1 files changed, 157 insertions, 0 deletions
diff --git a/xpcom/threads/SyncRunnable.h b/xpcom/threads/SyncRunnable.h
new file mode 100644
index 0000000000..943765afac
--- /dev/null
+++ b/xpcom/threads/SyncRunnable.h
@@ -0,0 +1,157 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_SyncRunnable_h
+#define mozilla_SyncRunnable_h
+
+#include <utility>
+
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/dom/JSExecutionManager.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+/**
+ * This class will wrap a nsIRunnable and dispatch it to the target thread
+ * synchronously. This is different from nsIEventTarget.DISPATCH_SYNC:
+ * this class does not spin the event loop waiting for the event to be
+ * dispatched. This means that you don't risk reentrance from pending
+ * messages, but you must be sure that the target thread does not ever block
+ * on this thread, or else you will deadlock.
+ *
+ * Typical usage:
+ *   RefPtr<SyncRunnable> sr = new SyncRunnable(new myrunnable...());
+ *   sr->DispatchToThread(t);
+ *
+ * We also provide convenience wrappers:
+ *   SyncRunnable::DispatchToThread(pThread, new myrunnable...());
+ *   SyncRunnable::DispatchToThread(pThread, NS_NewRunnableFunction(...));
+ *
+ */
+class SyncRunnable : public Runnable {
+ public:
+  explicit SyncRunnable(nsIRunnable* aRunnable)
+      : Runnable("SyncRunnable"),
+        mRunnable(aRunnable),
+        mMonitor("SyncRunnable"),
+        mDone(false) {}
+
+  explicit SyncRunnable(already_AddRefed<nsIRunnable> aRunnable)
+      : Runnable("SyncRunnable"),
+        mRunnable(std::move(aRunnable)),
+        mMonitor("SyncRunnable"),
+        mDone(false) {}
+
+  nsresult DispatchToThread(nsIEventTarget* aThread,
+                            bool aForceDispatch = false) {
+    nsresult rv;
+    bool on;
+
+    if (!aForceDispatch) {
+      rv = aThread->IsOnCurrentThread(&on);
+      MOZ_ASSERT(NS_SUCCEEDED(rv));
+      if (NS_SUCCEEDED(rv) && on) {
+        mRunnable->Run();
+        return NS_OK;
+      }
+    }
+
+    rv = aThread->Dispatch(this, NS_DISPATCH_NORMAL);
+    if (NS_SUCCEEDED(rv)) {
+      mozilla::MonitorAutoLock lock(mMonitor);
+      // This could be synchronously dispatching to a thread currently waiting
+      // for JS execution clearance. Yield JS execution.
+      dom::AutoYieldJSThreadExecution yield;
+
+      while (!mDone) {
+        lock.Wait();
+      }
+    }
+    return rv;
+  }
+
+  nsresult DispatchToThread(AbstractThread* aThread,
+                            bool aForceDispatch = false) {
+    if (!aForceDispatch && aThread->IsCurrentThreadIn()) {
+      mRunnable->Run();
+      return NS_OK;
+    }
+
+    // Check we don't have tail dispatching here. Otherwise we will deadlock
+    // ourself when spinning the loop below.
+    MOZ_ASSERT(!aThread->RequiresTailDispatchFromCurrentThread());
+
+    nsresult rv = aThread->Dispatch(RefPtr<nsIRunnable>(this).forget());
+    if (NS_SUCCEEDED(rv)) {
+      mozilla::MonitorAutoLock lock(mMonitor);
+      while (!mDone) {
+        lock.Wait();
+      }
+    }
+    return rv;
+  }
+
+  static nsresult DispatchToThread(nsIEventTarget* aThread,
+                                   nsIRunnable* aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(aRunnable));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  static nsresult DispatchToThread(AbstractThread* aThread,
+                                   nsIRunnable* aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(aRunnable));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  static nsresult DispatchToThread(nsIEventTarget* aThread,
+                                   already_AddRefed<nsIRunnable> aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(std::move(aRunnable)));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  static nsresult DispatchToThread(AbstractThread* aThread,
+                                   already_AddRefed<nsIRunnable> aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(std::move(aRunnable)));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  // These deleted overloads prevent accidentally (if harmlessly) double-
+  // wrapping SyncRunnable, which was previously a common anti-pattern.
+  static nsresult DispatchToThread(nsIEventTarget* aThread,
+                                   SyncRunnable* aRunnable,
+                                   bool aForceDispatch = false) = delete;
+  static nsresult DispatchToThread(AbstractThread* aThread,
+                                   SyncRunnable* aRunnable,
+                                   bool aForceDispatch = false) = delete;
+
+ protected:
+  NS_IMETHOD Run() override {
+    mRunnable->Run();
+
+    mozilla::MonitorAutoLock lock(mMonitor);
+    MOZ_ASSERT(!mDone);
+
+    mDone = true;
+    mMonitor.Notify();
+
+    return NS_OK;
+  }
+
+ private:
+  nsCOMPtr<nsIRunnable> mRunnable;
+  mozilla::Monitor mMonitor;
+  bool mDone MOZ_GUARDED_BY(mMonitor);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_SyncRunnable_h