refs: introduce the ref_transaction_update_reflog function

Introduce a new function `ref_transaction_update_reflog`, for clients to
add a reflog update to a transaction. While the existing function
`ref_transaction_update` also allows clients to add a reflog entry, this
function does a few things more, It:
  - Enforces that only a reflog entry is added and does not update the
  ref itself.
  - Allows the users to also provide the committer information. This
  means clients can add reflog entries with custom committer
  information.

The `transaction_refname_valid()` function also modifies the error
message selectively based on the type of the update. This change also
affects reflog updates which go through `ref_transaction_update()`.

A follow up commit will utilize this function to add reflog support to
`git refs migrate`.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1207,14 +1207,14 @@ static int transaction_refname_valid(const char *refname,
 		return 1;
 
 	if (is_pseudo_ref(refname)) {
-		strbuf_addf(err, _("refusing to update pseudoref '%s'"),
-			    refname);
+		const char *what = flags & REF_LOG_ONLY ? "reflog for pseudoref" : "pseudoref";
+		strbuf_addf(err, _("refusing to update %s '%s'"), what, refname);
 		return 0;
 	} else if ((new_oid && !is_null_oid(new_oid)) ?
 		 check_refname_format(refname, REFNAME_ALLOW_ONELEVEL) :
 		 !refname_is_safe(refname)) {
-		strbuf_addf(err, _("refusing to update ref with bad name '%s'"),
-			    refname);
+		const char *what = flags & REF_LOG_ONLY ? "reflog with bad name" : "ref with bad name";
+		strbuf_addf(err, _("refusing to update %s '%s'"), what, refname);
 		return 0;
 	}
 
@@ -1257,6 +1257,37 @@ int ref_transaction_update(struct ref_transaction *transaction,
 	ref_transaction_add_update(transaction, refname, flags,
 				   new_oid, old_oid, new_target,
 				   old_target, NULL, msg);
+
+	return 0;
+}
+
+int ref_transaction_update_reflog(struct ref_transaction *transaction,
+				  const char *refname,
+				  const struct object_id *new_oid,
+				  const struct object_id *old_oid,
+				  const char *committer_info, unsigned int flags,
+				  const char *msg, unsigned int index,
+				  struct strbuf *err)
+{
+	struct ref_update *update;
+
+	assert(err);
+
+	flags |= REF_LOG_ONLY | REF_NO_DEREF;
+
+	if (!transaction_refname_valid(refname, new_oid, flags, err))
+		return -1;
+
+	update = ref_transaction_add_update(transaction, refname, flags,
+					    new_oid, old_oid, NULL, NULL,
+					    committer_info, msg);
+	/*
+	 * While we do set the old_oid value, we unset the flag to skip
+	 * old_oid verification which only makes sense for refs.
+	 */
+	update->flags &= ~REF_HAVE_OLD;
+	update->index = index;
+
 	return 0;
 }
 

refs.h

@@ -727,6 +727,20 @@ int ref_transaction_update(struct ref_transaction *transaction,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err);
 
+/*
+ * Similar to`ref_transaction_update`, but this function is only for adding
+ * a reflog update. Supports providing custom committer information. The index
+ * field can be utiltized to order updates as desired. When not used, the
+ * updates default to being ordered by refname.
+ */
+int ref_transaction_update_reflog(struct ref_transaction *transaction,
+				  const char *refname,
+				  const struct object_id *new_oid,
+				  const struct object_id *old_oid,
+				  const char *committer_info, unsigned int flags,
+				  const char *msg, unsigned int index,
+				  struct strbuf *err);
+
 /*
  * Add a reference creation to transaction. new_oid is the value that
  * the reference should have after the update; it must not be

refs/files-backend.c

@@ -3080,10 +3080,12 @@ static int files_transaction_finish_initial(struct files_ref_store *refs,
 		}
 
 		/*
-		 * packed-refs don't support symbolic refs and root refs, so we
-		 * have to queue these references via the loose transaction.
+		 * packed-refs don't support symbolic refs, root refs and reflogs,
+		 * so we have to queue these references via the loose transaction.
 		 */
-		if (update->new_target || is_root_ref(update->refname)) {
+		if (update->new_target ||
+		    is_root_ref(update->refname) ||
+		    (update->flags & REF_LOG_ONLY)) {
 			if (!loose_transaction) {
 				loose_transaction = ref_store_transaction_begin(&refs->base, 0, err);
 				if (!loose_transaction) {
@@ -3092,6 +3094,12 @@ static int files_transaction_finish_initial(struct files_ref_store *refs,
 				}
 			}
 
+			if (update->flags & REF_LOG_ONLY)
+				ref_transaction_add_update(loose_transaction, update->refname,
+							   update->flags, &update->new_oid,
+							   &update->old_oid, NULL, NULL,
+							   update->committer_info, update->msg);
+			else
 				ref_transaction_add_update(loose_transaction, update->refname,
 							   update->flags & ~REF_HAVE_OLD,
 							   update->new_target ? NULL : &update->new_oid, NULL,