[PATCH v6 3/6] refs: receive and use the reference storage payload

From: Karthik Nayak <karthik.188@gmail.com>
Date: Feb 14, 2026, 22:34 UTC
Message-ID: <20260214-kn-alternate-ref-dir-v6-3-86a82c77cf59@gmail.com>
In-Reply-To: <20260214-kn-alternate-ref-dir-v6-0-86a82c77cf59@gmail.com>

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>
---
 refs.c                  | 40 +++++++++++++++++++++++++++++++++++++++-
 refs/files-backend.c    | 17 ++++++++++++-----
 refs/packed-backend.c   |  5 +++++
 refs/packed-backend.h   |  1 +
 refs/refs-internal.h    | 14 ++++++++++++++
 refs/reftable-backend.c | 24 ++++++++++++++----------
 6 files changed, 85 insertions(+), 16 deletions(-)

diff --git a/refs.c b/refs.c
index 77b93d655b..11d028232b 100644
--- a/refs.c
+++ b/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"
@@ -2247,7 +2248,7 @@ 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;
 }
 
@@ -3425,3 +3426,40 @@ 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);
+}
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 240d3c3b26..b40d6feb1f 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -106,19 +106,24 @@ 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);
 
@@ -126,6 +131,8 @@ static struct ref_store *files_ref_store_init(struct repository *repo,
 	chdir_notify_reparent("files-backend $GIT_COMMONDIR",
 			      &refs->gitcommondir);
 
+	strbuf_release(&refdir);
+
 	return ref_store;
 }
 
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4ea0c12299..e7bb9f10f9 100644
--- a/refs/packed-backend.c
+++ b/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)
 {
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index 9481d5e7c2..2c2377a356 100644
--- a/refs/packed-backend.h
+++ b/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);
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index c7d2a6e50b..9a635f4e6c 100644
--- a/refs/refs-internal.h
+++ b/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 store files 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 */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index d8651fe779..964b0b50fc 100644
--- a/refs/reftable-backend.c
+++ b/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;
 }

-- 
2.52.0