shallow: reset commit grafts when shallow is reset

When reset_repository_shallow() is called, Git clears its cache of
shallow information, so that if shallow information is re-requested, Git
will read fresh data from disk instead of reusing its stale cached data.
However, the cache of commit grafts is not likewise cleared, even though
there are commit grafts created from shallow information.

This means that if on-disk shallow information were to be updated and
then a commit-graft-using codepath were run (for example, a revision
walk), Git would be using stale commit graft information. This can be
seen from the test in this patch, in which Git performs a revision walk
(to check for changed submodules) after a fetch with --update-shallow.

Therefore, clear the cache of commit grafts whenever
reset_repository_shallow() is called.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -249,6 +249,16 @@ int for_each_commit_graft(each_commit_graft_fn fn, void *cb_data)
 	return ret;
 }
 
+void reset_commit_grafts(struct repository *r)
+{
+	int i;
+
+	for (i = 0; i < r->parsed_objects->grafts_nr; i++)
+		free(r->parsed_objects->grafts[i]);
+	r->parsed_objects->grafts_nr = 0;
+	r->parsed_objects->commit_graft_prepared = 0;
+}
+
 struct commit_buffer {
 	void *buffer;
 	unsigned long size;

commit.h

@@ -249,6 +249,7 @@ int commit_graft_pos(struct repository *r, const struct object_id *oid);
 int register_commit_graft(struct repository *r, struct commit_graft *, int);
 void prepare_commit_graft(struct repository *r);
 struct commit_graft *lookup_commit_graft(struct repository *r, const struct object_id *oid);
+void reset_commit_grafts(struct repository *r);
 
 struct commit *get_fork_point(const char *refname, struct commit *commit);
 

shallow.c

@@ -90,6 +90,7 @@ static void reset_repository_shallow(struct repository *r)
 {
 	r->parsed_objects->is_shallow = -1;
 	stat_validity_clear(r->parsed_objects->shallow_stat);
+	reset_commit_grafts(r);
 }
 
 int commit_shallow_file(struct repository *r, struct shallow_lock *lk)

submodule.c

@@ -22,6 +22,7 @@
 #include "parse-options.h"
 #include "object-store.h"
 #include "commit-reach.h"
+#include "shallow.h"
 
 static int config_update_recurse_submodules = RECURSE_SUBMODULES_OFF;
 static int initialized_fetch_ref_tips;

t/t5537-fetch-shallow.sh

@@ -161,6 +161,15 @@ test_expect_success 'fetch --update-shallow' '
 	)
 '
 
+test_expect_success 'fetch --update-shallow into a repo with submodules' '
+	git init a-submodule &&
+	test_commit -C a-submodule foo &&
+	git init repo-with-sub &&
+	git -C repo-with-sub submodule add ../a-submodule a-submodule &&
+	git -C repo-with-sub commit -m "added submodule" &&
+	git -C repo-with-sub fetch --update-shallow ../shallow/.git refs/heads/*:refs/remotes/shallow/*
+'
+
 test_expect_success 'fetch --update-shallow (with fetch.writeCommitGraph)' '
 	(
 	cd shallow &&