1 files changed, 57 insertions, 1 deletions

diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index ebcc711db9..04a810c206 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -10,6 +10,8 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math/rand"
+	"strconv"
 	"strings"
 	"testing"
 	"testing/iotest"
@@ -608,6 +610,37 @@ func TestWriter(t *testing.T) {
 	}
 }
 
+func TestWriterAppend(t *testing.T) {
+	got := new(bytes.Buffer)
+	var want []byte
+	rn := rand.New(rand.NewSource(0))
+	w := NewWriterSize(got, 64)
+	for i := 0; i < 100; i++ {
+		// Obtain a buffer to append to.
+		b := w.AvailableBuffer()
+		if w.Available() != cap(b) {
+			t.Fatalf("Available() = %v, want %v", w.Available(), cap(b))
+		}
+
+		// While not recommended, it is valid to append to a shifted buffer.
+		// This forces Write to copy the the input.
+		if rn.Intn(8) == 0 && cap(b) > 0 {
+			b = b[1:1:cap(b)]
+		}
+
+		// Append a random integer of varying width.
+		n := int64(rn.Intn(1 << rn.Intn(30)))
+		want = append(strconv.AppendInt(want, n, 10), ' ')
+		b = append(strconv.AppendInt(b, n, 10), ' ')
+		w.Write(b)
+	}
+	w.Flush()
+
+	if !bytes.Equal(got.Bytes(), want) {
+		t.Errorf("output mismatch:\ngot  %s\nwant %s", got.Bytes(), want)
+	}
+}
+
 // Check that write errors are returned properly.
 
 type errorWriterTest struct {
@@ -1312,6 +1345,7 @@ func TestReaderReset(t *testing.T) {
 	if string(buf) != "foo" {
 		t.Errorf("buf = %q; want foo", buf)
 	}
+
 	r.Reset(strings.NewReader("bar bar"))
 	all, err := io.ReadAll(r)
 	if err != nil {
@@ -1320,12 +1354,23 @@ func TestReaderReset(t *testing.T) {
 	if string(all) != "bar bar" {
 		t.Errorf("ReadAll = %q; want bar bar", all)
 	}
+
+	*r = Reader{} // zero out the Reader
+	r.Reset(strings.NewReader("bar bar"))
+	all, err = io.ReadAll(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(all) != "bar bar" {
+		t.Errorf("ReadAll = %q; want bar bar", all)
+	}
 }
 
 func TestWriterReset(t *testing.T) {
-	var buf1, buf2 bytes.Buffer
+	var buf1, buf2, buf3 bytes.Buffer
 	w := NewWriter(&buf1)
 	w.WriteString("foo")
+
 	w.Reset(&buf2) // and not flushed
 	w.WriteString("bar")
 	w.Flush()
@@ -1335,6 +1380,17 @@ func TestWriterReset(t *testing.T) {
 	if buf2.String() != "bar" {
 		t.Errorf("buf2 = %q; want bar", buf2.String())
 	}
+
+	*w = Writer{}  // zero out the Writer
+	w.Reset(&buf3) // and not flushed
+	w.WriteString("bar")
+	w.Flush()
+	if buf1.String() != "" {
+		t.Errorf("buf1 = %q; want empty", buf1.String())
+	}
+	if buf3.String() != "bar" {
+		t.Errorf("buf3 = %q; want bar", buf3.String())
+	}
 }
 
 func TestReaderDiscard(t *testing.T) {