Exec(Params|Prepared) return ResultReader with FieldDescriptions loaded

Previously, it wasn't loaded until NextRow was called the first time.

pgconn.go

@@ -84,6 +84,8 @@ type PgConn struct {
 	bufferingReceiveMsg pgproto3.BackendMessage
 	bufferingReceiveErr error
 
+	peekedMsg pgproto3.BackendMessage
+
 	// Reusable / preallocated resources
 	wbuf              []byte // write buffer
 	resultReader      ResultReader
@@ -427,8 +429,12 @@ func (pgConn *PgConn) ReceiveMessage(ctx context.Context) (pgproto3.BackendMessa
 	return msg, err
 }
 
-// receiveMessage receives a message without setting up context cancellation
-func (pgConn *PgConn) receiveMessage() (pgproto3.BackendMessage, error) {
+// peekMessage peeks at the next message without setting up context cancellation.
+func (pgConn *PgConn) peekMessage() (pgproto3.BackendMessage, error) {
+	if pgConn.peekedMsg != nil {
+		return pgConn.peekedMsg, nil
+	}
+
 	var msg pgproto3.BackendMessage
 	var err error
 	if pgConn.bufferingReceive {
@@ -455,6 +461,23 @@ func (pgConn *PgConn) receiveMessage() (pgproto3.BackendMessage, error) {
 		return nil, err
 	}
 
+	pgConn.peekedMsg = msg
+	return msg, nil
+}
+
+// receiveMessage receives a message without setting up context cancellation
+func (pgConn *PgConn) receiveMessage() (pgproto3.BackendMessage, error) {
+	msg, err := pgConn.peekMessage()
+	if err != nil {
+		// Close on anything other than timeout error - everything else is fatal
+		if err, ok := err.(net.Error); !(ok && err.Timeout()) {
+			pgConn.asyncClose()
+		}
+
+		return nil, err
+	}
+	pgConn.peekedMsg = nil
+
 	switch msg := msg.(type) {
 	case *pgproto3.ReadyForQuery:
 		pgConn.txStatus = msg.TxStatus
@@ -1044,7 +1067,10 @@ func (pgConn *PgConn) execExtendedSuffix(buf []byte, result *ResultReader) {
 		pgConn.contextWatcher.Unwatch()
 		result.closed = true
 		pgConn.unlock()
+		return
 	}
+
+	result.readUntilRowDescription()
 }
 
 // CopyTo executes the copy command sql and copies the results to w.
@@ -1454,6 +1480,26 @@ func (rr *ResultReader) Close() (CommandTag, error) {
 	return rr.commandTag, rr.err
 }
 
+// readUntilRowDescription ensures the ResultReader's fieldDescriptions are loaded. It does not return an error as any
+// error will be stored in the ResultReader.
+func (rr *ResultReader) readUntilRowDescription() {
+	for !rr.commandConcluded {
+		// Peek before receive to avoid consuming a DataRow if the result set does not include a RowDescription method.
+		// This should never happen under normal pgconn usage, but it is possible if SendBytes and ReceiveResults are
+		// manually used to construct a query that does not issue a describe statement.
+		msg, _ := rr.pgConn.peekMessage()
+		if _, ok := msg.(*pgproto3.DataRow); ok {
+			return
+		}
+
+		// Consume the message
+		msg, _ = rr.receiveMessage()
+		if _, ok := msg.(*pgproto3.RowDescription); ok {
+			return
+		}
+	}
+}
+
 func (rr *ResultReader) receiveMessage() (msg pgproto3.BackendMessage, err error) {
 	if rr.multiResultReader == nil {
 		msg, err = rr.pgConn.receiveMessage()

pgconn_test.go

@@ -481,6 +481,34 @@ func TestConnExecMultipleQueries(t *testing.T) {
 	ensureConnValid(t, pgConn)
 }
 
+func TestConnExecMultipleQueriesEagerFieldDescriptions(t *testing.T) {
+	t.Parallel()
+
+	pgConn, err := pgconn.Connect(context.Background(), os.Getenv("PGX_TEST_CONN_STRING"))
+	require.NoError(t, err)
+	defer closeConn(t, pgConn)
+
+	mrr := pgConn.Exec(context.Background(), "select 'Hello, world' as msg; select 1 as num")
+
+	require.True(t, mrr.NextResult())
+	require.Len(t, mrr.ResultReader().FieldDescriptions(), 1)
+	assert.Equal(t, []byte("msg"), mrr.ResultReader().FieldDescriptions()[0].Name)
+	_, err = mrr.ResultReader().Close()
+	require.NoError(t, err)
+
+	require.True(t, mrr.NextResult())
+	require.Len(t, mrr.ResultReader().FieldDescriptions(), 1)
+	assert.Equal(t, []byte("num"), mrr.ResultReader().FieldDescriptions()[0].Name)
+	_, err = mrr.ResultReader().Close()
+	require.NoError(t, err)
+
+	require.False(t, mrr.NextResult())
+
+	require.NoError(t, mrr.Close())
+
+	ensureConnValid(t, pgConn)
+}
+
 func TestConnExecMultipleQueriesError(t *testing.T) {
 	t.Parallel()
 
@@ -578,7 +606,10 @@ func TestConnExecParams(t *testing.T) {
 	require.NoError(t, err)
 	defer closeConn(t, pgConn)
 
-	result := pgConn.ExecParams(context.Background(), "select $1::text", [][]byte{[]byte("Hello, world")}, nil, nil, nil)
+	result := pgConn.ExecParams(context.Background(), "select $1::text as msg", [][]byte{[]byte("Hello, world")}, nil, nil, nil)
+	require.Len(t, result.FieldDescriptions(), 1)
+	assert.Equal(t, []byte("msg"), result.FieldDescriptions()[0].Name)
+
 	rowCount := 0
 	for result.NextRow() {
 		rowCount += 1
@@ -734,13 +765,16 @@ func TestConnExecPrepared(t *testing.T) {
 	require.NoError(t, err)
 	defer closeConn(t, pgConn)
 
-	psd, err := pgConn.Prepare(context.Background(), "ps1", "select $1::text", nil)
+	psd, err := pgConn.Prepare(context.Background(), "ps1", "select $1::text as msg", nil)
 	require.NoError(t, err)
 	require.NotNil(t, psd)
 	assert.Len(t, psd.ParamOIDs, 1)
 	assert.Len(t, psd.Fields, 1)
 
 	result := pgConn.ExecPrepared(context.Background(), "ps1", [][]byte{[]byte("Hello, world")}, nil, nil)
+	require.Len(t, result.FieldDescriptions(), 1)
+	assert.Equal(t, []byte("msg"), result.FieldDescriptions()[0].Name)
+
 	rowCount := 0
 	for result.NextRow() {
 		rowCount += 1