Send Describe in Batch.ExecStatement when cached fields are empty

This applies the same fix to Batch.ExecStatement that was previously
applied to Pipeline.SendQueryStatement and PgConn.ExecStatement. Empty
cached fields may mean the results were not knowable at prepare time,
e.g. a FETCH from a cursor that did not exist yet, so a Describe is sent
and the server-provided row description is used.

#2626

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jackc

pgconn/pgconn.go

@@ -2022,7 +2022,9 @@ func (batch *Batch) ExecPrepared(stmtName string, paramValues [][]byte, paramFor
 //
 // This differs from ExecPrepared in that it takes a *StatementDescription instead of just the prepared statement name.
 // Because it has the *StatementDescription it can avoid the Describe Portal message that ExecPrepared must send to get
-// the result column descriptions.
+// the result column descriptions. However, if the statement description has no fields then a Describe is still sent, as
+// an empty Fields may mean the results were not knowable at prepare time, e.g. a FETCH from a cursor that did not exist
+// yet.
 func (batch *Batch) ExecStatement(statementDescription *StatementDescription, paramValues [][]byte, paramFormats, resultFormats []int16) {
 	if batch.err != nil {
 		return
@@ -2033,6 +2035,16 @@ func (batch *Batch) ExecStatement(statementDescription *StatementDescription, pa
 		return
 	}
 
+	if len(statementDescription.Fields) == 0 {
+		// The cached field descriptions are empty, which can occur when the statement's result set was not known at
+		// prepare time, e.g. a FETCH from a cursor that did not exist yet. Send a Describe so the server supplies the
+		// actual row description when the statement is executed.
+		batch.buf, batch.err = (&pgproto3.Describe{ObjectType: 'P'}).Encode(batch.buf)
+		if batch.err != nil {
+			return
+		}
+	}
+
 	batch.statementDescriptions = append(batch.statementDescriptions, statementDescription)
 	batch.resultFormats = append(batch.resultFormats, resultFormats)
 

pgconn/pgconn_test.go

@@ -1977,6 +1977,54 @@ func TestConnExecBatchStatementNoRows(t *testing.T) {
 	ensureConnValid(t, pgConn)
 }
 
+// https://github.com/jackc/pgx/issues/2626
+func TestConnExecBatchStatementCursorFetch(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
+	defer cancel()
+
+	pgConn, err := pgconn.Connect(ctx, os.Getenv("PGX_TEST_DATABASE"))
+	require.NoError(t, err)
+	defer closeConn(t, pgConn)
+
+	if pgConn.ParameterStatus("crdb_version") != "" {
+		t.Skip("Server does not support cursors in implicit transactions")
+	}
+
+	// Prepare the FETCH before the cursor exists. The server describes the result as NoData so the statement
+	// description has no fields. The actual fields are only known at execution time.
+	sd, err := pgConn.Prepare(ctx, "ps_batch_fetch", `fetch all in "exec_batch_cursor"`, nil)
+	require.NoError(t, err)
+	require.Empty(t, sd.Fields)
+
+	// DECLARE CURSOR requires an explicit transaction block.
+	_, err = pgConn.Exec(ctx, "begin").ReadAll()
+	require.NoError(t, err)
+
+	batch := &pgconn.Batch{}
+	batch.ExecParams(`declare "exec_batch_cursor" cursor for select n, n::text from generate_series(1, 3) n`, nil, nil, nil, nil)
+	batch.ExecStatement(sd, nil, nil, nil)
+
+	results, err := pgConn.ExecBatch(ctx, batch).ReadAll()
+	require.NoError(t, err)
+	require.Len(t, results, 2)
+
+	fetchResult := results[1]
+	require.NoError(t, fetchResult.Err)
+	require.Len(t, fetchResult.FieldDescriptions, 2)
+	require.Equal(t, uint32(pgtype.Int4OID), fetchResult.FieldDescriptions[0].DataTypeOID)
+	require.Equal(t, uint32(pgtype.TextOID), fetchResult.FieldDescriptions[1].DataTypeOID)
+	require.Len(t, fetchResult.Rows, 3)
+	require.Equal(t, "1", string(fetchResult.Rows[0][0]))
+	require.Equal(t, "3", string(fetchResult.Rows[2][1]))
+
+	_, err = pgConn.Exec(ctx, "rollback").ReadAll()
+	require.NoError(t, err)
+
+	ensureConnValid(t, pgConn)
+}
+
 type mockConnection struct {
 	net.Conn
 	writeLatency *time.Duration