refs: receive and use the reference storage payload

An upcoming commit will add support for providing an URI via the
'extensions.refStorage' config. The URI will contain the reference
backend and a corresponding payload. The payload can be then used for
providing an alternate locations for the reference backend.

To prepare for this, modify the existing backends to accept such an
argument when initializing via the 'init()' function. Both the files
and reftable backends will parse the information to be filesystem paths
to store references. Given that no callers pass any payload yet this is
essentially a no-op change for now.

To enable this, provide a 'refs_compute_filesystem_location()' function
which will parse the current 'gitdir' and the 'payload' to provide the
final reference directory and common reference directory (if working in
a linked worktree).

The documentation and tests will be added alongside the extension of the
config variable.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: KarthikNayak

refs.c

@@ -5,6 +5,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "abspath.h"
 #include "advice.h"
 #include "config.h"
 #include "environment.h"
@@ -2290,7 +2291,7 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, gitdir, flags);
+	refs = be->init(repo, NULL, gitdir, flags);
 	return refs;
 }
 
@@ -3468,3 +3469,40 @@ const char *ref_transaction_error_msg(enum ref_transaction_error err)
 		return "unknown failure";
 	}
 }
+
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	*is_worktree = get_common_dir_noenv(ref_common_dir, gitdir);
+
+	if (!payload) {
+		/*
+		 * We can use the 'gitdir' as the 'refdir' without appending the
+		 * worktree path, as the 'gitdir' here is already the worktree
+		 * path and is different from 'commondir' denoted by 'ref_common_dir'.
+		 */
+		strbuf_addstr(refdir, gitdir);
+		return;
+	}
+
+	if (!is_absolute_path(payload)) {
+		strbuf_addf(&sb, "%s/%s", ref_common_dir->buf, payload);
+		strbuf_realpath(ref_common_dir, sb.buf, 1);
+	} else {
+		strbuf_realpath(ref_common_dir, payload, 1);
+	}
+
+	strbuf_addbuf(refdir, ref_common_dir);
+
+	if (*is_worktree) {
+		const char *wt_id = strrchr(gitdir, '/');
+		if (!wt_id)
+			BUG("worktree path does not contain slash");
+		strbuf_addf(refdir, "/worktrees/%s", wt_id + 1);
+	}
+
+	strbuf_release(&sb);
+}

refs/files-backend.c

@@ -106,26 +106,33 @@ static void clear_loose_ref_cache(struct files_ref_store *refs)
  * set of caches.
  */
 static struct ref_store *files_ref_store_init(struct repository *repo,
+					      const char *payload,
 					      const char *gitdir,
 					      unsigned int flags)
 {
 	struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
 	struct ref_store *ref_store = (struct ref_store *)refs;
-	struct strbuf sb = STRBUF_INIT;
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
+	bool is_worktree;
+
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
 
-	base_ref_store_init(ref_store, repo, gitdir, &refs_be_files);
+	base_ref_store_init(ref_store, repo, refdir.buf, &refs_be_files);
 	refs->store_flags = flags;
-	get_common_dir_noenv(&sb, gitdir);
-	refs->gitcommondir = strbuf_detach(&sb, NULL);
+	refs->gitcommondir = strbuf_detach(&ref_common_dir, NULL);
 	refs->packed_ref_store =
-		packed_ref_store_init(repo, refs->gitcommondir, flags);
+		packed_ref_store_init(repo, NULL, refs->gitcommondir, flags);
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
 	repo_config_get_bool(repo, "core.prefersymlinkrefs", &refs->prefer_symlink_refs);
 
 	chdir_notify_reparent("files-backend $GIT_DIR", &refs->base.gitdir);
 	chdir_notify_reparent("files-backend $GIT_COMMONDIR",
 			      &refs->gitcommondir);
 
+	strbuf_release(&refdir);
+
 	return ref_store;
 }
 

refs/packed-backend.c

@@ -211,7 +211,12 @@ static size_t snapshot_hexsz(const struct snapshot *snapshot)
 	return snapshot->refs->base.repo->hash_algo->hexsz;
 }
 
+/*
+ * Since packed-refs is only stored in the common dir, don't parse the
+ * payload and rely on the files-backend to set 'gitdir' correctly.
+ */
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload UNUSED,
 					const char *gitdir,
 					unsigned int store_flags)
 {

refs/packed-backend.h

@@ -14,6 +14,7 @@ struct ref_transaction;
  */
 
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload,
 					const char *gitdir,
 					unsigned int store_flags);
 

refs/refs-internal.h

@@ -389,6 +389,7 @@ struct ref_store;
  * the ref_store and to record the ref_store for later lookup.
  */
 typedef struct ref_store *ref_store_init_fn(struct repository *repo,
+					    const char *payload,
 					    const char *gitdir,
 					    unsigned int flags);
 /*
@@ -666,4 +667,17 @@ enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs
 					  unsigned int initial_transaction,
 					  struct strbuf *err);
 
+/*
+ * Given a gitdir and the reference storage payload provided, retrieve the
+ * 'refdir' and 'ref_common_dir'. The former is where references should be
+ * stored for the current worktree, the latter is the common reference
+ * directory if working with a linked worktree. If working with the main
+ * worktree, both values will be the same.
+ *
+ * This is used by backends that store references in the repository directly.
+ */
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir);
+
 #endif /* REFS_REFS_INTERNAL_H */

refs/reftable-backend.c

@@ -372,18 +372,24 @@ static int reftable_be_fsync(int fd)
 }
 
 static struct ref_store *reftable_be_init(struct repository *repo,
+					  const char *payload,
 					  const char *gitdir,
 					  unsigned int store_flags)
 {
 	struct reftable_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
 	struct strbuf path = STRBUF_INIT;
-	int is_worktree;
+	bool is_worktree;
 	mode_t mask;
 
 	mask = umask(0);
 	umask(mask);
 
-	base_ref_store_init(&refs->base, repo, gitdir, &refs_be_reftable);
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
+
+	base_ref_store_init(&refs->base, repo, refdir.buf, &refs_be_reftable);
 	strmap_init(&refs->worktree_backends);
 	refs->store_flags = store_flags;
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
@@ -419,14 +425,11 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	/*
 	 * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.
 	 * This stack contains both the shared and the main worktree refs.
-	 *
-	 * Note that we don't try to resolve the path in case we have a
-	 * worktree because `get_common_dir_noenv()` already does it for us.
 	 */
-	is_worktree = get_common_dir_noenv(&path, gitdir);
+	strbuf_addbuf(&path, &ref_common_dir);
 	if (!is_worktree) {
 		strbuf_reset(&path);
-		strbuf_realpath(&path, gitdir, 0);
+		strbuf_realpath(&path, ref_common_dir.buf, 0);
 	}
 	strbuf_addstr(&path, "/reftable");
 	refs->err = reftable_backend_init(&refs->main_backend, path.buf,
@@ -443,10 +446,9 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	 * do it efficiently.
 	 */
 	if (is_worktree) {
-		strbuf_reset(&path);
-		strbuf_addf(&path, "%s/reftable", gitdir);
+		strbuf_addstr(&refdir, "/reftable");
 
-		refs->err = reftable_backend_init(&refs->worktree_backend, path.buf,
+		refs->err = reftable_backend_init(&refs->worktree_backend, refdir.buf,
 						  &refs->write_options);
 		if (refs->err)
 			goto done;
@@ -456,6 +458,8 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 
 done:
 	assert(refs->err != REFTABLE_API_ERROR);
+	strbuf_release(&ref_common_dir);
+	strbuf_release(&refdir);
 	strbuf_release(&path);
 	return &refs->base;
 }