Fix fastpath / largeobjects query counting

fastpath.Call was not incrementing pendingReadyForQueryCount when it
sent a function call. But it was being decremented when the function
call was finished and the server sent the ready for query message. This
caused pendingReadyForQueryCount to go negative. This meant that any
subsequent activity that depended on ensureConnectionReadyForQuery would
not operate correctly because the connection would be considered ready
before it had read all previous data off the wire.

fixes #403

fastpath.go

@@ -76,6 +76,8 @@ func (f *fastpath) Call(oid pgtype.OID, args []fpArg) (res []byte, err error) {
 		return nil, err
 	}
 
+	f.cn.pendingReadyForQueryCount++
+
 	for {
 		msg, err := f.cn.rxMsg()
 		if err != nil {

large_objects_test.go

@@ -119,3 +119,147 @@ func TestLargeObjects(t *testing.T) {
 		t.Errorf("Expected undefined_object error (42704), got %#v", err)
 	}
 }
+
+func TestLargeObjectsMultipleTransactions(t *testing.T) {
+	t.Parallel()
+
+	conn, err := pgx.Connect(*defaultConnConfig)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tx, err := conn.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	lo, err := tx.LargeObjects()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	id, err := lo.Create(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	obj, err := lo.Open(id, pgx.LargeObjectModeWrite)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n, err := obj.Write([]byte("testing"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 7 {
+		t.Errorf("Expected n to be 7, got %d", n)
+	}
+
+	// Commit the first transaction
+	err = tx.Commit()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// IMPORTANT: Use the same connection for another query
+	query := `select n from generate_series(1,10) n`
+	rows, err := conn.Query(query)
+	if err != nil {
+		t.Fatal(err)
+	}
+	rows.Close()
+
+	// Start a new transaction
+	tx2, err := conn.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	lo2, err := tx2.LargeObjects()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Reopen the large object in the new transaction
+	obj2, err := lo2.Open(id, pgx.LargeObjectModeRead|pgx.LargeObjectModeWrite)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	pos, err := obj2.Seek(1, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pos != 1 {
+		t.Errorf("Expected pos to be 1, got %d", pos)
+	}
+
+	res := make([]byte, 6)
+	n, err = obj2.Read(res)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(res) != "esting" {
+		t.Errorf(`Expected res to be "esting", got %q`, res)
+	}
+	if n != 6 {
+		t.Errorf("Expected n to be 6, got %d", n)
+	}
+
+	n, err = obj2.Read(res)
+	if err != io.EOF {
+		t.Error("Expected io.EOF, go nil")
+	}
+	if n != 0 {
+		t.Errorf("Expected n to be 0, got %d", n)
+	}
+
+	pos, err = obj2.Tell()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pos != 7 {
+		t.Errorf("Expected pos to be 7, got %d", pos)
+	}
+
+	err = obj2.Truncate(1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	pos, err = obj2.Seek(-1, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pos != 0 {
+		t.Errorf("Expected pos to be 0, got %d", pos)
+	}
+
+	res = make([]byte, 2)
+	n, err = obj2.Read(res)
+	if err != io.EOF {
+		t.Errorf("Expected err to be io.EOF, got %v", err)
+	}
+	if n != 1 {
+		t.Errorf("Expected n to be 1, got %d", n)
+	}
+	if res[0] != 't' {
+		t.Errorf("Expected res[0] to be 't', got %v", res[0])
+	}
+
+	err = obj2.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = lo2.Unlink(id)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = lo2.Open(id, pgx.LargeObjectModeRead)
+	if e, ok := err.(pgx.PgError); !ok || e.Code != "42704" {
+		t.Errorf("Expected undefined_object error (42704), got %#v", err)
+	}
+}