fix(lib): return an error instead of panic if execute fails

If executing an internal task fails, a new variant of `hyper::Error` is
returned to the user, with improved messaging.

If a non-critical task fails to spawn, it no longer panics, instead just
logging a warning.

Closes #1566

src/client/mod.rs

@@ -270,13 +270,20 @@ where C: Connect + Sync + 'static,
                                 .http2_only(pool_key.1 == Ver::Http2)
                                 .handshake(io)
                                 .and_then(move |(tx, conn)| {
-                                    executor.execute(conn.map_err(|e| {
+                                    let bg = executor.execute(conn.map_err(|e| {
                                         debug!("client connection error: {}", e)
                                     }));
 
+                                    // This task is critical, so an execute error
+                                    // should be returned.
+                                    if let Err(err) = bg {
+                                        warn!("error spawning critical client task: {}", err);
+                                        return Either::A(future::err(err));
+                                    }
+
                                     // Wait for 'conn' to ready up before we
                                     // declare this tx as usable
-                                    tx.when_ready()
+                                    Either::B(tx.when_ready())
                                 })
                                 .map(move |tx| {
                                     pool.pooled(connecting, PoolClient {
@@ -373,26 +380,32 @@ where C: Connect + Sync + 'static,
                         } else if !res.body().is_end_stream() {
                             let (delayed_tx, delayed_rx) = oneshot::channel();
                             res.body_mut().delayed_eof(delayed_rx);
-                            executor.execute(
-                                future::poll_fn(move || {
-                                    pooled.poll_ready()
-                                })
+                            let on_idle = future::poll_fn(move || {
+                                pooled.poll_ready()
+                            })
                                 .then(move |_| {
                                     // At this point, `pooled` is dropped, and had a chance
                                     // to insert into the pool (if conn was idle)
                                     drop(delayed_tx);
                                     Ok(())
-                                })
-                            );
+                                });
+
+                            if let Err(err) = executor.execute(on_idle) {
+                                // This task isn't critical, so just log and ignore.
+                                warn!("error spawning task to insert idle connection: {}", err);
+                            }
                         } else {
                             // There's no body to delay, but the connection isn't
                             // ready yet. Only re-insert when it's ready
-                            executor.execute(
-                                future::poll_fn(move || {
-                                    pooled.poll_ready()
-                                })
-                                .then(|_| Ok(()))
-                            );
+                            let on_idle = future::poll_fn(move || {
+                                pooled.poll_ready()
+                            })
+                                .then(|_| Ok(()));
+
+                            if let Err(err) = executor.execute(on_idle) {
+                                // This task isn't critical, so just log and ignore.
+                                warn!("error spawning task to insert idle connection: {}", err);
+                            }
                         }
                         Ok(res)
                     });

src/client/pool.rs

@@ -407,12 +407,16 @@ impl<T: Poolable> Connections<T> {
 
         let start = Instant::now() + dur;
 
-        let interval = Interval::new(start, dur);
-        self.exec.execute(IdleInterval {
-            interval: interval,
+        let interval = IdleInterval {
+            interval: Interval::new(start, dur),
             pool: WeakOpt::downgrade(pool_ref),
             pool_drop_notifier: rx,
-        });
+        };
+
+        if let Err(err) = self.exec.execute(interval) {
+            // This task isn't critical, so simply log and ignore.
+            warn!("error spawning connection pool idle interval: {}", err);
+        }
     }
 }
 

src/common/exec.rs

@@ -13,15 +13,21 @@ pub(crate) enum Exec {
 
 
 impl Exec {
-    pub(crate) fn execute<F>(&self, fut: F)
+    pub(crate) fn execute<F>(&self, fut: F) -> ::Result<()>
     where
         F: Future<Item=(), Error=()> + Send + 'static,
     {
         match *self {
             Exec::Default => {
                 #[cfg(feature = "runtime")]
                 {
-                    ::tokio_executor::spawn(fut)
+                    use ::tokio_executor::Executor;
+                    ::tokio_executor::DefaultExecutor::current()
+                        .spawn(Box::new(fut))
+                        .map_err(|err| {
+                            warn!("executor error: {:?}", err);
+                            ::Error::new_execute()
+                        })
                 }
                 #[cfg(not(feature = "runtime"))]
                 {
@@ -30,10 +36,11 @@ impl Exec {
                 }
             },
             Exec::Executor(ref e) => {
-                let _ = e.execute(Box::new(fut))
+                e.execute(Box::new(fut))
                     .map_err(|err| {
-                        panic!("executor error: {:?}", err.kind());
-                    });
+                        warn!("executor error: {:?}", err.kind());
+                        ::Error::new_execute()
+                    })
             },
         }
     }

src/error.rs

@@ -67,6 +67,9 @@ pub(crate) enum Kind {
 
     /// User polled for an upgrade, but low-level API is not using upgrades.
     ManualUpgrade,
+
+    /// Error trying to call `Executor::execute`.
+    Execute,
 }
 
 #[derive(Debug, PartialEq)]
@@ -114,7 +117,8 @@ impl Error {
             Kind::Closed |
             Kind::UnsupportedVersion |
             Kind::UnsupportedRequestMethod |
-            Kind::NoUpgrade => true,
+            Kind::NoUpgrade |
+            Kind::Execute => true,
             _ => false,
         }
     }
@@ -130,7 +134,7 @@ impl Error {
     }
 
     /// Returns the error's cause.
-    /// 
+    ///
     /// This is identical to `Error::cause` except that it provides extra
     /// bounds required to be able to downcast the error.
     pub fn cause2(&self) -> Option<&(StdError + 'static + Sync + Send)> {
@@ -244,6 +248,10 @@ impl Error {
         Error::new(Kind::Shutdown, Some(Box::new(cause)))
     }
 
+    pub(crate) fn new_execute() -> Error {
+        Error::new(Kind::Execute, None)
+    }
+
     pub(crate) fn new_h2(cause: ::h2::Error) -> Error {
         Error::new(Kind::Http2, Some(Box::new(cause)))
     }
@@ -297,6 +305,7 @@ impl StdError for Error {
             Kind::UnsupportedRequestMethod => "request has unsupported HTTP method",
             Kind::NoUpgrade => "no upgrade available",
             Kind::ManualUpgrade => "upgrade expected but low level API in use",
+            Kind::Execute => "executor failed to spawn task",
 
             Kind::Io => "an IO error occurred",
         }

src/proto/h2/client.rs

@@ -94,7 +94,7 @@ where
                             }
                             Err(Either::B((never, _))) => match never {},
                         });
-                    self.executor.execute(fut);
+                    self.executor.execute(fut)?;
                     State::Ready(request_tx, tx)
                 },
                 State::Ready(ref mut tx, ref conn_dropper) => {
@@ -129,7 +129,7 @@ where
                                         drop(conn_drop_ref);
                                         x
                                     });
-                                self.executor.execute(pipe);
+                                self.executor.execute(pipe)?;
                             }
 
                             let fut = fut
@@ -148,7 +148,7 @@ where
                                     }
                                     Ok(())
                                 });
-                            self.executor.execute(fut);
+                            self.executor.execute(fut)?;
                             continue;
                         },
 

src/proto/h2/server.rs

@@ -132,7 +132,7 @@ where
                 ::Body::h2(stream, content_length)
             });
             let fut = H2Stream::new(service.call(req), respond);
-            exec.execute(fut);
+            exec.execute(fut)?;
         }
 
         // no more incoming streams...

src/server/conn.rs

@@ -641,7 +641,7 @@ where
                     // flatten basically
                     .and_then(|conn| conn.with_upgrades())
                     .map_err(|err| debug!("conn error: {}", err));
-                self.serve.protocol.exec.execute(fut);
+                self.serve.protocol.exec.execute(fut)?;
             } else {
                 return Ok(Async::Ready(()))
             }