proposal: bytes: guarantee a useful zero value of Reader

[FiloSottile]

A zero bytes.Reader is almost functional (like a bytes.NewReader(nil)), except for the prevRune offset which is only used by UnreadRune. It would be nice to make the zero value useful and document it as such. For example we could make prevRune 1-indexed and have 0 be the sentinel previous operation was not ReadRune value. It already suspiciously truncates an int64 index into an int without checking anyway (which might be a bug AFAICT).

I found myself wishing for this in https://go-review.googlesource.com/c/go/+/142818 where I had a bytes.Reader field that would meaningfully sit empty at times, including at object creation.

[bradfitz]

To be clear, you mean a bytes.Reader value and not a nil *bytes.Reader pointer, right? (Because it's the pointer type that implements io.Reader, etc.)

So you mean that a (perhaps implicit) pointer to a zero-value bytes.Reader should act like a bytes.NewReader(nil)?

If so, that might be okay (if we also do strings.Reader), but I would object to making a nil pointer act like an always-EOF reader.

[FiloSottile]

Yes, correct, a zero bytes.Reader value, not a nil pointer. The idea is to avoid initialization of a bytes.Reader field, like bytes.Buffer and sync.Mutex allow.

The zero value for Buffer is an empty buffer ready to use.

The zero value for a Mutex is an unlocked mutex.

Looks like strings.Reader can be adapted just as easily.

[rsc]

We should definitely make new(bytes.Reader) and bytes.NewReader(nil) behave identically. There's no need to redefine prevRune necessarily - there's enough other info in the struct to understand that there is no previous (i==0 for example). It looks like UnreadRune just needs the same 3-line if statement as in UnreadByte added to it (not replacing the existing if statement, which is something else).

Marking accepted from an API point of view.

[gopherbot]

Change https://golang.org/cl/145098 mentions this issue: bytes: fix Reader.UnreadRune returning without error on a zero Reader

[gopherbot]

Change https://golang.org/cl/145737 mentions this issue: crypto/tls: remove unneeded calls to bytes.NewReader