PPAPI: Make TrackedCallback more threadsafe

ppapi/shared_impl/tracked_callback.h

Index: ppapi/shared_impl/tracked_callback.h
diff --git a/ppapi/shared_impl/tracked_callback.h b/ppapi/shared_impl/tracked_callback.h
index 294c9b99b4e3b89492e0c5d5c9fe438b6f078c3e..2131584228e75f60cce8ec5cb823d9ff7cf0424c 100644
--- a/ppapi/shared_impl/tracked_callback.h
+++ b/ppapi/shared_impl/tracked_callback.h
@@ -13,6 +13,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/synchronization/condition_variable.h"
+#include "base/synchronization/lock.h"
 #include "ppapi/c/pp_completion_callback.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_resource.h"
@@ -52,13 +53,12 @@ class EnterBase;
 // the "owning" |CallbackTracker| will keep a reference until the callback is
 // completed.
 //
-// Subclasses must do several things:
-//  - They must ensure that the callback is executed at most once (by looking at
-//    |completed()| before running the callback).
-//  - They must ensure that the callback is run abortively if it is marked as to
-//    be aborted (by looking at |aborted()| before running the callback).
-//  - They must call |MarkAsCompleted()| immediately before actually running the
-//    callback; see the comment for |MarkAsCompleted()| for a caveat.
+// A note on threading:
+// TrackedCallback is usable on any thread. It is *mostly* only used when
+// ppapi::ProxyLock is held. However, it's necessary that Run() can be called
+// without the ProxyLock. This is used to allow signalling the callback from

[bbudge, 2015/02/25 23:51:31]

It might not be clear to readers what "signalling the callback" means.

+// the IO thread. In particular, blocking callbacks may not have a message loop
+// to which we could post.
 class PPAPI_SHARED_EXPORT TrackedCallback
     : public base::RefCountedThreadSafe<TrackedCallback> {
  public:
@@ -80,6 +80,7 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   // (as determined by target_loop_). If invoked on a different thread, the
   // callback will be scheduled to run later on target_loop_.
   void Run(int32_t result);
+  void AcquireProxyLockAndRun(int32_t result);
   // PostRun is like Run(), except it guarantees that the callback will be run
   // later. In particular, if you invoke PostRun on the same thread on which the
   // callback is targeted to run, it will *not* be run immediately.
@@ -93,7 +94,8 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   typedef base::Callback<int32_t(int32_t /* result */)> CompletionTask;
 
   // Sets a task that is run just before calling back into the plugin. This
-  // should only be called once.
+  // should only be called once. Note that the CompletionTask always runs while
+  // holding the ppapi::ProxyLock.
   void set_completion_task(const CompletionTask& completion_task);
 
   // Returns the ID of the resource which "owns" the callback, or 0 if the
@@ -101,17 +103,31 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   PP_Resource resource_id() const { return resource_id_; }
 
   // Returns true if the callback was completed (possibly aborted).
-  bool completed() const { return completed_; }
+  bool completed() const {
+    base::AutoLock acquire(lock_);
+    return completed_;
+  }
 
   // Returns true if the callback was or should be aborted; this will be the
   // case whenever |Abort()| or |PostAbort()| is called before a non-abortive
   // completion.
-  bool aborted() const { return aborted_; }
+  bool aborted() const {
+    base::AutoLock acquire(lock_);
+    return aborted_;
+  }
 
   // Returns true if this is a blocking callback.
-  bool is_blocking() { return !callback_.func; }
+  bool is_blocking() const {
+    // This is set on construction and never changes after that, so there is
+    // no need to lock.
+    return !callback_.func;
+  }
 
-  MessageLoopShared* target_loop() const { return target_loop_.get(); }
+  MessageLoopShared* target_loop() const {
+    // This is set on construction and never changes after that, so there is
+    // no need to lock.
+    return target_loop_.get();
+  }
 
   // Determines if the given callback is pending. A callback is pending if it
   // has not completed and has not been aborted. When receiving a plugin call,
@@ -127,7 +143,7 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   // message loop.
   static bool IsScheduledToRun(const scoped_refptr<TrackedCallback>& callback);
 
- protected:
+ private:
   bool is_required() {
     return (callback_.func &&
             !(callback_.flags & PP_COMPLETIONCALLBACK_FLAG_OPTIONAL));
@@ -138,7 +154,11 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   }
   bool has_null_target_loop() const { return target_loop_.get() == NULL; }
 
- private:
+  // Same as PostRun(), but lock_ must already be held.
+  void PostRunImpl(int32_t result);
+
+  void SignalBlockingCallback(int32_t result);
+
   // TrackedCallback and EnterBase manage dealing with how to invoke callbacks
   // appropriately. Pepper interface implementations and proxies should not have
   // to check the type of callback, block, or mark them complete explicitly.

[bbudge, 2015/02/25 23:51:31]

Some of our resources do check whether callbacks are blocking. Is that what this
comment means by "type"?

[dmichael (off chromium), 2015/02/27 22:04:40]

Right. I tried improving & updating the callback. I think I wrote that comment
before the FileIO optimization you did.

@@ -152,10 +172,13 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   // be called once. Note that running this may result in this object being
   // deleted (so keep a reference if it'll still be needed).
   void MarkAsCompleted();
+  void MarkAsCompletedWithLock();
 
   // This class is ref counted.
   friend class base::RefCountedThreadSafe<TrackedCallback>;
-  virtual ~TrackedCallback();
+  ~TrackedCallback();
+
+  mutable base::Lock lock_;
 
   // Flag used by |PostAbort()| and |PostRun()| to check that we don't schedule
   // the callback more than once.