refs: add function to translate errors to strings

The commit 76e760b999 (refs: introduce enum-based transaction error
types, 2025-04-08) introduced enum-based transaction error types. The
refs transaction logic was also modified to propagate these errors. For
clients of the ref transaction system, it would be beneficial to provide
human readable messages for these errors.

There is already an existing mapping in 'builtin/update-ref.c', move it
to 'refs.c' as `ref_transaction_error_msg()` and use the same within the
'builtin/update-ref.c'.

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

3 files changed, 26 insertions, 24 deletions

diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 2b1e336ba1..1e6131e04a 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -575,30 +575,7 @@ static void print_rejected_refs(const char *refname,
 				void *cb_data UNUSED)
 {
 	struct strbuf sb = STRBUF_INIT;
-	const char *reason = "";
-
-	switch (err) {
-	case REF_TRANSACTION_ERROR_NAME_CONFLICT:
-		reason = "refname conflict";
-		break;
-	case REF_TRANSACTION_ERROR_CREATE_EXISTS:
-		reason = "reference already exists";
-		break;
-	case REF_TRANSACTION_ERROR_NONEXISTENT_REF:
-		reason = "reference does not exist";
-		break;
-	case REF_TRANSACTION_ERROR_INCORRECT_OLD_VALUE:
-		reason = "incorrect old value provided";
-		break;
-	case REF_TRANSACTION_ERROR_INVALID_NEW_VALUE:
-		reason = "invalid new value provided";
-		break;
-	case REF_TRANSACTION_ERROR_EXPECTED_SYMREF:
-		reason = "expected symref but found regular ref";
-		break;
-	default:
-		reason = "unkown failure";
-	}
+	const char *reason = ref_transaction_error_msg(err);
 
 	strbuf_addf(&sb, "rejected %s %s %s %s\n", refname,
 		    new_oid ? oid_to_hex(new_oid) : new_target,
diff --git a/refs.c b/refs.c
index dce5c49ca2..3d580d03bb 100644
--- a/refs.c
+++ b/refs.c
@@ -3314,3 +3314,23 @@ 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);
 }
+
+const char *ref_transaction_error_msg(enum ref_transaction_error err)
+{
+	switch (err) {
+	case REF_TRANSACTION_ERROR_NAME_CONFLICT:
+		return "refname conflict";
+	case REF_TRANSACTION_ERROR_CREATE_EXISTS:
+		return "reference already exists";
+	case REF_TRANSACTION_ERROR_NONEXISTENT_REF:
+		return "reference does not exist";
+	case REF_TRANSACTION_ERROR_INCORRECT_OLD_VALUE:
+		return "incorrect old value provided";
+	case REF_TRANSACTION_ERROR_INVALID_NEW_VALUE:
+		return "invalid new value provided";
+	case REF_TRANSACTION_ERROR_EXPECTED_SYMREF:
+		return "expected symref but found regular ref";
+	default:
+		return "unknown failure";
+	}
+}
diff --git a/refs.h b/refs.h
index 46a6008e07..2d58af3d88 100644
--- a/refs.h
+++ b/refs.h
@@ -908,6 +908,11 @@ void ref_transaction_for_each_rejected_update(struct ref_transaction *transactio
 					      void *cb_data);
 
 /*
+ * Translate errors to human readable error messages.
+ */
+const char *ref_transaction_error_msg(enum ref_transaction_error err);
+
+/*
  * Free `*transaction` and all associated data.
  */
 void ref_transaction_free(struct ref_transaction *transaction);