Go version: tip.

The documentation for Read says

It calls Read at most once on the underlying Reader, hence n may be less than len(p).

but Read calls fill, and fill calls the underlying reader up to maxConsecutiveEmptyReads (100) times.

I realize the point of this is actually to say that n may be less than len(p), because Read stops after it gets one block of data (rather than continuing to read more data to fill p, as one might assume). The "no progress" case is unusual, and it generally doesn't matter to the caller, but if the user is supplying the underlying reader, this statement might be misleading.