clean up the receive window check

[marten-seemann]

The receive window was performed in TryReserve:

go-yamux/util.go

Lines 125 to 133 in fad228e

	func (s *segmentedBuffer) TryReserve(space uint32) bool {
	s.bm.Lock()
	defer s.bm.Unlock()
	if s.cap < s.pending+space {
	return false
	}
	s.pending += space
	return true
	}

Note the uint32 overflow.

The only reason this overflow can't be exploited is that it's not possible that pending is unequal 0 after calling Append (unless Append errors, in which case we close the connection anyway). This is the only way we use those two functions:

go-yamux/stream.go

Lines 425 to 435 in fad228e

	// Validate it's okay to copy
	if !s.recvBuf.TryReserve(length) {
	s.session.logger.Printf("[ERR] yamux: receive window exceeded (stream: %d, remain: %d, recv: %d)", s.id, s.recvBuf.Cap(), length)
	return ErrRecvWindowExceeded
	}
	// Copy into buffer
	if err := s.recvBuf.Append(conn, int(length)); err != nil {
	s.session.logger.Printf("[ERR] yamux: Failed to read stream data: %v", err)
	return err
	}

That means that the pending member variable is not actually used for anything useful, and can be removed by merging TryReserve and Append. This is what this PR does.

[Stebalien]

👍