Avoid double loops to zero out padding in events.slice (#41810)

lib/events/stream.go

@@ -841,9 +841,10 @@ func (s *slice) reader() (io.ReadSeeker, error) {
 	// non last slices should be at least min upload bytes (as limited by S3 API spec)
 	if !s.isLast && wroteBytes < s.proto.cfg.MinUploadBytes {
 		paddingBytes = s.proto.cfg.MinUploadBytes - wroteBytes
-		if _, err := s.buffer.ReadFrom(utils.NewRepeatReader(byte(0), int(paddingBytes))); err != nil {
-			return nil, trace.Wrap(err)
-		}
+		s.buffer.Grow(int(paddingBytes))
+		padding := s.buffer.AvailableBuffer()[:paddingBytes]
+		clear(padding)
+		s.buffer.Write(padding)
 	}
 	data := s.buffer.Bytes()
 	// when the slice was created, the first bytes were reserved

lib/utils/utils_test.go

@@ -21,7 +21,6 @@ package utils
 import (
 	"bytes"
 	"fmt"
-	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -562,45 +561,6 @@ func TestStringsSet(t *testing.T) {
 	require.NotNil(t, out)
 }
 
-// TestRepeatReader tests repeat reader
-func TestRepeatReader(t *testing.T) {
-	t.Parallel()
-
-	type tc struct {
-		name     string
-		repeat   byte
-		count    int
-		expected string
-	}
-	tcs := []tc{
-		{
-			name:     "repeat once",
-			repeat:   byte('a'),
-			count:    1,
-			expected: "a",
-		},
-		{
-			name:     "repeat zero times",
-			repeat:   byte('a'),
-			count:    0,
-			expected: "",
-		},
-		{
-			name:     "repeat multiple times",
-			repeat:   byte('a'),
-			count:    3,
-			expected: "aaa",
-		},
-	}
-	for _, tc := range tcs {
-		t.Run(tc.name, func(t *testing.T) {
-			data, err := io.ReadAll(NewRepeatReader(tc.repeat, tc.count))
-			require.NoError(t, err)
-			require.Equal(t, tc.expected, string(data))
-		})
-	}
-}
-
 func TestReadAtMost(t *testing.T) {
 	t.Parallel()