Merge pull request #58817 from karlhungus/bugfix_yaml_decoder_short_buf

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add test/fix for ErrShortBuffer edgecase

**What this PR does / why we need it**:

Found a bug with YAMLToJSONDecoder where subsequent reads after `io.ErrShortBuffer` would return values from the next yaml section, rather than the rest of the section I was reading.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #59055 

https://github.com/kubernetes/kubernetes/issues/59055

**Special notes for your reviewer**:

**Release note**:

```release-note
YAMLDecoder Read now tracks rest of buffer on io.ErrShortBuffer
```

staging/src/k8s.io/apimachinery/pkg/util/yaml/decoder.go

@@ -126,8 +126,8 @@ func (d *YAMLDecoder) Read(data []byte) (n int, err error) {
 	}
 
 	// caller will need to reread
-	copy(data, d.remaining[:left])
+	copy(data, d.remaining[:len(data)])
-	d.remaining = d.remaining[left:]
+	d.remaining = d.remaining[len(data):]
 	return len(data), io.ErrShortBuffer
 }
 

staging/src/k8s.io/apimachinery/pkg/util/yaml/decoder_test.go

@@ -54,6 +54,36 @@ stuff: 1
 	}
 }
 
+func TestYAMLDecoderCallsAfterErrShortBufferRestOfFrame(t *testing.T) {
+	d := `---
+stuff: 1
+	test-foo: 1`
+	r := NewDocumentDecoder(ioutil.NopCloser(bytes.NewReader([]byte(d))))
+	b := make([]byte, 12)
+	n, err := r.Read(b)
+	if err != io.ErrShortBuffer || n != 12 {
+		t.Fatalf("expected ErrShortBuffer: %d / %v", n, err)
+	}
+	expected := "---\nstuff: 1"
+	if string(b) != expected {
+		t.Fatalf("expected bytes read to be: %s  got: %s", expected, string(b))
+	}
+	b = make([]byte, 13)
+	n, err = r.Read(b)
+	if err != nil || n != 13 {
+		t.Fatalf("expected nil: %d / %v", n, err)
+	}
+	expected = "\n\ttest-foo: 1"
+	if string(b) != expected {
+		t.Fatalf("expected bytes read to be: '%s'  got: '%s'", expected, string(b))
+	}
+	b = make([]byte, 15)
+	n, err = r.Read(b)
+	if err != io.EOF || n != 0 {
+		t.Fatalf("expected EOF: %d / %v", n, err)
+	}
+}
+
 func TestSplitYAMLDocument(t *testing.T) {
 	testCases := []struct {
 		input  string