Don't hang when Cargo's worker threads panic

This shouldn't ever happen during normal development, but happens from
time to time while developing Cargo itself.

Closes #6433

Author: alexcrichton

Diff for: src/cargo/core/compiler/job_queue.rs

@@ -2,10 +2,12 @@ use std::cell::Cell;
 use std::collections::{HashMap, HashSet};
 use std::io;
 use std::marker;
+use std::mem;
 use std::sync::mpsc::{channel, Receiver, Sender};
 use std::sync::Arc;
 
 use crossbeam_utils::thread::Scope;
+use failure::format_err;
 use jobserver::{Acquired, HelperThread};
 use log::{debug, info, trace};
 
@@ -490,7 +492,13 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
                 rmeta_required: Cell::new(rmeta_required),
                 _marker: marker::PhantomData,
             };
-            let res = job.run(&state);
+
+            let mut sender = FinishOnDrop {
+                tx: &my_tx,
+                id,
+                result: Err(format_err!("worker panicked")),
+            };
+            sender.result = job.run(&state);
 
             // If the `rmeta_required` wasn't consumed but it was set
             // previously, then we either have:
@@ -504,13 +512,28 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
             // we'll just naturally abort the compilation operation but for 1
             // we need to make sure that the metadata is flagged as produced so
             // send a synthetic message here.
-            if state.rmeta_required.get() && res.is_ok() {
+            if state.rmeta_required.get() && sender.result.is_ok() {
                 my_tx
                     .send(Message::Finish(id, Artifact::Metadata, Ok(())))
                     .unwrap();
             }
 
-            my_tx.send(Message::Finish(id, Artifact::All, res)).unwrap();
+            // Use a helper struct with a `Drop` implementation to guarantee
+            // that a `Finish` message is sent even if our job panics. We
+            // shouldn't panic unless there's a bug in Cargo, so we just need
+            // to make sure nothing hangs by accident.
+            struct FinishOnDrop<'a> {
+                tx: &'a Sender<Message>,
+                id: u32,
+                result: CargoResult<()>,
+            }
+
+            impl Drop for FinishOnDrop<'_> {
+                fn drop(&mut self) {
+                    let msg = mem::replace(&mut self.result, Ok(()));
+                    drop(self.tx.send(Message::Finish(self.id, Artifact::All, msg)));
+                }
+            }
         };
 
         if !cx.bcx.build_config.build_plan {