DB-817 add a txn method to get the child of the txn if any

buildheader/make_tdb.cc

@@ -586,6 +586,7 @@ static void print_db_txn_struct (void) {
         "void (*set_client_id)(DB_TXN *, uint64_t client_id)",
         "uint64_t (*get_client_id)(DB_TXN *)",
         "bool (*is_prepared)(DB_TXN *)",
+        "DB_TXN *(*get_child)(DB_TXN *)",
 	NULL};
     sort_and_dump_fields("db_txn", false, extra);
 }

src/ydb_txn.cc

@@ -428,6 +428,10 @@ static bool toku_txn_is_prepared(DB_TXN *txn) {
     return toku_txn_get_state(ttxn) == TOKUTXN_PREPARING;
 }
 
+static DB_TXN *toku_txn_get_child(DB_TXN *txn) {
+    return db_txn_struct_i(txn)->child;
+}
+
 static inline void txn_func_init(DB_TXN *txn) {
 #define STXN(name) txn->name = locked_txn_ ## name
     STXN(abort);
@@ -445,6 +449,7 @@ static inline void txn_func_init(DB_TXN *txn) {
 #undef SUTXN
     txn->id64 = toku_txn_id64;
     txn->is_prepared = toku_txn_is_prepared;
+    txn->get_child = toku_txn_get_child;
 }
 
 //