sync: add Convar wrapper that panics instead of returning Result

The Condvar wrapper exposed by this change is analogous to the Mutex
wrapper in this crate. Instead of a Result being returned in the case of
a poisoned Mutex, a panic is triggered.

TEST=cargo build
BUG=chromium:920875

Change-Id: Id8bd6bc2891bfc5c8ce334fbdb482ef40500f2d7
Reviewed-on: https://chromium-review.googlesource.com/1416316
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: David Tolnay <dtolnay@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

sync/src/condvar.rs

@@ -0,0 +1,45 @@
+// Copyright 2018 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+use std::fmt::{self, Debug};
+use std::sync::{Condvar as StdCondvar, MutexGuard};
+
+/// A Condition Variable.
+#[derive(Default)]
+pub struct Condvar {
+    std: StdCondvar,
+}
+
+impl Condvar {
+    /// Creates a new condvar that is ready to be waited on.
+    pub fn new() -> Condvar {
+        Condvar {
+            std: StdCondvar::new(),
+        }
+    }
+
+    /// Waits on a condvar, blocking the current thread until it is notified.
+    pub fn wait<'a, T>(&self, guard: MutexGuard<'a, T>) -> MutexGuard<'a, T> {
+        match self.std.wait(guard) {
+            Ok(guard) => guard,
+            Err(_) => panic!("condvar is poisoned"),
+        }
+    }
+
+    /// Notifies one thread blocked by this condvar.
+    pub fn notify_one(&self) {
+        self.std.notify_one();
+    }
+
+    /// Notifies all threads blocked by this condvar.
+    pub fn notify_all(&self) {
+        self.std.notify_all();
+    }
+}
+
+impl Debug for Condvar {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        Debug::fmt(&self.std, formatter)
+    }
+}

sync/src/lib.rs

@@ -2,6 +2,27 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+//! Sync primitive types whose methods panic rather than returning error in case of poison.
+//!
+//! The Mutex/Condvar type in this crates wraps the standard library versions and mirrors the same
+//! methods, except that they panic where the standard library would return an Error. This API
+//! codifies our error handling strategy around poisoned mutexes in crosvm.
+//!
+//! - Crosvm releases are built with panic=abort so poisoning never occurs. A panic while a mutex is
+//!   held (or ever) takes down the entire process. Thus we would like for code not to have to
+//!   consider the possibility of poison.
+//!
+//! - We could ask developers to always write `.lock().unwrap()` on a standard library mutex.
+//!   However, we would like to stigmatize the use of unwrap. It is confusing to permit unwrap but
+//!   only on mutex lock results. During code review it may not always be obvious whether a
+//!   particular unwrap is unwrapping a mutex lock result or a different error that should be
+//!   handled in a more principled way.
+//!
+//! Developers should feel free to use types defined in this crate anywhere in crosvm that they
+//! would otherwise be using the corresponding types in std::sync.
+
+mod condvar;
 mod mutex;
 
+pub use condvar::Condvar;
 pub use mutex::{Mutex, WouldBlock};