1405,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
                struct string_list include = STRING_LIST_INIT_NODUP;
                midx_included_packs(&include, &existing, &names, &geometry);
 
-               ret = write_midx_included_packs(&include, &geometry,
+               ret = write_midx_included_packs(&include, &geometry, &names,
                                                refs_snapshot ? get_tempfile_path(refs_snapshot) : NULL,
                                                show_progress, write_bitmaps > 0);
 

diff --git a/t/t7704-repack-cruft.sh b/t/t7704-repack-cruft.sh
index dc86ca8269bb1ac89d9e71f634dfd4f983dec33f..be3735dff0837a6050780082f3acc4e1f7da37dc 100755 (executable)
--- a/t/t7704-repack-cruft.sh
+++ b/t/t7704-repack-cruft.sh
@@ -372,4 +372,43 @@ test_expect_success '--max-cruft-size ignores non-local packs' '
        )
 '
 
+test_expect_success 'reachable packs are preferred over cruft ones' '
+       repo="cruft-preferred-packs" &&
+       git init "$repo" &&
+       (
+               cd "$repo" &&
+
+               # This test needs to exercise careful control over when a MIDX
+               # is and is not written. Unset the corresponding TEST variable
+               # accordingly.
+               sane_unset GIT_TEST_MULTI_PACK_INDEX &&
+
+               test_commit base &&
+               test_commit --no-tag cruft &&
+
+               non_cruft="$(echo base | git pack-objects --revs $packdir/pack)" &&
+               # Write a cruft pack which both (a) sorts ahead of the non-cruft
+               # pack in lexical order, and (b) has an older mtime to appease
+               # the MIDX preferred pack selection routine.
+               cruft="$(echo pack-$non_cruft.pack | git pack-objects --cruft $packdir/pack-A)" &&
+               test-tool chmtime -1000 $packdir/pack-A-$cruft.pack &&
+
+               test_commit other &&
+               git repack -d &&
+
+               git repack --geometric 2 -d --write-midx --write-bitmap-index &&
+
+               # After repacking, there are two packs left: one reachable one
+               # (which is the result of combining both of the existing two
+               # non-cruft packs), and one cruft pack.
+               find .git/objects/pack -type f -name "*.pack" >packs &&
+               test_line_count = 2 packs &&
+
+               # Make sure that the pack we just wrote is marked as preferred,
+               # not the cruft one.
+               pack="$(test-tool read-midx --preferred-pack $objdir)" &&
+               test_path_is_missing "$packdir/$(basename "$pack" ".idx").mtimes"
+       )
+'
+
 test_done