Handle closed stdio streams

[nikic]

Detect closed stdio streams on startup and open a dummy stream for
them on which all operations fail. This avoids random FDs being
adopted as stdio streams.

Fixes bug #74252.

[nikic]

cc @kelunik @bwoebi

Overall I kind of get the impression that closed stdio streams are really not supported, even at the C level (the stdin etc FILE handles point to closed FDs).

I think this is going to make STDIN etc behave in a somewhat more predictable way, but there are probably enough other places that will exhibit broken behavior (things directly using stdout for example, or inheriting FDs in proc_open or whatever).

[nikic]

We might be better off not defining STDIN and friends at all if the FDs are closed.

[bwoebi]

I think it's better to leave STDIN defined. there's no value in distinguishing between stdin closed at program start or closed after a few ms. It would just add a burden of an additional check - feof() isn't enough then.
If it's that broken, then we should maybe set stdin to a fake closed resource.
I would like to keep the details of this out of userland.

[kelunik]

I also think that STDIN should be defined even if the stream is closed. LGTM.

[nikic]

Unfortunately it looks like STDIN still gets overridden on CI. Something is opening an FD before the validity check runs.

[nikic]

I'm now checking stdio stream validity directly on entry into main() and something is still overwriting FD 0. Probably some library we link is opening /dev/urandom or so during startup.

I think at this point this issue is simply "Won't Fix". Don't close stdio streams, the behavior is ill-defined. Redirect to /dev/null instead.