backend/epoll: implement eventfd wakeup notification

Tries to mimic what happens in backend/kqueue.

Closes mitchellh#4

src/backend/epoll.zig

@@ -21,6 +21,12 @@ pub const Loop = struct {
 
     fd: posix.fd_t,
 
+    /// The eventfd that this epoll queue always has a filter for. Writing
+    /// an empty message to this eventfd can be used to wake up the loop
+    /// at any time. Waking up the loop via this eventfd won't trigger any
+    /// particular completion, it just forces tick to cycle.
+    eventfd: xev.Async,
+
     /// The number of active completions. This DOES NOT include completions that
     /// are queued in the submissions queue.
     active: usize = 0,
@@ -56,8 +62,12 @@ pub const Loop = struct {
     } = .{},
 
     pub fn init(options: xev.Options) !Loop {
+        var eventfd = try xev.Async.init();
+        errdefer eventfd.deinit();
+
         var res: Loop = .{
             .fd = try posix.epoll_create1(std.os.linux.EPOLL.CLOEXEC),
+            .eventfd = eventfd,
             .thread_pool = options.thread_pool,
             .thread_pool_completions = undefined,
             .cached_now = undefined,
@@ -68,6 +78,7 @@ pub const Loop = struct {
 
     pub fn deinit(self: *Loop) void {
         posix.close(self.fd);
+        self.eventfd.deinit();
     }
 
     /// Run the event loop. See RunMode documentation for details on modes.
@@ -262,9 +273,26 @@ pub const Loop = struct {
         // Initialize
         if (!self.flags.init) {
             self.flags.init = true;
+
             if (self.thread_pool != null) {
                 self.thread_pool_completions.init();
             }
+
+            var ev: linux.epoll_event = .{
+                .events = linux.EPOLL.IN | linux.EPOLL.RDHUP,
+                .data = .{ .ptr = 0 },
+            };
+            posix.epoll_ctl(
+                self.fd,
+                linux.EPOLL.CTL_ADD,
+                self.eventfd.fd,
+                &ev,
+            ) catch |err| {
+                // We reset initialization because we can't do anything
+                // safely unless we get this mach port registered!
+                self.flags.init = false;
+                return err;
+            };
         }
 
         // Submit all the submissions. We copy the submission queue so that
@@ -369,6 +397,10 @@ pub const Loop = struct {
 
             // Process all our events and invoke their completion handlers
             for (events[0..n]) |ev| {
+                // Zero data values are internal events that we do nothing
+                // on such as the eventfd wakeup.
+                if (ev.data.ptr == 0) continue;
+
                 const c: *Completion = @ptrFromInt(@as(usize, @intCast(ev.data.ptr)));
 
                 // We get the fd and mark this as in progress we can properly
@@ -415,6 +447,7 @@ pub const Loop = struct {
         const pool = self.thread_pool orelse return error.ThreadPoolRequired;
 
         // Setup our completion state so that thread_perform can do stuff
+        c.task_loop = self;
         c.task_completions = &self.thread_pool_completions;
         c.task = .{ .callback = Loop.thread_perform };
 
@@ -436,6 +469,14 @@ pub const Loop = struct {
 
         // Add to our completion queue
         c.task_completions.push(c);
+
+        // Wake up our main loop
+        c.task_loop.wakeup() catch {};
+    }
+
+    /// Sends an empty message to this loop's eventfd so that it wakes up.
+    fn wakeup(self: *Loop) !void {
+        try self.eventfd.notify();
     }
 
     fn start(self: *Loop, completion: *Completion) void {
@@ -800,6 +841,7 @@ pub const Completion = struct {
     /// reliable way to get access to the loop and shouldn't be used
     /// except internally.
     task: ThreadPool.Task = undefined,
+    task_loop: *Loop = undefined,
     task_completions: *Loop.TaskCompletionQueue = undefined,
     task_result: Result = undefined,