bytes: add Buffer.Peek

Fixes #73794

Change-Id: I0a57db05aacfa805213fe8278fc727e76eb8a65e
GitHub-Last-Rev: 3494d93f80
GitHub-Pull-Request: golang/go#73795
Reviewed-on: https://go-review.googlesource.com/c/go/+/674415
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

api/next/73794.txt

@@ -0,0 +1 @@
+pkg bytes, method (*Buffer) Peek(int) ([]uint8, error) #73794

doc/next/6-stdlib/99-minor/bytes/73794.md

@@ -0,0 +1,2 @@
+The new [Buffer.Peek] method returns the next n bytes from the buffer without
+advancing it.

src/bytes/buffer.go

@@ -77,6 +77,18 @@ func (b *Buffer) String() string {
 	return string(b.buf[b.off:])
 }
 
+// Peek returns the next n bytes without advancing the buffer.
+// If Peek returns fewer than n bytes, it also returns [io.EOF].
+// The slice is only valid until the next call to a read or write method.
+// The slice aliases the buffer content at least until the next buffer modification,
+// so immediate changes to the slice will affect the result of future reads.
+func (b *Buffer) Peek(n int) ([]byte, error) {
+	if b.Len() < n {
+		return b.buf[b.off:], io.EOF
+	}
+	return b.buf[b.off:n], nil
+}
+
 // empty reports whether the unread portion of the buffer is empty.
 func (b *Buffer) empty() bool { return len(b.buf) <= b.off }
 

src/bytes/buffer_test.go

@@ -531,6 +531,34 @@ func TestReadString(t *testing.T) {
 	}
 }
 
+var peekTests = []struct {
+	buffer   string
+	n        int
+	expected string
+	err      error
+}{
+	{"", 0, "", nil},
+	{"aaa", 3, "aaa", nil},
+	{"foobar", 2, "fo", nil},
+	{"a", 2, "a", io.EOF},
+}
+
+func TestPeek(t *testing.T) {
+	for _, test := range peekTests {
+		buf := NewBufferString(test.buffer)
+		bytes, err := buf.Peek(test.n)
+		if string(bytes) != test.expected {
+			t.Errorf("expected %q, got %q", test.expected, bytes)
+		}
+		if err != test.err {
+			t.Errorf("expected error %v, got %v", test.err, err)
+		}
+		if buf.Len() != len(test.buffer) {
+			t.Errorf("bad length after peek: %d, want %d", buf.Len(), len(test.buffer))
+		}
+	}
+}
+
 func BenchmarkReadString(b *testing.B) {
 	const n = 32 << 10