Explicitly explain failure points in allow_threads implementation for…

… better debuggability.

src/marker.rs

@@ -318,7 +318,7 @@ impl<'py> Python<'py> {
         use std::mem::transmute;
         use std::panic::{catch_unwind, resume_unwind, AssertUnwindSafe};
         use std::sync::mpsc::{sync_channel, SendError, SyncSender};
-        use std::thread::{spawn, Result};
+        use std::thread::{Builder, Result};
         use std::time::Duration;
 
         use parking_lot::{const_mutex, Mutex};
@@ -344,11 +344,15 @@ impl<'py> Python<'py> {
         let mut f = Some(f);
 
         let mut task = || {
-            let f = f.take().unwrap();
+            let f = f
+                .take()
+                .expect("allow_threads closure called more than once");
 
             let result = catch_unwind(AssertUnwindSafe(f));
 
-            result_sender.send(result).unwrap();
+            result_sender
+                .send(result)
+                .expect("allow_threads runtime task was abandoned");
         };
 
         // SAFETY: the current thread will block until the closure has returned
@@ -371,22 +375,27 @@ impl<'py> Python<'py> {
 
             let (task_sender, task_receiver) = sync_channel::<Task>(0);
 
-            spawn(move || {
-                let mut next_task = Ok(task);
+            Builder::new()
+                .name("pyo3 allow_threads runtime".to_owned())
+                .spawn(move || {
+                    let mut next_task = Ok(task);
 
-                while let Ok(task) = next_task {
-                    // SAFETY: all data accessed by `task` will stay alive until it completes
-                    unsafe { (*task.0)() };
+                    while let Ok(task) = next_task {
+                        // SAFETY: all data accessed by `task` will stay alive until it completes
+                        unsafe { (*task.0)() };
 
-                    next_task = task_receiver.recv_timeout(Duration::from_secs(60));
-                }
-            });
+                        next_task = task_receiver.recv_timeout(Duration::from_secs(60));
+                    }
+                })
+                .expect("failed to create allow_threads runtime thread");
 
             task_sender
         };
 
         // 3. Wait for completion and check result
-        let result = result_receiver.recv().unwrap();
+        let result = result_receiver
+            .recv()
+            .expect("allow_threads runtime thread died unexpectedly");
 
         trap.disarm();