os/notify: make epoll wrapper thread safe (#10473)

vlib/os/notify/backend_linux.c.v

@@ -24,14 +24,15 @@ fn C.epoll_ctl(int, int, int, &C.epoll_event) int
 
 fn C.epoll_wait(int, &C.epoll_event, int, int) int
 
+// EpollNotifier provides methods that implement FdNotifier using the
+// epoll I/O event notification facility (linux only)
 [noinit]
 struct EpollNotifier {
 	epoll_fd int
-mut:
-	num_watching int // heuristic
-	events       []C.epoll_event
 }
 
+// EpollEvent describes an event that occurred for a file descriptor in
+// the watch list
 [noinit]
 struct EpollEvent {
 pub:
@@ -39,6 +40,9 @@ pub:
 	kind FdEventType
 }
 
+// new creates a new EpollNotifier
+// The FdNotifier interface is returned to allow OS specific
+// implementations without exposing the concrete type
 pub fn new() ?FdNotifier {
 	fd := C.epoll_create1(0) // 0 indicates default behavior
 	if fd == -1 {
@@ -64,6 +68,7 @@ const (
 	epoll_exclusive    = u32(C.EPOLLEXCLUSIVE)
 )
 
+// ctl is a helper method for add, modify, and remove
 fn (mut en EpollNotifier) ctl(fd int, op int, mask u32) ? {
 	event := C.epoll_event{
 		events: mask
@@ -76,45 +81,43 @@ fn (mut en EpollNotifier) ctl(fd int, op int, mask u32) ? {
 	}
 }
 
+// add adds a file descriptor to the watch list
 fn (mut en EpollNotifier) add(fd int, events FdEventType, conf ...FdConfigFlags) ? {
 	mask := flags_to_mask(events, ...conf)
 	en.ctl(fd, C.EPOLL_CTL_ADD, mask) ?
-	en.num_watching++
 }
 
+// modify sets an existing entry in the watch list to the provided events and configuration
 fn (mut en EpollNotifier) modify(fd int, events FdEventType, conf ...FdConfigFlags) ? {
 	mask := flags_to_mask(events, ...conf)
 	en.ctl(fd, C.EPOLL_CTL_MOD, mask) ?
 }
 
+// remove removes a file descriptor from the watch list
 fn (mut en EpollNotifier) remove(fd int) ? {
 	en.ctl(fd, C.EPOLL_CTL_DEL, 0) ?
-	if en.num_watching > 0 {
-		en.num_watching--
-	}
 }
 
-[direct_array_access]
+// wait waits to be notified of events on the watch list,
+// returns at most 512 events
 fn (mut en EpollNotifier) wait(timeout time.Duration) []FdEvent {
-	if en.events.cap < en.num_watching {
-		en.events.grow_cap(en.num_watching - en.events.cap)
-	}
-	// populate en.events.data with the new events
+	// arbitrary 512 limit; events will round robin on successive
+	// waits if the number exceeds this
+	// NOTE: we use a fixed size array here for stack allocation; this has
+	//       the added bonus of making EpollNotifier thread safe
+	events := [512]C.epoll_event{}
+	// populate events with the new events
 	to := match timeout {
 		time.infinite { -1 }
 		else { int(timeout / time.millisecond) }
 	}
-	count := C.epoll_wait(en.epoll_fd, en.events.data, en.events.cap, to)
-
-	// set len to count
-	en.events.clear()
-	unsafe { en.events.grow_len(count) }
+	count := C.epoll_wait(en.epoll_fd, &events[0], events.len, to)
 
 	if count > 0 {
 		mut arr := []FdEvent{cap: count}
 		for i := 0; i < count; i++ {
-			fd := unsafe { en.events[i].data.fd }
-			kind := event_mask_to_flag(en.events[i].events)
+			fd := unsafe { events[i].data.fd }
+			kind := event_mask_to_flag(events[i].events)
 			if kind.is_empty() {
 				// NOTE: tcc only reports the first event for some
 				// reason, leaving subsequent structs in the array as 0
@@ -131,12 +134,16 @@ fn (mut en EpollNotifier) wait(timeout time.Duration) []FdEvent {
 	return []
 }
 
+// close closes the EpollNotifier,
+// any successive calls to add, modify, remove, and wait should fail
 fn (mut en EpollNotifier) close() ? {
 	if C.close(en.epoll_fd) == -1 {
 		return error(os.posix_get_error_msg(C.errno))
 	}
 }
 
+// event_mask_to_flag is a helper function that converts a bitmask
+// returned by epoll_wait to FdEventType
 fn event_mask_to_flag(mask u32) FdEventType {
 	mut flags := FdEventType{}
 
@@ -162,6 +169,8 @@ fn event_mask_to_flag(mask u32) FdEventType {
 	return flags
 }
 
+// flags_to_mask is a helper function that converts FdEventType and
+// FdConfigFlags to a bitmask used by the C functions
 fn flags_to_mask(events FdEventType, confs ...FdConfigFlags) u32 {
 	mut mask := u32(0)
 	if events.has(.read) {