bytes: add Buffer.Peek

Fixes golang#73794

Author: icholy

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