Fix Drop for Server deadlock behavior (#75)

Summary:
This fixes two possible deadlocks that cause `elp server` to hang during exit. I was on the right track in #74 but there are actually two deadlocks:

* The first happens because the logger backend has a clone of the `Sender` of the `Connection` (because the `LspLogger` clones it one). This can cause a deadlock when a message is _not_ logged following the drop of `Server` on `io_threads.join()` because the `Connection`'s thread for receiving messages from the `Connection`'s `Receiver` awaits messages indefinitely. The solution for this deadlock is to remove the `LspLogger` from the backend on `Drop` of the server (or any time earlier). That decrements the reference count on the `Sender` and causes the `Receiver`'s iterator to return `None` resulting in the termination of its thread.
* The second I don't really understand fully. The `Drop for Server` hangs because the cache task pool's drop joins its threads (by design) and there is an outstanding task that seems to be frozen, I think related to Salsa's cancellation mechanism. Explicitly requesting cancellation during drop eliminates this problem. Plus it matches what rust-analyzer does.

Fixes #36

Pull Request resolved: #75

Reviewed By: michalmuskala

Differential Revision: D69115301

Pulled By: alanz

fbshipit-source-id: 3b0a5b39f49d668199a2370f1b23bc78f0ae6d17

Author: the-mikedavis

crates/elp/src/server.rs

@@ -256,6 +256,16 @@ pub struct Server {
     vfs_config_version: u32,
 }
 
+impl Drop for Server {
+    fn drop(&mut self) {
+        // Remove the LSP logger. This prevents a deadlock because the logger has a strong
+        // reference to the `Connection`'s `Sender`.
+        self.logger.remove_logger(LOGGER_NAME);
+        // Cancel any ongoing analyses.
+        self.analysis_host.request_cancellation();
+    }
+}
+
 impl Server {
     #[allow(clippy::too_many_arguments)]
     pub fn new(
@@ -603,7 +613,6 @@ impl Server {
         RequestDispatcher::new(self, req)
             .on_sync::<request::Shutdown>(|this, ()| {
                 this.transition(Status::ShuttingDown);
-                this.analysis_host.request_cancellation();
                 Ok(())
             })?
             .on::<request::CodeActionRequest>(handlers::handle_code_action)

crates/elp_log/src/lib.rs

@@ -72,6 +72,13 @@ impl Logger {
         log::set_max_level(shared.max_level());
     }
 
+    pub fn remove_logger(&self, name: &str) {
+        let mut shared = self.shared.write();
+        shared.writers.remove(name.into());
+
+        log::set_max_level(shared.max_level());
+    }
+
     pub fn reconfigure(&self, name: &str, filter: Builder) {
         let mut shared = self.shared.write();