refs: extract out refname verification in transactions

Unless the `REF_SKIP_REFNAME_VERIFICATION` flag is set for an update,
the refname of the update is verified for:

  - Ensuring it is not a pseudoref.
  - Checking the refname format.

These checks will also be needed in a following commit where the
function to add reflog updates to the transaction is introduced. Extract
the code out into a new static function.

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

Author: KarthikNayak

refs.c

@@ -1196,6 +1196,28 @@ struct ref_update *ref_transaction_add_update(
 	return update;
 }
 
+static int transaction_refname_valid(const char *refname,
+				     const struct object_id *new_oid,
+				     unsigned int flags, struct strbuf *err)
+{
+	if (flags & REF_SKIP_REFNAME_VERIFICATION)
+		return 1;
+
+	if (is_pseudo_ref(refname)) {
+		strbuf_addf(err, _("refusing to update pseudoref '%s'"),
+			    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);
+		return 0;
+	}
+
+	return 1;
+}
+
 int ref_transaction_update(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *new_oid,
@@ -1213,21 +1235,8 @@ int ref_transaction_update(struct ref_transaction *transaction,
 		return -1;
 	}
 
-	if (!(flags & REF_SKIP_REFNAME_VERIFICATION) &&
-	    ((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);
+	if (!transaction_refname_valid(refname, new_oid, flags, err))
 		return -1;
-	}
-
-	if (!(flags & REF_SKIP_REFNAME_VERIFICATION) &&
-	    is_pseudo_ref(refname)) {
-		strbuf_addf(err, _("refusing to update pseudoref '%s'"),
-			    refname);
-		return -1;
-	}
 
 	if (flags & ~REF_TRANSACTION_UPDATE_ALLOWED_FLAGS)
 		BUG("illegal flags 0x%x passed to ref_transaction_update()", flags);