Allow to cast filtered streams to PHP_STREAM_AS_FD_FOR_SELECT

[cmb69]

Just a PoC to check for obvious issues.

[nikic]

With this change a) is it possible for select to report a readable stream, but reading it will read zero bytes (because filter is waiting for data)? b) if yes, does that violate the select contract (does it allow "spurious wake"?)

[cmb69]

Oh, that can happen indeed. So we cannot cast filtered streams to PHP_STREAM_AS_FD_FOR_SELECT.

[nikic]

Well, that depends on what the answer to the second question is ^^ Maybe it's okay...

[cmb69]

if yes, does that violate the select contract (does it allow "spurious wake"?)

I don't know, and failed to find any relevant info about that. We're usually (in practise always?) calling recv(2) with MSG_DONTWAIT, so that may not cause spurious wakes, but we also have a fallback definition (https://github.com/php/php-src/blob/php-7.4.18/main/streams/xp_socket.c#L32-L34), so …

[bukka]

I have been looking into this and think the mentioned "spurious wake" can already happen for TLS if there is less than the TLS record size read. I might need to verify it but don't think it's an issue. The trickier part is to find out if anything is buffered in the filters and "wake" should happen even though nothing ready to read from fd. I have a feeling that solution for that would be probably for master only but will check that more.

[bukka]

For the reference I just created a PR for this with tests in #20540 . I did further investigation and I think it's safe (at least for master which I'm targeting).