bufio: reject UnreadByte or UnreadRune after a Discard or WriteTo

Discard is not really a read operation, and in theory it could
Seek the underlying Reader without actually reading anything,
so an UnreadByte following a Discard is disallowed.

Similarly, although WriteTo usually does end up calling Read on the
underlying buffer, if the underlying Reader implements io.WriterTo it
may instead terminate in a call to WriteTo, without ever buffering or
even seeing the last byte written. (It is conceptually read-like, but
not strictly “a read operation”.)

Fixes #48446

Change-Id: Ide6f2b157332b423486810399f66140c914144e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/351810
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Joe Tsai <joetsai@digital-static.net>

1 files changed, 34 insertions, 0 deletions

diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index 04a810c206..8e8a8a1778 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -304,6 +304,40 @@ func TestNoUnreadByteAfterPeek(t *testing.T) {
 	}
 }
 
+func TestNoUnreadRuneAfterDiscard(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadRune()
+	br.Discard(1)
+	if err := br.UnreadRune(); err == nil {
+		t.Error("UnreadRune didn't fail after Discard")
+	}
+}
+
+func TestNoUnreadByteAfterDiscard(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadByte()
+	br.Discard(1)
+	if err := br.UnreadByte(); err == nil {
+		t.Error("UnreadByte didn't fail after Discard")
+	}
+}
+
+func TestNoUnreadRuneAfterWriteTo(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.WriteTo(io.Discard)
+	if err := br.UnreadRune(); err == nil {
+		t.Error("UnreadRune didn't fail after WriteTo")
+	}
+}
+
+func TestNoUnreadByteAfterWriteTo(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.WriteTo(io.Discard)
+	if err := br.UnreadByte(); err == nil {
+		t.Error("UnreadByte didn't fail after WriteTo")
+	}
+}
+
 func TestUnreadByte(t *testing.T) {
 	segments := []string{"Hello, ", "world"}
 	r := NewReader(&StringReader{data: segments})