[FD Leak] asyncdispatch: dispatcher's epoll/kqueue fd can never be closed and leaks (one fd per dispatcher, one per terminated thread) #26102

Nim Version

Nim Compiler Version 2.3.1 [MacOSX: arm64] (devel, also reproduces with 2.2.10)

Description

A PDispatcher's selector owns an OS resource: an epoll fd (Linux), a kqueue
fd plus a helper socket (macOS/BSD), or an IO completion port handle
(Windows). There is no way to release it:

• asyncdispatch exposes no close/destroy for PDispatcher
  (std/selectors has a working close(), but it is unreachable through
  the asyncdispatch API)
• no destructor/finalizer is attached, so a garbage-collected dispatcher
  leaks the fd
• nothing closes the thread-local dispatcher when a thread terminates

Every dropped dispatcher therefore leaks at least one fd. The practical
worst case is thread pools: any thread that touches async (e.g. a
waitFor in a worker) permanently leaks one fd when it exits, so
long-running servers with short-lived worker threads exhaust the fd limit.

import std/[asyncdispatch, os]

proc openFds(): int =
  for _ in walkDir("/proc/self/fd"): inc result

echo "fds at start: ", openFds()
for i in 0..<10:
  let d = newDispatcher()
  setGlobalDispatcher(d)
  waitFor sleepAsync(1)   # force selector creation/use
  setGlobalDispatcher(nil)
  GC_fullCollect()
echo "fds after 10 dispatcher cycles: ", openFds()

Compiled and run on Linux with:

nim c --mm:orc -d:useMalloc --debugger:native displeak.nim
valgrind --track-fds=yes ./displeak

Output (program + valgrind fd tracking):

fds at start: 27          # baseline includes valgrind's own fds
fds after 10 dispatcher cycles: 37

==440== FILE DESCRIPTORS: 29 open (3 std) at exit.
==440== Open file descriptor 28:
==440==    at 0x4973487: epoll_create1 (syscall-template.S:120)
==440==    by 0x1200A6: selectors::newSelector (ioselectors_epoll.nim:86)
==440==    by 0x123ECC: asyncdispatch::newDispatcher (asyncdispatch.nim:1209)
==440==    by 0x127488: NimMainModule (displeak.nim:8)
==440==    by 0x127599: NimMainInner (system.nim:959)
==440==    by 0x1275AC: NimMain (system.nim:970)
==440==    by 0x1275CE: main (system.nim:978)
[... 9 more identical entries, fds 19-27, one per loop iteration ...]

Current Output

Expected Output

Known Workarounds

No response

Additional Information

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