fix: panic with kubectl when timestamps are enabled (#7947)

Fixes a bug where using the `--timestamps` flag with `kubectl` deploy
panics with:

> error:panic: runtime error: slice bounds out of range

With timestamps enabled, `skaffoldWriter.Write` includes the length of
the timestamp in its return value. But `Writer` implementations should
return the number of bytes _consumed_, not written. This results in
`textio.PrefixWriter`[^1] writing `N` bytes to its buffer and then
discarding `N+len(timestamp)` bytes, causing a panic.

[^1]: Used by `pkg/skaffold/deploy/kubectl/kubectl.go`

Relevant `textio.PrefixWriter` source code (ref:
https://github.com/segmentio/textio/blob/master/prefix.go#L54-L55):

```go
    c, err = w.writeLine(chunk)
    w.discard(c)
```

Fixes: #7947

Author: kdeloachbayer

pkg/skaffold/output/output.go

@@ -38,14 +38,13 @@ type skaffoldWriter struct {
 }
 
 func (s skaffoldWriter) Write(p []byte) (int, error) {
-	written := 0
+	if len(p) == 0 {
+		return 0, nil
+	}
 	if s.timestamps {
-		t, err := s.MainWriter.Write([]byte(time.Now().Format(timestampFormat) + " "))
-		if err != nil {
-			return t, err
+		if _, err := io.WriteString(s.MainWriter, time.Now().Format(timestampFormat+" ")); err != nil {
+			return 0, err
 		}
-
-		written += t
 	}
 
 	n, err := s.MainWriter.Write(p)
@@ -56,11 +55,9 @@ func (s skaffoldWriter) Write(p []byte) (int, error) {
 		return n, io.ErrShortWrite
 	}
 
-	written += n
-
 	s.EventWriter.Write(p)
 
-	return written, nil
+	return n, nil
 }
 
 func GetWriter(ctx context.Context, out io.Writer, defaultColor int, forceColors bool, timestamps bool) io.Writer {

pkg/skaffold/output/output_test.go

@@ -163,11 +163,17 @@ func TestWriteWithTimeStamps(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
+		testutil.Run(t, test.name, func(t *testutil.T) {
 			var buf bytes.Buffer
 			out := test.writer(&buf)
 			Default.Fprintf(out, "testing!")
-			testutil.CheckDeepEqual(t, test.expectedLen, len(buf.String()))
+			t.CheckDeepEqual(test.expectedLen, len(buf.String()))
+
+			// Consume same number of bytes regardless of writer options.
+			out2 := test.writer(io.Discard)
+			n, err := out2.Write([]byte("testing!"))
+			t.CheckNoError(err)
+			t.CheckDeepEqual(len("testing!"), n)
 		})
 	}
 }