[VTQueryServer] MySQLProxy drops transaction on error

It's totally valid to have an error mid-transaction and then continue to
act on the transaction. Unfortunately, the vtqueryserver mysql plugin
handler drops the transaction ID (vis a vis ProxySession) on error.

Signed-off-by: Daniel Tahara <tahara@dropbox.com>

go/vt/mysqlproxy/mysqlproxy.go

@@ -77,11 +77,10 @@ func (mp *Proxy) Execute(ctx context.Context, session *ProxySession, sql string,
 		result, err = mp.executeOther(ctx, session, sql, bindVariables)
 	}
 
-	if err != nil {
-		return nil, nil, err
-	}
-
-	return session, result, nil
+	// N.B. You must return session, even on error. Modeled after vtgate mysql plugin, the
+	// vtqueryserver plugin expects you to return a new or updated session and not drop it on the
+	// floor during an error.
+	return session, result, err
 }
 
 // Rollback rolls back the session

go/vt/vtqueryserver/endtoend_test.go

@@ -367,6 +367,40 @@ func TestTransactionsInProcess(t *testing.T) {
 
 }
 
+func TestLostTransaction(t *testing.T) {
+	ctx := context.Background()
+	conn, err := mysql.Connect(ctx, &proxyConnParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conn2, err := mysql.Connect(ctx, &proxyConnParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testDML(t, conn, "begin", 1, 0)
+	testDML(t, conn, "insert into test (id, val) values(1, 'hello')", 1, 1)
+	_, err = conn.ExecuteFetch("select this is garbage", 1, false)
+	if err == nil {
+		t.Log("Expected error for garbage sql request")
+		t.FailNow()
+	}
+	// First connection should still see its data.
+	testFetch(t, conn, "select * from test", 1)
+	// But second won't, since it's uncommitted.
+	testFetch(t, conn2, "select * from test", 0)
+
+	// Commit works still.
+	testDML(t, conn, "commit", 1, 0)
+	testFetch(t, conn, "select * from test", 1)
+	testFetch(t, conn2, "select * from test", 1)
+
+	// Cleanup
+	testDML(t, conn2, "begin", 1, 0)
+	testDML(t, conn2, "delete from test", 2, 3)
+	testDML(t, conn2, "commit", 1, 0)
+}
+
 func TestOther(t *testing.T) {
 	ctx := context.Background()
 	conn, err := mysql.Connect(ctx, &proxyConnParams)