midx-write.c: tolerate `--preferred-pack` without bitmaps

When passing a preferred pack to the MIDX write machinery, we ensure
that the given preferred pack is non-empty since 5d3cd09a808 (midx:
reject empty `--preferred-pack`'s, 2021-08-31).

However packs are only loaded (via `write_midx_internal()`, though a
subsequent patch will refactor this code out to its own function) when
the `MIDX_WRITE_REV_INDEX` flag is set.

So if a caller runs:

    $ git multi-pack-index write --preferred-pack=...

with both (a) an existing MIDX, and (b) specifies a pack from that MIDX
as the preferred one, without passing `--bitmap`, then the check added
in 5d3cd09a808 will result in a segfault.

Note that packs loaded from disk which don't appear in an existing MIDX
do not trigger this issue, as those packs are loaded unconditionally. We
conditionally load packs from a MIDX since we tolerate MIDXs whose
packs do not resolve (i.e., via the MIDX write after removing
unreferenced packs via 'git multi-pack-index expire').

In practice, this isn't possible to trigger when running `git
multi-pack-index write` from `git repack`, as the latter always passes
`--stdin-packs`, which prevents us from loading an existing MIDX, as it
forces all packs to be read from disk.

But a future commit in this series will change that behavior to
unconditionally load an existing MIDX, even with `--stdin-packs`, making
this behavior trigger-able from 'repack' much more easily.

Prevent this from being an issue by removing the segfault altogether by
calling `prepare_midx_pack()` on packs loaded from an existing MIDX when
either the `MIDX_WRITE_REV_INDEX` flag is set *or* we specified a
`--preferred-pack`.

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

1 files changed, 7 insertions, 1 deletions

diff --git a/midx-write.c b/midx-write.c
index 65e69d2de7..fface1e93a 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -929,11 +929,17 @@ static int write_midx_internal(const char *object_dir,
 		for (i = 0; i < ctx.m->num_packs; i++) {
 			ALLOC_GROW(ctx.info, ctx.nr + 1, ctx.alloc);
 
-			if (flags & MIDX_WRITE_REV_INDEX) {
+			if (flags & MIDX_WRITE_REV_INDEX ||
+			    preferred_pack_name) {
 				/*
 				 * If generating a reverse index, need to have
 				 * packed_git's loaded to compare their
 				 * mtimes and object count.
+				 *
+				 * If a preferred pack is specified,
+				 * need to have packed_git's loaded to
+				 * ensure the chosen preferred pack has
+				 * a non-zero object count.
 				 */
 				if (prepare_midx_pack(the_repository, ctx.m, i)) {
 					error(_("could not load pack"));