list-objects: handle NULL function pointers

If a caller to traverse_commit_list() specifies the options for the
--objects flag but does not specify a show_object function pointer, the
result is a segfault. This is currently visible by running 'git bundle
create --objects HEAD'.

We could fix this problem by supplying a no-op callback in
builtin/bundle.c, but that only solves the problem for one builtin,
leaving this segfault open for other callers.

Replace all callers of the show_commit and show_object function pointers
in list-objects.c to call helper functions show_commit() and
show_object() which check that the given context has non-NULL functions
before passing the necessary data. One extra benefit is that it reduces
duplication due to passing ctx->show_data to every caller.

Test that this segfault no longer occurs for 'git bundle'.

Co-authored-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bundle.c

@@ -544,6 +544,8 @@ int create_bundle(struct repository *r, const char *path,
 		die("revision walk setup failed");
 	bpi.fd = bundle_fd;
 	bpi.pending = &revs_copy.pending;
+
+	revs.blob_objects = revs.tree_objects = 0;
 	traverse_commit_list(&revs, write_bundle_prerequisites, NULL, &bpi);
 	object_array_remove_duplicates(&revs_copy.pending);
 

list-objects.c

@@ -21,6 +21,23 @@ struct traversal_context {
 	struct filter *filter;
 };
 
+static void show_commit(struct traversal_context *ctx,
+			struct commit *commit)
+{
+	if (!ctx->show_commit)
+		return;
+	ctx->show_commit(commit, ctx->show_data);
+}
+
+static void show_object(struct traversal_context *ctx,
+			struct object *object,
+			const char *name)
+{
+	if (!ctx->show_object)
+		return;
+	ctx->show_object(object, name, ctx->show_data);
+}
+
 static void process_blob(struct traversal_context *ctx,
 			 struct blob *blob,
 			 struct strbuf *path,
@@ -60,7 +77,7 @@ static void process_blob(struct traversal_context *ctx,
 	if (r & LOFR_MARK_SEEN)
 		obj->flags |= SEEN;
 	if (r & LOFR_DO_SHOW)
-		ctx->show_object(obj, path->buf, ctx->show_data);
+		show_object(ctx, obj, path->buf);
 	strbuf_setlen(path, pathlen);
 }
 
@@ -194,7 +211,7 @@ static void process_tree(struct traversal_context *ctx,
 	if (r & LOFR_MARK_SEEN)
 		obj->flags |= SEEN;
 	if (r & LOFR_DO_SHOW)
-		ctx->show_object(obj, base->buf, ctx->show_data);
+		show_object(ctx, obj, base->buf);
 	if (base->len)
 		strbuf_addch(base, '/');
 
@@ -210,7 +227,7 @@ static void process_tree(struct traversal_context *ctx,
 	if (r & LOFR_MARK_SEEN)
 		obj->flags |= SEEN;
 	if (r & LOFR_DO_SHOW)
-		ctx->show_object(obj, base->buf, ctx->show_data);
+		show_object(ctx, obj, base->buf);
 
 	strbuf_setlen(base, baselen);
 	free_tree_buffer(tree);
@@ -228,7 +245,7 @@ static void process_tag(struct traversal_context *ctx,
 	if (r & LOFR_MARK_SEEN)
 		tag->object.flags |= SEEN;
 	if (r & LOFR_DO_SHOW)
-		ctx->show_object(&tag->object, name, ctx->show_data);
+		show_object(ctx, &tag->object, name);
 }
 
 static void mark_edge_parents_uninteresting(struct commit *commit,
@@ -402,7 +419,7 @@ static void do_traverse(struct traversal_context *ctx)
 		if (r & LOFR_MARK_SEEN)
 			commit->object.flags |= SEEN;
 		if (r & LOFR_DO_SHOW)
-			ctx->show_commit(commit, ctx->show_data);
+			show_commit(ctx, commit);
 
 		if (ctx->revs->tree_blobs_in_commit_order)
 			/*

t/t6020-bundle-misc.sh

@@ -475,4 +475,16 @@ test_expect_success 'clone from bundle' '
 	test_cmp expect actual
 '
 
+test_expect_success 'unfiltered bundle with --objects' '
+	git bundle create all-objects.bdl \
+		--all --objects &&
+	git bundle create all.bdl \
+		--all &&
+
+	# Compare the headers of these files.
+	sed -n -e "/^$/q" -e "p" all.bdl >expect &&
+	sed -n -e "/^$/q" -e "p" all-objects.bdl >actual &&
+	test_cmp expect actual
+'
+
 test_done