fsevent should join on thread shutdown

We had some test failures because crossbeam-channel may panic when trying
to call recv() during thread shutdown. This seems to be similar to this
upstream bug: crossbeam-rs/crossbeam#321.
Unfortunately it seems that some operating systems may tear down thread-local
storage early, rust-lang/rust#28129, which can
trigger panics if trying to interact with TLS during a drop.

To avoid this issue, this switches from using a channel to signal the thread
shutdown to just using the join handle (which we should have been doing
anyway).

src/fsevent.rs

@@ -16,7 +16,7 @@
 
 use crate::event::*;
 use crate::{Config, Error, EventFn, RecursiveMode, Result, Watcher};
-use crossbeam_channel::{unbounded, Receiver, Sender};
+use crossbeam_channel::{unbounded, Sender};
 use fsevent_sys as fs;
 use fsevent_sys::core_foundation as cf;
 use std::collections::HashMap;
@@ -65,7 +65,7 @@ pub struct FsEventWatcher {
     latency: cf::CFTimeInterval,
     flags: fs::FSEventStreamCreateFlags,
     event_fn: Arc<Mutex<dyn EventFn>>,
-    runloop: Option<(cf::CFRunLoopRef, Receiver<()>)>,
+    runloop: Option<(cf::CFRunLoopRef, thread::JoinHandle<()>)>,
     recursive_info: HashMap<PathBuf, bool>,
 }
 
@@ -275,7 +275,7 @@ impl FsEventWatcher {
             return;
         }
 
-        if let Some((runloop, done)) = self.runloop.take() {
+        if let Some((runloop, thread_handle)) = self.runloop.take() {
             unsafe {
                 let runloop = runloop as *mut raw::c_void;
 
@@ -286,11 +286,8 @@ impl FsEventWatcher {
                 cf::CFRunLoopStop(runloop);
             }
 
-            // sync done channel
-            match done.recv() {
-                Ok(()) => (),
-                Err(_) => panic!("the runloop may not be finished!"),
-            }
+            // Wait for the thread to shut down.
+            thread_handle.join().expect("thread to shut down");
         }
     }
 
@@ -366,9 +363,6 @@ impl FsEventWatcher {
             return Err(Error::path_not_found());
         }
 
-        // done channel is used to sync quit status of runloop thread
-        let (done_tx, done_rx) = unbounded();
-
         let info = StreamContextInfo {
             event_fn: self.event_fn.clone(),
             recursive_info: self.recursive_info.clone(),
@@ -424,7 +418,7 @@ impl FsEventWatcher {
         // channel to pass runloop around
         let (rl_tx, rl_rx) = unbounded();
 
-        thread::spawn(move || {
+        let thread_handle = thread::spawn(move || {
             let stream = stream.0;
 
             unsafe {
@@ -452,13 +446,9 @@ impl FsEventWatcher {
                 //   longer references the context pointer.
                 let _context = Box::from_raw(thread_context_ptr.0);
             }
-
-            done_tx
-                .send(())
-                .expect("error while signal run loop is done");
         });
         // block until runloop has been sent
-        self.runloop = Some((rl_rx.recv().unwrap().0, done_rx));
+        self.runloop = Some((rl_rx.recv().unwrap().0, thread_handle));
 
         Ok(())
     }