refs: allow loose files without packed-refs

The extensions.refFormat extension is a multi-valued config that
specifies which ref formats are available to the current repository. By
default, Git assumes the list of "files" and "packed", unless there is
at least one of these extensions specified.

With the current values, it is possible for a user to specify only
"files" or only "packed". The only-"packed" option was already ruled as
invalid since Git's current code has too many places that require a
loose reference. This could change in the future.

However, we can now allow the user to specify extensions.refFormat=files
alone, making it impossible to create a packed-refs file (or to read one
that might exist).

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

Author: derrickstolee

refs/files-backend.c

@@ -1207,6 +1207,12 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
+	if (!packed_refs_enabled(refs->store_flags)) {
+		warning(_("refusing to create '%s' file because '%s' is not set"),
+			"packed-refs", "extensions.refFormat=packed");
+		return -1;
+	}
+
 	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
 	if (!transaction)
 		return -1;

refs/packed-backend.c

@@ -478,6 +478,9 @@ static int load_contents(struct snapshot *snapshot)
 	size_t size;
 	ssize_t bytes_read;
 
+	if (!packed_refs_enabled(snapshot->refs->store_flags))
+		return 0;
+
 	fd = open(snapshot->refs->path, O_RDONLY);
 	if (fd < 0) {
 		if (errno == ENOENT) {

refs/refs-internal.h

@@ -524,6 +524,11 @@ struct ref_store;
 #define REF_STORE_FORMAT_FILES		(1 << 8) /* can use loose ref files */
 #define REF_STORE_FORMAT_PACKED		(1 << 9) /* can use packed-refs file */
 
+static inline int packed_refs_enabled(int flags)
+{
+	return flags & REF_STORE_FORMAT_PACKED;
+}
+
 /*
  * Initialize the ref_store for the specified gitdir. These functions
  * should call base_ref_store_init() to initialize the shared part of

t/t3212-ref-formats.sh

@@ -36,4 +36,24 @@ test_expect_success 'extensions.refFormat=packed only' '
 	)
 '
 
+test_expect_success 'extensions.refFormat=files only' '
+	test_commit T &&
+	git pack-refs --all &&
+	git init only-loose &&
+	(
+		cd only-loose &&
+		git config core.repositoryFormatVersion 1 &&
+		git config extensions.refFormat files &&
+		test_commit A &&
+		test_commit B &&
+		test_must_fail git pack-refs 2>err &&
+		grep "refusing to create" err &&
+		test_path_is_missing .git/packed-refs &&
+
+		# Refuse to parse a packed-refs file.
+		cp ../.git/packed-refs .git/packed-refs &&
+		test_must_fail git rev-parse refs/tags/T
+	)
+'
+
 test_done