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 5d3cd09a80 (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 5d3cd09a80 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>

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"));

t/t5319-multi-pack-index.sh

@@ -350,6 +350,29 @@ test_expect_success 'preferred packs must be non-empty' '
 	)
 '
 
+test_expect_success 'preferred pack from existing MIDX without bitmaps' '
+	git init preferred-without-bitmaps &&
+	(
+		cd preferred-without-bitmaps &&
+
+		test_commit one &&
+		pack="$(git pack-objects --all $objdir/pack/pack </dev/null)" &&
+
+		git multi-pack-index write &&
+
+		# make another pack so that the subsequent MIDX write
+		# has something to do
+		test_commit two &&
+		git repack -d &&
+
+		# write a new MIDX without bitmaps reusing the singular
+		# pack from the existing MIDX as the preferred pack in
+		# the new MIDX
+		git multi-pack-index write --preferred-pack=pack-$pack.pack
+	)
+
+'
+
 test_expect_success 'verify multi-pack-index success' '
 	git multi-pack-index verify --object-dir=$objdir
 '