Handle absence of EVFILT_USER with kevent backend.

Some kqueue implementations do not have EVFILT_USER. We can work around this by creating a timer implementation with a very small timeout.[1] [1] Thanks to this post, which provided the idea for the workaround: https://lists.macosforge.org/pipermail/libdispatch-dev/2009-September/000010.html

Author: 3405691582

src/event/event_kevent.c

@@ -104,7 +104,9 @@ _evfiltstr(short filt)
 #ifdef EVFILT_FS
 _evfilt2(EVFILT_FS);
 #endif
+#ifdef EVFILT_USER
 _evfilt2(EVFILT_USER);
+#endif
 #ifdef EVFILT_SOCK
 _evfilt2(EVFILT_SOCK);
 #endif
@@ -532,11 +534,22 @@ _dispatch_kevent_merge_muxed(dispatch_kevent_t ke)
 }
 }
 
+/*
+ * If the kevent implementation doesn't support EVFILT_USER for
+ * signaling, then we use EVFILT_TIMER with EV_ONESHOT with this ident
+ * to make do.
+ */
+#define DISPATCH_KEVENT_ERSATZ_EVFILT_USER_IDENT (~0ull << 9)
+
 DISPATCH_NOINLINE
 static void
 _dispatch_kevent_drain(dispatch_kevent_t ke)
 {
+#ifdef EVFILT_USER
 if (ke->filter == EVFILT_USER) {
+#else
+if (ke->filter == EVFILT_TIMER && ke->ident == DISPATCH_KEVENT_ERSATZ_EVFILT_USER_IDENT) {
+#endif
 _dispatch_kevent_mgr_debug("received", ke);
 return;
 }
@@ -583,10 +596,17 @@ static void
 _dispatch_kq_create(intptr_t *fd_ptr)
 {
 static const dispatch_kevent_s kev = {
+#ifdef EVFILT_USER
 .ident = 1,
 .filter = EVFILT_USER,
 .flags = EV_ADD|EV_CLEAR,
 .udata = (dispatch_kevent_udata_t)DISPATCH_WLH_MANAGER,
+#else
+.ident = DISPATCH_KEVENT_ERSATZ_EVFILT_USER_IDENT,
+.filter = EVFILT_TIMER,
+.flags = EV_ADD|EV_DISABLE|EV_ONESHOT,
+.data = 1,
+#endif
 };
 int kqfd;
 
@@ -789,9 +809,15 @@ _dispatch_kq_drain(dispatch_wlh_t wlh, dispatch_kevent_t ke, int n,
 
 #if DISPATCH_DEBUG
 for (r = 0; r < n; r++) {
+#ifdef EVFILT_USER
 if (ke[r].filter != EVFILT_USER || DISPATCH_MGR_QUEUE_DEBUG) {
 _dispatch_kevent_debug_n(NULL, ke + r, r, n);
 }
+#else
+if (DISPATCH_MGR_QUEUE_DEBUG) {
+_dispatch_kevent_debug_n(NULL, ke + r, r, n);
+}
+#endif
 }
 #endif
 
@@ -923,9 +949,13 @@ _dispatch_kq_deferred_update(dispatch_wlh_t wlh, dispatch_kevent_t ke)
 ke->udata);
 dispatch_kevent_t dk = _dispatch_kq_deferred_reuse_slot(wlh, ddi, slot);
 *dk = *ke;
+#ifdef EVFILT_USER
 if (ke->filter != EVFILT_USER) {
 _dispatch_kevent_mgr_debug("deferred", ke);
 }
+#else
+_dispatch_kevent_mgr_debug("deferred", ke);
+#endif
 } else {
 _dispatch_kq_update_one(wlh, ke);
 }
@@ -1887,10 +1917,17 @@ _dispatch_event_loop_poke(dispatch_wlh_t wlh, uint64_t dq_state, uint32_t flags)
 {
 if (wlh == DISPATCH_WLH_MANAGER) {
 dispatch_kevent_s ke = (dispatch_kevent_s){
+#ifdef EVFILT_USER
 .ident = 1,
 .filter = EVFILT_USER,
 .fflags = NOTE_TRIGGER,
 .udata = (dispatch_kevent_udata_t)DISPATCH_WLH_MANAGER,
+#else
+.ident = DISPATCH_KEVENT_ERSATZ_EVFILT_USER_IDENT,
+.filter = EVFILT_TIMER,
+.flags = EV_ADD|EV_ENABLE|EV_ONESHOT,
+.data = 1
+#endif
 };
 return _dispatch_kq_deferred_update(DISPATCH_WLH_ANON, &ke);
 } else if (wlh && wlh != DISPATCH_WLH_ANON) {