Support parallel captures from the StackSamplingProfiler.

base/profiler/stack_sampling_profiler.h

Index: base/profiler/stack_sampling_profiler.h
diff --git a/base/profiler/stack_sampling_profiler.h b/base/profiler/stack_sampling_profiler.h
index 622f6e5764a5b4e59fbf0a6eed6c7d328d737fc3..6f25d31fad8bffa8fcb34cab70641872468cb7a6 100644
--- a/base/profiler/stack_sampling_profiler.h
+++ b/base/profiler/stack_sampling_profiler.h
@@ -25,12 +25,20 @@ namespace base {
 
 class NativeStackSampler;
 class NativeStackSamplerTestDelegate;
+class WaitableEvent;
 
 // StackSamplingProfiler periodically stops a thread to sample its stack, for
 // the purpose of collecting information about which code paths are
 // executing. This information is used in aggregate by UMA to identify hot
 // and/or janky code paths.
 //
+// IMPORTANT: Only threads guaranteed to live beyond the lifetime of the
+// profiler object can be safely sampled. Sampling the activity of the current
+// thread within the scope of this object is safe, but sampling any other
+// thread could cause Bad Things(tm) to occur should that thread exit before
+// sampling is complete; ensure the profiler object gets destructed before
+// exit of the thread under test. USE WITH CAUTION!
+//

[Mike Wittman, 2017/02/13 22:35:57]

If we refactor the constructors to:

  StackSamplingProfiler(const SamplingParams& params,
                        const CompletedCallback& callback,
                        NativeStackSamplerTestDelegate* test_delegate =
nullptr);

for the A-samples-A case, and

  StackSamplingProfiler(PlatformThreadId thread_id,
                        const SamplingParams& params,
                        const CompletedCallback& callback,
                        NativeStackSamplerTestDelegate* test_delegate =
nullptr);

for the A-samples-B case, then this comment can go directly on the second
constructor where it's more likely to be noticed.

[bcwhite, 2017/02/14 14:33:02]

I like it.  Done.

 // Sample StackSamplingProfiler usage:
 //
 //   // Create and customize params as desired.
@@ -205,13 +213,6 @@ class BASE_EXPORT StackSamplingProfiler {
   // Stops any profiling currently taking place before destroying the profiler.
   ~StackSamplingProfiler();
 
-  // The fire-and-forget interface: starts a profiler and allows it to complete
-  // without the caller needing to manage the profiler lifetime. May be invoked
-  // from any thread, but requires that the calling thread has a message loop.
-  static void StartAndRunAsync(PlatformThreadId thread_id,
-                               const SamplingParams& params,
-                               const CompletedCallback& callback);
-
   // Initializes the profiler and starts sampling.
   void Start();
 
@@ -221,6 +222,27 @@ class BASE_EXPORT StackSamplingProfiler {
   // whichever occurs first.
   void Stop();
 
+  // Stops all active profiles and cleans up any resources in anticipation of
+  // a shutdown of the current process.
+  static void Shutdown();
+
+  // Resets the "shutdown" state so that the sampling thread can be restarted.
+  static void UndoShutdownForTesting();
+
+  // Returns whether the sampling thread is currently running or not.
+  static bool IsSamplingThreadRunningForTesting();
+
+  // Sets the auto-shutdown delay time for the sampling thread, in ms. Set this
+  // to zero to disable it.
+  static void SetSamplingThreadIdleShutdownTimeForTesting(int shutdown_ms);
+
+  // Initiates an idle shutdown task, as though the idle timer had expired,
+  // causing the thread to exit if there are no more sampling tasks pending.
+  // This returns immediately. Watch IsSamplingThreadRunningForTesting() to
+  // know when the thread has exited, though it will never do so if it is
+  // not idle.
+  static void InitiateSamplingThreadIdleShutdownForTesting();
+
   // Set the current system state that is recorded with each captured stack
   // frame. This is thread-safe so can be called from anywhere. The parameter
   // value should be from an enumeration of the appropriate type with values
@@ -233,44 +255,7 @@ class BASE_EXPORT StackSamplingProfiler {
  private:
   // SamplingThread is a separate thread used to suspend and sample stacks from
   // the target thread.
-  class SamplingThread : public PlatformThread::Delegate {
-   public:
-    // Samples stacks using |native_sampler|. When complete, invokes
-    // |completed_callback| with the collected call stack profiles.
-    // |completed_callback| must be callable on any thread.
-    SamplingThread(std::unique_ptr<NativeStackSampler> native_sampler,
-                   const SamplingParams& params,
-                   const CompletedCallback& completed_callback);
-    ~SamplingThread() override;
-
-    // PlatformThread::Delegate:
-    void ThreadMain() override;
-
-    void Stop();
-
-   private:
-    // Collects |profile| from a single burst. If the profiler was stopped
-    // during collection, sets |was_stopped| and provides the set of samples
-    // collected up to that point.
-    void CollectProfile(CallStackProfile* profile, TimeDelta* elapsed_time,
-                        bool* was_stopped);
-
-    // Collects call stack profiles from all bursts, or until the sampling is
-    // stopped. If stopped before complete, the last profile in
-    // |call_stack_profiles| may contain a partial burst.
-    void CollectProfiles(CallStackProfiles* profiles);
-
-    std::unique_ptr<NativeStackSampler> native_sampler_;
-    const SamplingParams params_;
-
-    // If Stop() is called, it signals this event to force the sampling to
-    // terminate before all the samples specified in |params_| are collected.
-    WaitableEvent stop_event_;
-
-    const CompletedCallback completed_callback_;
-
-    DISALLOW_COPY_AND_ASSIGN(SamplingThread);
-  };
+  class SamplingThread;
 
   // Adds annotations to a Sample.
   static void RecordAnnotations(Sample* sample);
@@ -287,11 +272,18 @@ class BASE_EXPORT StackSamplingProfiler {
 
   const SamplingParams params_;
 
-  std::unique_ptr<SamplingThread> sampling_thread_;
-  PlatformThreadHandle sampling_thread_handle_;
-
   const CompletedCallback completed_callback_;
 
+  // An event signaled when all sampling is complete and the callback done.
+  WaitableEvent finished_event_;
+
+  // Object that does the native sampling. This is created during construction
+  // and later passed to the sampling thread when profiling is started.
+  std::unique_ptr<NativeStackSampler> native_sampler_;
+
+  // An ID uniquely identifying this collection to the sampling thread.
+  int collection_id_ = -1;
+
   // Stored until it can be passed to the NativeStackSampler created in Start().
   NativeStackSamplerTestDelegate* const test_delegate_;