refs: introduce enum-based transaction error types

Replace preprocessor-defined transaction errors with a strongly-typed
enum `ref_transaction_error`. This change:

  - Improves type safety and function signature clarity.
  - Makes error handling more explicit and discoverable.
  - Maintains existing error cases, while adding new error cases for
    common scenarios.

This refactoring paves the way for more comprehensive error handling
which we will utilize in the upcoming commits to add batch reference
update support.

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

Author: KarthikNayak

builtin/fetch.c

@@ -687,7 +687,7 @@ static int s_update_ref(const char *action,
 		switch (ref_transaction_commit(our_transaction, &err)) {
 		case 0:
 			break;
-		case TRANSACTION_NAME_CONFLICT:
+		case REF_TRANSACTION_ERROR_NAME_CONFLICT:
 			ret = STORE_REF_ERROR_DF_CONFLICT;
 			goto out;
 		default:

refs.c

@@ -2271,7 +2271,7 @@ int refs_update_symref_extended(struct ref_store *refs, const char *ref,
 					   REF_NO_DEREF, logmsg, &err))
 			goto error_return;
 		prepret = ref_transaction_prepare(transaction, &err);
-		if (prepret && prepret != TRANSACTION_CREATE_EXISTS)
+		if (prepret && prepret != REF_TRANSACTION_ERROR_CREATE_EXISTS)
 			goto error_return;
 	} else {
 		if (ref_transaction_update(transaction, ref, NULL, NULL,
@@ -2289,7 +2289,7 @@ int refs_update_symref_extended(struct ref_store *refs, const char *ref,
 		}
 	}
 
-	if (prepret == TRANSACTION_CREATE_EXISTS)
+	if (prepret == REF_TRANSACTION_ERROR_CREATE_EXISTS)
 		goto cleanup;
 
 	if (ref_transaction_commit(transaction, &err))
@@ -2425,7 +2425,7 @@ int ref_transaction_prepare(struct ref_transaction *transaction,
 
 	string_list_sort(&transaction->refnames);
 	if (ref_update_reject_duplicates(&transaction->refnames, err))
-		return TRANSACTION_GENERIC_ERROR;
+		return REF_TRANSACTION_ERROR_GENERIC;
 
 	ret = refs->be->transaction_prepare(refs, transaction, err);
 	if (ret)
@@ -2497,19 +2497,19 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 	return ret;
 }
 
-int refs_verify_refnames_available(struct ref_store *refs,
-				   const struct string_list *refnames,
-				   const struct string_list *extras,
-				   const struct string_list *skip,
-				   unsigned int initial_transaction,
-				   struct strbuf *err)
+enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs,
+					  const struct string_list *refnames,
+					  const struct string_list *extras,
+					  const struct string_list *skip,
+					  unsigned int initial_transaction,
+					  struct strbuf *err)
 {
 	struct strbuf dirname = STRBUF_INIT;
 	struct strbuf referent = STRBUF_INIT;
 	struct string_list_item *item;
 	struct ref_iterator *iter = NULL;
 	struct strset dirnames;
-	int ret = -1;
+	int ret = REF_TRANSACTION_ERROR_NAME_CONFLICT;
 
 	/*
 	 * For the sake of comments in this function, suppose that
@@ -2625,12 +2625,13 @@ int refs_verify_refnames_available(struct ref_store *refs,
 	return ret;
 }
 
-int refs_verify_refname_available(struct ref_store *refs,
-				  const char *refname,
-				  const struct string_list *extras,
-				  const struct string_list *skip,
-				  unsigned int initial_transaction,
-				  struct strbuf *err)
+enum ref_transaction_error refs_verify_refname_available(
+	struct ref_store *refs,
+	const char *refname,
+	const struct string_list *extras,
+	const struct string_list *skip,
+	unsigned int initial_transaction,
+	struct strbuf *err)
 {
 	struct string_list_item item = { .string = (char *) refname };
 	struct string_list refnames = {
@@ -2818,26 +2819,28 @@ int ref_update_has_null_new_value(struct ref_update *update)
 	return !update->new_target && is_null_oid(&update->new_oid);
 }
 
-int ref_update_check_old_target(const char *referent, struct ref_update *update,
-				struct strbuf *err)
+enum ref_transaction_error ref_update_check_old_target(const char *referent,
+						       struct ref_update *update,
+						       struct strbuf *err)
 {
 	if (!update->old_target)
 		BUG("called without old_target set");
 
 	if (!strcmp(referent, update->old_target))
 		return 0;
 
-	if (!strcmp(referent, ""))
+	if (!strcmp(referent, "")) {
 		strbuf_addf(err, "verifying symref target: '%s': "
 			    "reference is missing but expected %s",
 			    ref_update_original_update_refname(update),
 			    update->old_target);
-	else
-		strbuf_addf(err, "verifying symref target: '%s': "
-			    "is at %s but expected %s",
+		return REF_TRANSACTION_ERROR_NONEXISTENT_REF;
+	}
+
+	strbuf_addf(err, "verifying symref target: '%s': is at %s but expected %s",
 			    ref_update_original_update_refname(update),
 			    referent, update->old_target);
-	return -1;
+	return REF_TRANSACTION_ERROR_INCORRECT_OLD_VALUE;
 }
 
 struct migration_data {

refs.h

@@ -16,6 +16,23 @@ struct worktree;
 enum ref_storage_format ref_storage_format_by_name(const char *name);
 const char *ref_storage_format_to_name(enum ref_storage_format ref_storage_format);
 
+enum ref_transaction_error {
+	/* Default error code */
+	REF_TRANSACTION_ERROR_GENERIC = -1,
+	/* Ref name conflict like A vs A/B */
+	REF_TRANSACTION_ERROR_NAME_CONFLICT = -2,
+	/* Ref to be created already exists */
+	REF_TRANSACTION_ERROR_CREATE_EXISTS = -3,
+	/* ref expected but doesn't exist */
+	REF_TRANSACTION_ERROR_NONEXISTENT_REF = -4,
+	/* Provided old_oid or old_target of reference doesn't match actual */
+	REF_TRANSACTION_ERROR_INCORRECT_OLD_VALUE = -5,
+	/* Provided new_oid or new_target is invalid */
+	REF_TRANSACTION_ERROR_INVALID_NEW_VALUE = -6,
+	/* Expected ref to be symref, but is a regular ref */
+	REF_TRANSACTION_ERROR_EXPECTED_SYMREF = -7,
+};
+
 /*
  * Resolve a reference, recursively following symbolic references.
  *
@@ -117,24 +134,24 @@ int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
  *
  * extras and skip must be sorted.
  */
-int refs_verify_refname_available(struct ref_store *refs,
-				  const char *refname,
-				  const struct string_list *extras,
-				  const struct string_list *skip,
-				  unsigned int initial_transaction,
-				  struct strbuf *err);
+enum ref_transaction_error refs_verify_refname_available(struct ref_store *refs,
+						 const char *refname,
+						 const struct string_list *extras,
+						 const struct string_list *skip,
+						 unsigned int initial_transaction,
+						 struct strbuf *err);
 
 /*
  * Same as `refs_verify_refname_available()`, but checking for a list of
  * refnames instead of only a single item. This is more efficient in the case
  * where one needs to check multiple refnames.
  */
-int refs_verify_refnames_available(struct ref_store *refs,
-				   const struct string_list *refnames,
-				   const struct string_list *extras,
-				   const struct string_list *skip,
-				   unsigned int initial_transaction,
-				   struct strbuf *err);
+enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs,
+					  const struct string_list *refnames,
+					  const struct string_list *extras,
+					  const struct string_list *skip,
+					  unsigned int initial_transaction,
+					  struct strbuf *err);
 
 int refs_ref_exists(struct ref_store *refs, const char *refname);
 
@@ -830,13 +847,6 @@ int ref_transaction_verify(struct ref_transaction *transaction,
 			   unsigned int flags,
 			   struct strbuf *err);
 
-/* Naming conflict (for example, the ref names A and A/B conflict). */
-#define TRANSACTION_NAME_CONFLICT -1
-/* When only creation was requested, but the ref already exists. */
-#define TRANSACTION_CREATE_EXISTS -2
-/* All other errors. */
-#define TRANSACTION_GENERIC_ERROR -3
-
 /*
  * Perform the preparatory stages of committing `transaction`. Acquire
  * any needed locks, check preconditions, etc.; basically, do as much