Update read_transaction for gh#2132

contracts/eosiolib/transaction.h

@@ -67,7 +67,7 @@ extern "C" {
     * access a copy of the currently executing transaction
     *
     * @param buffer - a buffer to write the current transaction to
-    * @param size - the size of the buffer
+    * @param size - the size of the buffer, 0 to return required size
     * @return the size of the transaction written to the buffer
     */
    size_t read_transaction(char *buffer, size_t size);

contracts/test_api/test_transaction.cpp

@@ -149,6 +149,7 @@ void test_transaction::test_read_transaction() {
    auto size = transaction_size();
    char buf[size];
    uint32_t read = read_transaction( buf, size );
+   eosio_assert( size == read, "read_transaction failed");
    sha256(buf, read, &h);
    printhex( &h, sizeof(h) );
 }

libraries/chain/wasm_interface.cpp

@@ -1131,12 +1131,16 @@ class context_free_transaction_api : public context_aware_api {
       context_free_transaction_api( apply_context& ctx )
       :context_aware_api(ctx,true){}
 
-      int read_transaction( array_ptr<char> data, size_t data_len ) {
+      int read_transaction( array_ptr<char> data, size_t buffer_size ) {
          bytes trx = context.get_packed_transaction();
-         if (data_len >= trx.size()) {
-            memcpy(data, trx.data(), trx.size());
-         }
-         return trx.size();
+
+         auto s = trx.size();
+         if( buffer_size == 0) return s;
+
+         auto copy_size = std::min( buffer_size, s );
+         memcpy( data, trx.data(), copy_size );
+
+         return copy_size;
       }
 
       int transaction_size() {