pack-objects: fix segfault in --stdin-packs option

Fix a segfault in the --stdin-packs option added in
339bce27f4 (builtin/pack-objects.c: add '--stdin-packs' option,
2021-02-22).

The read_packs_list_from_stdin() function didn't check that the lines
it was reading were valid packs, and thus when doing the QSORT() with
pack_mtime_cmp() we'd have a NULL "util" field. The "util" field is
used to associate the names of included/excluded packs with the
packed_git structs they correspond to.

The logic error was in assuming that we could iterate all packs and
annotate the excluded and included packs we got, as opposed to
checking the lines we got on stdin. There was a check for excluded
packs, but included packs were simply assumed to be valid.

As noted in the test we'll not report the first bad line, but whatever
line sorted first according to the string-list.c API. In this case I
think that's fine. There was further discussion of alternate
approaches in [1].

Even though we're being lazy let's assert the line we do expect to get
in the test, since whoever changes this code in the future might miss
this case, and would want to update the test and comments.

1. http://lore.kernel.org/git/YND3h2l10PlnSNGJ@nand.local

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -3311,9 +3311,26 @@ static void read_packs_list_from_stdin(void)
 	}
 
 	/*
-	 * First handle all of the excluded packs, marking them as kept in-core
-	 * so that later calls to add_object_entry() discards any objects that
-	 * are also found in excluded packs.
+	 * Arguments we got on stdin may not even be packs. First
+	 * check that to avoid segfaulting later on in
+	 * e.g. pack_mtime_cmp(), excluded packs are handled below.
+	 *
+	 * Since we first parsed our STDIN and then sorted the input
+	 * lines the pack we error on will be whatever line happens to
+	 * sort first. This is lazy, it's enough that we report one
+	 * bad case here, we don't need to report the first/last one,
+	 * or all of them.
+	 */
+	for_each_string_list_item(item, &include_packs) {
+		struct packed_git *p = item->util;
+		if (!p)
+			die(_("could not find pack '%s'"), item->string);
+	}
+
+	/*
+	 * Then, handle all of the excluded packs, marking them as
+	 * kept in-core so that later calls to add_object_entry()
+	 * discards any objects that are also found in excluded packs.
 	 */
 	for_each_string_list_item(item, &exclude_packs) {
 		struct packed_git *p = item->util;

t/t5300-pack-object.sh

@@ -119,6 +119,25 @@ test_expect_success 'pack-object <stdin parsing: [|--revs] with --stdin' '
 	test_cmp err.expect err.actual
 '
 
+test_expect_success 'pack-object <stdin parsing: --stdin-packs handles garbage' '
+	cat >in <<-EOF &&
+	$(git -C pack-object-stdin rev-parse one)
+	$(git -C pack-object-stdin rev-parse two)
+	EOF
+
+	# That we get "two" and not "one" has to do with OID
+	# ordering. It happens to be the same here under SHA-1 and
+	# SHA-256. See commentary in pack-objects.c
+	cat >err.expect <<-EOF &&
+	fatal: could not find pack '"'"'$(git -C pack-object-stdin rev-parse two)'"'"'
+	EOF
+	test_must_fail git \
+		-C pack-object-stdin \
+		pack-objects stdin-with-stdin-option --stdin-packs \
+		<in 2>err.actual &&
+	test_cmp err.expect err.actual
+'
+
 # usage: check_deltas <stderr_from_pack_objects> <cmp_op> <nr_deltas>
 # e.g.: check_deltas stderr -gt 0
 check_deltas() {