Fix ioselectors_kqueue raising wrong exceptions (#24079)

kqueue will remove pipes automatically if their read end is closed.
Unfortunately this means that trying to unregister it (which is
necessary to clean up resources & for consistency with other ioselectors
implementations) will set an ENOENT error, which currently raises an
exception.

(ETA: in other words, it is currently impossible to call unregister on a
pipe fd without potentially getting the selector into an invalid state
on platforms with kqueue.)

Avoid this issue by ignoring ENOENT errors returned from kqueue.

(Tested on FreeBSD. I added a test case to the tioselectors file; the
seemingly unrelated change is to fix a race condition that doesn't
appear on Linux, so that it would run my code too.)

lib/pure/ioselects/ioselectors_kqueue.nim

@@ -194,7 +194,9 @@ when hasThreadSupport:
         if s.changesLength > 0:
           if kevent(s.kqFD, addr(s.changes[0]), cint(s.changesLength),
                     nil, 0, nil) == -1:
-            raiseIOSelectorsError(osLastError())
+            let res = osLastError()
+            if cint(res) != ENOENT: # ignore pipes whose read end is closed
+              raiseIOSelectorsError(res)
           s.changesLength = 0
 else:
   template modifyKQueue[T](s: Selector[T], nident: uint, nfilter: cshort,
@@ -211,7 +213,9 @@ else:
       if length > 0:
         if kevent(s.kqFD, addr(s.changes[0]), length,
                   nil, 0, nil) == -1:
-          raiseIOSelectorsError(osLastError())
+          let res = osLastError()
+          if cint(res) != ENOENT: # ignore pipes whose read end is closed
+            raiseIOSelectorsError(res)
         s.changes.setLen(0)
 
 proc registerHandle*[T](s: Selector[T], fd: int | SocketHandle,

tests/async/tioselectors.nim

@@ -58,7 +58,11 @@ when not defined(windows):
     registerHandle(selector, client_socket, {Event.Write}, 0)
 
     freeAddrInfo(aiList)
-    discard selector.select(100)
+
+    # make sure both sockets are selected
+    var nevs = 0
+    while nevs < 2:
+      nevs += selector.select(100).len
 
     var sockAddress: SockAddr
     var addrLen = sizeof(sockAddress).Socklen
@@ -427,6 +431,20 @@ when not defined(windows):
       doAssert(res[0].fd == dirfd and
                {Event.Vnode, Event.VnodeDelete} <= res[0].events)
 
+  proc pipe_test(): bool =
+    # closing the read end of a pipe will result in it automatically
+    # being removed from the kqueue; make sure no exception is raised
+    var s = newSelector[int]()
+    var fds: array[2, cint]
+    discard pipe(fds)
+    s.registerHandle(fds[1], {Write}, 0)
+    discard close(fds[0])
+    let res = s.select(-1)
+    doAssert(res.len == 1)
+    s.unregister(fds[1])
+    discard close(fds[1])
+    return true
+
   when hasThreadSupport:
 
     var counter = 0
@@ -468,6 +486,7 @@ when not defined(windows):
   when defined(macosx) or defined(freebsd) or defined(openbsd) or
        defined(netbsd):
     processTest("File notification test...", vnode_test())
+    processTest("Pipe test...", pipe_test())
   echo("All tests passed!")
 else:
   import nativesockets, winlean, os, osproc