improve shouldAcceptRequest

[timotheecour]

refs https://github.com/nim-lang/Nim/pull/16599/files#r553011143

is there a way to wait until a FD is closed to avoid a forced wait?

refs nim-lang#16603
refs nim-lang#18161
refs https://github.com/nim-lang/Nim/pull/16599/files#r552201712
refs nim-lang/RFCs#382

notes

• this could be improved by using C API to list dir entries instead of calling ls
• /proc/self/fd is probably simpler than /proc/$pid/fd
• https://unix.stackexchange.com/questions/278932/how-to-get-number-of-opened-file-descriptors-for-user

Exploring /proc (which is what lsof does under the hood) is as good as it gets. /proc is the Linux API to get information about processes.

• see also RLIMIT_NOFILE, RLIMIT_NPROC, lsof
• https://stackoverflow.com/questions/6583158/finding-open-file-descriptors-for-a-process-linux-c-code shows using opendir etc instead of calling ls

on windows

• GetProcessHandleCount
  https://stackoverflow.com/questions/15357961/retrieve-the-number-of-opened-file-descriptors-using-the-windows-api
  https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-getprocesshandlecount

on osx

lsof -p $pid + filter by FD column

• https://stackoverflow.com/questions/795236/in-mac-os-x-how-can-i-get-an-accurate-count-of-file-descriptor-usage