[t:4876], small change to toku_txn_begin relating to input flags

git-svn-id: file:///svn/toku/tokudb@44110 c7de825b-a66e-492c-adef-691d508d4ae1

src/ydb_txn.c

@@ -374,10 +374,6 @@ toku_txn_begin(DB_ENV *env, DB_TXN * stxn, DB_TXN ** txn, u_int32_t flags) {
     }
     flags &= ~iso_flags;
 
-    if (stxn) {
-        child_isolation = db_txn_struct_i(stxn)->iso;
-    }
-    else {
     switch (iso_flags) {
         case (DB_INHERIT_ISOLATION):
             if (stxn) {
@@ -401,14 +397,15 @@ toku_txn_begin(DB_ENV *env, DB_TXN * stxn, DB_TXN ** txn, u_int32_t flags) {
             child_isolation = TOKU_ISO_SNAPSHOT;
             break;
         case (DB_SERIALIZABLE):
-            case (0):
             child_isolation = TOKU_ISO_SERIALIZABLE;
             break;
+        case (0):
+            child_isolation = stxn ? db_txn_struct_i(stxn)->iso : TOKU_ISO_SERIALIZABLE;
+            break;
         default:
             assert(FALSE); // error path is above, so this should not happen
             break;
     }
-    }
     if (stxn && child_isolation != db_txn_struct_i(stxn)->iso) {
         return toku_ydb_do_error(
             env,