[PATCH v10 2/8] refs: atomically record overwritten ref in update_symref

[Bence Ferdinandy <bence@ferdinandy.com>]

When updating a symref with update_symref it's currently not possible to
know for sure what was the previous value that was overwritten. Extend
refs_update_symref under a new function name, to record the value after
the ref has been locked if the caller of refs_update_symref_extended
requests it via a new variable in the function call. Keep the original
refs_update_symref function with the same signature, but now as
a wrapper around refs_update_symref_extended.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
---

Notes:
    v4: new patch
    
    v5: - added before_target to reftables backend
        - added an extra safety check for transaction's existence in refs.c
    
    v6: - no change
    
    v7: - remove the whole before_target concept from the backends and
          handle checking it in refs.c instead (thanks Karthik)
        - rename the before_target to referent which is how the same concept
          is called in the backends
        - change commit prefix to be more in line with project standards
    
    v8: no change
    
    v9: - instead of adding parameters to refs_update_symref, rename what
          was in v8 as refs_update_symref_extended and make refs_update_symref
          a wrapper for that. This significantly reduces the number of files
          that need to be touched, and avoids adding a lot of dummy NULL-s
          in unrelated places.
    
    v10: no change

 refs.c | 19 ++++++++++++++++---
 refs.h |  4 ++++
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/refs.c b/refs.c
index 5f729ed412..24a4172cd2 100644
--- a/refs.c
+++ b/refs.c
@@ -2115,6 +2115,13 @@ int peel_iterated_oid(struct repository *r, const struct object_id *base, struct
 
 int refs_update_symref(struct ref_store *refs, const char *ref,
 		       const char *target, const char *logmsg)
+{
+	return refs_update_symref_extended(refs, ref, target, logmsg, NULL);
+}
+
+int refs_update_symref_extended(struct ref_store *refs, const char *ref,
+		       const char *target, const char *logmsg,
+		       struct strbuf *referent)
 {
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
@@ -2122,13 +2129,20 @@ int refs_update_symref(struct ref_store *refs, const char *ref,
 
 	transaction = ref_store_transaction_begin(refs, &err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, ref, NULL, NULL,
+		ref_transaction_update(transaction, ref, NULL, NULL,
 				   target, NULL, REF_NO_DEREF,
 				   logmsg, &err) ||
-	    ref_transaction_commit(transaction, &err)) {
+		ref_transaction_prepare(transaction, &err)) {
 		ret = error("%s", err.buf);
+		goto cleanup;
 	}
+	if (referent)
+		refs_read_symbolic_ref(refs, ref, referent);
 
+	if (ref_transaction_commit(transaction, &err))
+		ret = error("%s", err.buf);
+
+cleanup:
 	strbuf_release(&err);
 	if (transaction)
 		ref_transaction_free(transaction);
@@ -2948,4 +2962,3 @@ int ref_update_expects_existing_old_ref(struct ref_update *update)
 	return (update->flags & REF_HAVE_OLD) &&
 		(!is_null_oid(&update->old_oid) || update->old_target);
 }
-
diff --git a/refs.h b/refs.h
index 108dfc93b3..259191a485 100644
--- a/refs.h
+++ b/refs.h
@@ -573,6 +573,10 @@ int refs_copy_existing_ref(struct ref_store *refs, const char *oldref,
 int refs_update_symref(struct ref_store *refs, const char *refname,
 		       const char *target, const char *logmsg);
 
+int refs_update_symref_extended(struct ref_store *refs, const char *refname,
+		       const char *target, const char *logmsg,
+		       struct strbuf *referent);
+
 enum action_on_err {
 	UPDATE_REFS_MSG_ON_ERR,
 	UPDATE_REFS_DIE_ON_ERR,
-- 
2.47.0.94.g8861098b6d