tmp-objdir: disable ref updates when replacing the primary odb

When creating a subprocess with a temporary ODB, we set the
GIT_QUARANTINE_ENVIRONMENT env var to tell child Git processes not
to update refs, since the tmp-objdir may go away.

Introduce a similar mechanism for in-process temporary ODBs when
we call tmp_objdir_replace_primary_odb. Now both mechanisms set
the disable_ref_updates flag on the odb, which is queried by
the ref_transaction_prepare function.

Peff's test case [1] was invoking ref updates via the cachetextconv
setting. That particular code silently does nothing when a ref
update is forbidden. See the call to notes_cache_put in
fill_textconv where errors are ignored.

[1] https://lore.kernel.org/git/YVOn3hDsb5pnxR53@coredump.intra.peff.net/

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -177,6 +177,10 @@ void setup_git_env(const char *git_dir)
 	args.graft_file = getenv_safe(&to_free, GRAFT_ENVIRONMENT);
 	args.index_file = getenv_safe(&to_free, INDEX_ENVIRONMENT);
 	args.alternate_db = getenv_safe(&to_free, ALTERNATE_DB_ENVIRONMENT);
+	if (getenv(GIT_QUARANTINE_ENVIRONMENT)) {
+		args.disable_ref_updates = 1;
+	}
+
 	repo_set_gitdir(the_repository, git_dir, &args);
 	strvec_clear(&to_free);
 

object-file.c

@@ -767,6 +767,12 @@ struct object_directory *set_temporary_primary_odb(const char *dir, int will_des
 	 */
 	new_odb = xcalloc(1, sizeof(*new_odb));
 	new_odb->path = xstrdup(dir);
+
+	/*
+	 * Disable ref updates while a temporary odb is active, since
+	 * the objects in the database may roll back.
+	 */
+	new_odb->disable_ref_updates = 1;
 	new_odb->will_destroy = will_destroy;
 	new_odb->next = the_repository->objects->odb;
 	the_repository->objects->odb = new_odb;

object-store.h

@@ -27,6 +27,13 @@ struct object_directory {
 	uint32_t loose_objects_subdir_seen[8]; /* 256 bits */
 	struct oidtree *loose_objects_cache;
 
+	/*
+	 * This is a temporary object store created by the tmp_objdir
+	 * facility. Disable ref updates since the objects in the store
+	 * might be discarded on rollback.
+	 */
+	int disable_ref_updates;
+
 	/*
 	 * This object store is ephemeral, so there is no need to fsync.
 	 */

refs.c

@@ -2126,7 +2126,7 @@ int ref_transaction_prepare(struct ref_transaction *transaction,
 		break;
 	}
 
-	if (getenv(GIT_QUARANTINE_ENVIRONMENT)) {
+	if (refs->repo->objects->odb->disable_ref_updates) {
 		strbuf_addstr(err,
 			      _("ref updates forbidden inside quarantine environment"));
 		return -1;

repository.c

@@ -80,6 +80,8 @@ void repo_set_gitdir(struct repository *repo,
 	expand_base_dir(&repo->objects->odb->path, o->object_dir,
 			repo->commondir, "objects");
 
+	repo->objects->odb->disable_ref_updates = o->disable_ref_updates;
+
 	free(repo->objects->alternate_db);
 	repo->objects->alternate_db = xstrdup_or_null(o->alternate_db);
 	expand_base_dir(&repo->graft_file, o->graft_file,

repository.h

@@ -162,6 +162,7 @@ struct set_gitdir_args {
 	const char *graft_file;
 	const char *index_file;
 	const char *alternate_db;
+	int disable_ref_updates;
 };
 
 void repo_set_gitdir(struct repository *repo, const char *root,