Detect unsafe pgtype.DriverBytes usage

Add test for unsafe usage and test for correct usage that ensures driver
memory is actually used.

pgtype/bytea_test.go

@@ -36,7 +36,7 @@ func TestByteaCodec(t *testing.T) {
 	})
 }
 
-func TestDriverBytes(t *testing.T) {
+func TestDriverBytesQueryRow(t *testing.T) {
 	conn := testutil.MustConnectPgx(t)
 	defer testutil.MustCloseContext(t, conn)
 
@@ -44,14 +44,47 @@ func TestDriverBytes(t *testing.T) {
 
 	var buf []byte
 	err := conn.QueryRow(ctx, `select $1::bytea`, []byte{1, 2}).Scan((*pgtype.DriverBytes)(&buf))
+	require.EqualError(t, err, "cannot scan into *pgtype.DriverBytes from QueryRow")
+}
+
+func TestDriverBytes(t *testing.T) {
+	conn := testutil.MustConnectPgx(t)
+	defer testutil.MustCloseContext(t, conn)
+
+	ctx := context.Background()
+
+	argBuf := make([]byte, 128)
+	for i := range argBuf {
+		argBuf[i] = byte(i)
+	}
+
+	rows, err := conn.Query(ctx, `select $1::bytea from generate_series(1, 1000)`, argBuf)
+	require.NoError(t, err)
+	defer rows.Close()
+
+	rowCount := 0
+	resultBuf := argBuf
+	detectedResultMutation := false
+	for rows.Next() {
+		rowCount++
+
+		// At some point the buffer should be reused and change.
+		if bytes.Compare(argBuf, resultBuf) != 0 {
+			detectedResultMutation = true
+		}
+
+		err = rows.Scan((*pgtype.DriverBytes)(&resultBuf))
 		require.NoError(t, err)
 
-	require.Len(t, buf, 2)
+		require.Len(t, resultBuf, len(argBuf))
-	require.Equal(t, buf, []byte{1, 2})
+		require.Equal(t, resultBuf, argBuf)
-	require.Equalf(t, cap(buf), len(buf), "cap(buf) is larger than len(buf)")
+		require.Equalf(t, cap(resultBuf), len(resultBuf), "cap(resultBuf) is larger than len(resultBuf)")
+	}
 
-	// Don't actually have any way to be sure that the bytes are from the driver at the moment as underlying driver
+	require.True(t, detectedResultMutation)
-	// doesn't reuse buffers at the present.
+
+	err = rows.Err()
+	require.NoError(t, err)
 }
 
 func TestPreallocBytes(t *testing.T) {

rows.go

@@ -79,6 +79,13 @@ func (r *connRow) Scan(dest ...interface{}) (err error) {
 		return rows.Err()
 	}
 
+	for _, d := range dest {
+		if _, ok := d.(*pgtype.DriverBytes); ok {
+			rows.Close()
+			return fmt.Errorf("cannot scan into *pgtype.DriverBytes from QueryRow")
+		}
+	}
+
 	if !rows.Next() {
 		if rows.Err() == nil {
 			return ErrNoRows