Possible uasyncio write improvment

[git-n-pissed]

I have some code running on an ESP8266 which was based on an old commit of MicroPython (8f55a8f) which did not contain uasyncio. I rebased my code the other day on (ac5934c) and updated it to use the included uasyncio. Unfortunately I found that picoweb kept crashing due to a memory error on the last line of usayncio.stream.write. The current implementation:

    def write(self, buf):
        if not self.out_buf:
            # Try to write immediately to the underlying stream.
            ret = self.s.write(buf)
            if ret == len(buf):
                return
            if ret is not None:
                buf = buf[ret:]
        self.out_buf += buf

The problem with this is that the =+ causes a copy of buf to be made even if self.out_buf is already empty. This makes sense in cases where a developer desires to call write multiple times before drain, but in cases where drain is immediately called after write, the RAM used by the actual data in buf ends up being double what it needs to be. This is especially evident when awrite / stream_awrite is used, because it does call drain immediately after calling write. The following change fixed my problem and I believe prevents the described issue.

    def write(self, buf):
        if not self.out_buf:
            # Try to write immediately to the underlying stream.
            ret = self.s.write(buf)
            if ret == len(buf):
                return
            if ret is not None:
                buf = buf[ret:]

        if not self.out_buf:
            self.out_buf = buf
        else:
            self.out_buf += buf

[miketeachman]

There is an outstanding PR #7868 for this issue.
Perhaps you could add a supportive comment to this existing PR in the hope that it gets some attention.