io: prevent seeking to position prior to offsetwrite.base

We don't want to permit writing before the start of an OffsetWriter.

One of the goals of OffsetWriter is to restrict where data
can be written.

However, this rule can be violated by WriteAt() method of OffsetWriter
as the following code shows:

f, _ := os.Create("file.txt")
owr := io.NewOffsetWriter(f, 10)
owr.Write([]byte("world"))
owr.WriteAt([]byte("hello"), -10)

Change-Id: I6c7519fea68daefa641f25130cdd9803dc8aae22
GitHub-Last-Rev: a29d890d6f32fd5a1ecef84d012b8447b406e2e2
GitHub-Pull-Request: golang/go#60222
Reviewed-on: https://go-review.googlesource.com/c/go/+/495155
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Jabar Asadi <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

src/io/io.go

@@ -575,6 +575,10 @@ func (o *OffsetWriter) Write(p []byte) (n int, err error) {
 }
 
 func (o *OffsetWriter) WriteAt(p []byte, off int64) (n int, err error) {
+ if off < 0 {
+ return 0, errOffset
+ }
+
  off += o.base
  return o.w.WriteAt(p, off)
 }

src/io/io_test.go

@@ -608,6 +608,26 @@ func TestOffsetWriter_WriteAt(t *testing.T) {
  }
 }
 
+func TestWriteAt_PositionPriorToBase(t *testing.T) {
+ tmpdir := t.TempDir()
+ tmpfilename := "TestOffsetWriter_WriteAt"
+ tmpfile, err := os.CreateTemp(tmpdir, tmpfilename)
+ if err != nil {
+ t.Fatalf("CreateTemp(%s) failed: %v", tmpfilename, err)
+ }
+ defer tmpfile.Close()
+
+ // start writing position in OffsetWriter
+ offset := int64(10)
+ // position we want to write to the tmpfile
+ at := int64(-1)
+ w := NewOffsetWriter(tmpfile, offset)
+ _, e := w.WriteAt([]byte("hello"), at)
+ if e == nil {
+ t.Errorf("error expected to be not nil")
+ }
+}
+
 func TestOffsetWriter_Write(t *testing.T) {
  const content = "0123456789ABCDEF"
  contentSize := len(content)