refs: implement releasing ref storages

Ref storages are typically only initialized once for `the_repository`
and then never released. Until now we got away with that without causing
memory leaks because `the_repository` stays reachable, and because the
ref backend is reachable via `the_repository` its memory basically never
leaks.

This is about to change though because of the upcoming migration logic,
which will create a secondary ref storage. In that case, we will either
have to release the old or new ref storage to avoid leaks.

Implement a new `release` callback and expose it via a new
`ref_storage_release()` function.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2041,6 +2041,12 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	return refs;
 }
 
+void ref_store_release(struct ref_store *ref_store)
+{
+	ref_store->be->release(ref_store);
+	free(ref_store->gitdir);
+}
+
 struct ref_store *get_main_ref_store(struct repository *r)
 {
 	if (r->refs_private)

refs.h

@@ -118,6 +118,11 @@ int is_branch(const char *refname);
 
 int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err);
 
+/*
+ * Release all memory and resources associated with the ref store.
+ */
+void ref_store_release(struct ref_store *ref_store);
+
 /*
  * Return the peeled value of the oid currently being iterated via
  * for_each_ref(), etc. This is equivalent to calling:

refs/debug.c

@@ -33,6 +33,13 @@ struct ref_store *maybe_debug_wrap_ref_store(const char *gitdir, struct ref_stor
 	return (struct ref_store *)res;
 }
 
+static void debug_release(struct ref_store *refs)
+{
+	struct debug_ref_store *drefs = (struct debug_ref_store *)refs;
+	drefs->refs->be->release(drefs->refs);
+	trace_printf_key(&trace_refs, "release\n");
+}
+
 static int debug_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err)
 {
 	struct debug_ref_store *drefs = (struct debug_ref_store *)refs;
@@ -427,6 +434,7 @@ static int debug_reflog_expire(struct ref_store *ref_store, const char *refname,
 struct ref_storage_be refs_be_debug = {
 	.name = "debug",
 	.init = NULL,
+	.release = debug_release,
 	.create_on_disk = debug_create_on_disk,
 
 	/*

refs/files-backend.c

@@ -149,6 +149,14 @@ static struct files_ref_store *files_downcast(struct ref_store *ref_store,
 	return refs;
 }
 
+static void files_ref_store_release(struct ref_store *ref_store)
+{
+	struct files_ref_store *refs = files_downcast(ref_store, 0, "release");
+	free_ref_cache(refs->loose);
+	free(refs->gitcommondir);
+	ref_store_release(refs->packed_ref_store);
+}
+
 static void files_reflog_path(struct files_ref_store *refs,
 			      struct strbuf *sb,
 			      const char *refname)
@@ -3284,7 +3292,9 @@ static int files_ref_store_create_on_disk(struct ref_store *ref_store,
 struct ref_storage_be refs_be_files = {
 	.name = "files",
 	.init = files_ref_store_init,
+	.release = files_ref_store_release,
 	.create_on_disk = files_ref_store_create_on_disk,
+
 	.transaction_prepare = files_transaction_prepare,
 	.transaction_finish = files_transaction_finish,
 	.transaction_abort = files_transaction_abort,

refs/packed-backend.c

@@ -252,6 +252,15 @@ static void clear_snapshot(struct packed_ref_store *refs)
 	}
 }
 
+static void packed_ref_store_release(struct ref_store *ref_store)
+{
+	struct packed_ref_store *refs = packed_downcast(ref_store, 0, "release");
+	clear_snapshot(refs);
+	rollback_lock_file(&refs->lock);
+	delete_tempfile(&refs->tempfile);
+	free(refs->path);
+}
+
 static NORETURN void die_unterminated_line(const char *path,
 					   const char *p, size_t len)
 {
@@ -1707,7 +1716,9 @@ static struct ref_iterator *packed_reflog_iterator_begin(struct ref_store *ref_s
 struct ref_storage_be refs_be_packed = {
 	.name = "packed",
 	.init = packed_ref_store_init,
+	.release = packed_ref_store_release,
 	.create_on_disk = packed_ref_store_create_on_disk,
+
 	.transaction_prepare = packed_transaction_prepare,
 	.transaction_finish = packed_transaction_finish,
 	.transaction_abort = packed_transaction_abort,

refs/refs-internal.h

@@ -529,6 +529,10 @@ struct ref_store;
 typedef struct ref_store *ref_store_init_fn(struct repository *repo,
 					    const char *gitdir,
 					    unsigned int flags);
+/*
+ * Release all memory and resources associated with the ref store.
+ */
+typedef void ref_store_release_fn(struct ref_store *refs);
 
 typedef int ref_store_create_on_disk_fn(struct ref_store *refs,
 					int flags,
@@ -668,6 +672,7 @@ typedef int read_symbolic_ref_fn(struct ref_store *ref_store, const char *refnam
 struct ref_storage_be {
 	const char *name;
 	ref_store_init_fn *init;
+	ref_store_release_fn *release;
 	ref_store_create_on_disk_fn *create_on_disk;
 
 	ref_transaction_prepare_fn *transaction_prepare;

refs/reftable-backend.c

@@ -293,6 +293,27 @@ done:
 	return &refs->base;
 }
 
+static void reftable_be_release(struct ref_store *ref_store)
+{
+	struct reftable_ref_store *refs = reftable_be_downcast(ref_store, 0, "release");
+	struct strmap_entry *entry;
+	struct hashmap_iter iter;
+
+	if (refs->main_stack) {
+		reftable_stack_destroy(refs->main_stack);
+		refs->main_stack = NULL;
+	}
+
+	if (refs->worktree_stack) {
+		reftable_stack_destroy(refs->worktree_stack);
+		refs->worktree_stack = NULL;
+	}
+
+	strmap_for_each_entry(&refs->worktree_stacks, &iter, entry)
+		reftable_stack_destroy(entry->value);
+	strmap_clear(&refs->worktree_stacks, 0);
+}
+
 static int reftable_be_create_on_disk(struct ref_store *ref_store,
 				      int flags UNUSED,
 				      struct strbuf *err UNUSED)
@@ -2248,7 +2269,9 @@ done:
 struct ref_storage_be refs_be_reftable = {
 	.name = "reftable",
 	.init = reftable_be_init,
+	.release = reftable_be_release,
 	.create_on_disk = reftable_be_create_on_disk,
+
 	.transaction_prepare = reftable_be_transaction_prepare,
 	.transaction_finish = reftable_be_transaction_finish,
 	.transaction_abort = reftable_be_transaction_abort,