Initial GetSample implementation.

src/sampler.cc

Index: src/sampler.cc
diff --git a/src/sampler.cc b/src/sampler.cc
index 0ed4b379626a9746038b6e05b1661cee522f4f1f..3eb3e988204db4926a3c78bcf0b5e9e33d266af4 100644
--- a/src/sampler.cc
+++ b/src/sampler.cc
@@ -1,7 +1,6 @@
 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-
 #include "src/sampler.h"
 
 #if V8_OS_POSIX && !V8_OS_CYGWIN
@@ -42,6 +41,8 @@
 
 #endif
 
+#include "include/v8-sampler.h"
+
 #include "src/v8.h"
 
 #include "src/base/platform/platform.h"
@@ -269,8 +270,21 @@ class SimulatorHelper {
 
 class SignalHandler : public AllStatic {
  public:
-  static void SetUp() { if (!mutex_) mutex_ = new base::Mutex(); }
-  static void TearDown() { delete mutex_; }
+  static void SetUp() {
+    CHECK(mutex_ == NULL);

[Benedikt Meurer, 2014/08/18 04:22:04]

Nit: DCHECK_EQ(NULL, mutex_)

[gholap, 2014/08/18 19:51:59]

Done.

+    CHECK(sampling_semaphore_ == NULL);

[Benedikt Meurer, 2014/08/18 04:22:04]

Nit: DCHECK_EQ(NULL, sampling_semaphore_)

[gholap, 2014/08/18 19:52:00]

Done.

+
+    mutex_ = new base::Mutex();
+    sampling_semaphore_ = new base::Semaphore(0);
+  }
+
+  static void TearDown() {
+    delete mutex_;
+    delete sampling_semaphore_;
+
+    mutex_ = NULL;
+    sampling_semaphore_ = NULL;
+  }
 
   static void IncreaseSamplerCount() {
     base::LockGuard<base::Mutex> lock_guard(mutex_);
@@ -308,14 +322,45 @@ class SignalHandler : public AllStatic {
   }
 
   static void HandleProfilerSignal(int signal, siginfo_t* info, void* context);
+
+  // The SIGPROF could have come from DoSample or from GetSample.
+  // GetSample is the internal endpoint for the public
+  // GetSample API, which aims to bypass all the internal buffers
+  // and return just one sample instead, rightaway.
+  // TODO(gholap): Eventually, get rid of DoSample.
+  //               Only GetSample should remain.
+
+  // This is the sample which will be filled by the call from GetSample.
+  static TickSample sample_;
+
+  // For the SIGPROF handler to know whether the call was from GetSample.
+  static volatile bool called_from_get_sample_;
+
+  // GetSample waits synchronously till the SIGPROF handler returns.
+  // this semaphore is used to signal GetSample.
+  static base::Semaphore* sampling_semaphore_;
+
+  // It is not that every time HandleProfilerSignal is invoked,
+  // it succeeds in obtaining a sample.
+  // This provides GetSample with the information whether
+  // the handler finished with or without getting the sample.
+  static bool sample_available_;
+
   // Protects the process wide state below.
   static base::Mutex* mutex_;
   static int client_count_;
   static bool signal_handler_installed_;
   static struct sigaction old_signal_handler_;
+
+  friend class Sampler;
 };
 
 
+TickSample SignalHandler::sample_;
+volatile bool SignalHandler::called_from_get_sample_ = false;
+base::Semaphore* SignalHandler::sampling_semaphore_ = NULL;
+bool SignalHandler::sample_available_ = false;
+
 base::Mutex* SignalHandler::mutex_ = NULL;
 int SignalHandler::client_count_ = 0;
 struct sigaction SignalHandler::old_signal_handler_;
@@ -329,6 +374,11 @@ void SignalHandler::HandleProfilerSignal(int signal, siginfo_t* info,
   // is disabled.
   return;
 #else
+  // Even if we return prematurely,
+  // we need to signal the sampling_semaphore_
+  sample_available_ = false;
+  ScopedSemaphore scoped_semaphore(sampling_semaphore_);
+
   USE(info);
   if (signal != SIGPROF) return;
   Isolate* isolate = Isolate::UncheckedReentrantCurrent();
@@ -466,7 +516,12 @@ void SignalHandler::HandleProfilerSignal(int signal, siginfo_t* info,
 #endif  // V8_HOST_ARCH_*
 #endif  // V8_OS_QNX
 #endif  // USE_SIMULATOR
-  sampler->SampleStack(state);
+  if (called_from_get_sample_) {
+    sample_.Init(sampler->isolate(), state);
+  } else {
+    sampler->SampleStack(state);
+  }
+  sample_available_ = true;
 #endif  // V8_OS_NACL
 }
 
@@ -686,6 +741,15 @@ void Sampler::SampleStack(const RegisterState& state) {
 }
 
 
+void Sampler::CopyTickSampleToSample(TickSample* tick_sample,
+                                     v8::Sample* sample) {
+  for (int i = 0; i < tick_sample->frames_count; i++) {
+    sample->stack[i] = tick_sample->stack[i];
+  }
+  sample->frames_count = tick_sample->frames_count;
+}
+
+
 #if defined(USE_SIGNALS)
 
 void Sampler::DoSample() {
@@ -693,19 +757,35 @@ void Sampler::DoSample() {
   pthread_kill(platform_data()->vm_tid(), SIGPROF);
 }
 
+
+v8::Sample* Sampler::GetSample(v8::Sample* sample) {
+  if (!SignalHandler::Installed()) return NULL;
+  SignalHandler::called_from_get_sample_ = true;
+  pthread_kill(platform_data()->vm_tid(), SIGPROF);
+  SignalHandler::sampling_semaphore_->Wait();
+  if (SignalHandler::sample_available_) {
+    CopyTickSampleToSample(&SignalHandler::sample_, sample);
+  } else {
+    sample = NULL;
+  }
+  SignalHandler::called_from_get_sample_ = false;
+  return sample;
+}
+
 #elif V8_OS_WIN || V8_OS_CYGWIN
 
-void Sampler::DoSample() {
+void Sampler::GetSampleHelper(TickSample * sample,
+                              bool called_from_get_sample) {
   HANDLE profiled_thread = platform_data()->profiled_thread();
-  if (profiled_thread == NULL) return;
+  if (profiled_thread == NULL) return NULL;
 
 #if defined(USE_SIMULATOR)
   SimulatorHelper helper;
-  if (!helper.Init(this, isolate())) return;
+  if (!helper.Init(this, isolate())) return NULL;
 #endif
 
   const DWORD kSuspendFailed = static_cast<DWORD>(-1);
-  if (SuspendThread(profiled_thread) == kSuspendFailed) return;
+  if (SuspendThread(profiled_thread) == kSuspendFailed) return NULL;
 
   // Context used for sampling the register state of the profiled thread.
   CONTEXT context;
@@ -726,11 +806,31 @@ void Sampler::DoSample() {
     state.fp = reinterpret_cast<Address>(context.Ebp);
 #endif
 #endif  // USE_SIMULATOR
-    SampleStack(state);
+    if (called_from_get_sample) {
+      sample->Init(sampler->isolate(), state);
+    } else {
+      SampleStack(state);
+    }
   }
   ResumeThread(profiled_thread);
 }
 
+
+void Sampler::DoSample() {
+  GetSampleHelper(NULL, false);
+}
+
+
+v8::Sample* Sampler::GetSample(v8::Sample* sample) {
+  // We need this tick_sample because,
+  // GetSampleHelper calls Init, which must be called on
+  // a TickSample and never on Sample.
+  TickSample tick_sample;
+  GetSampleHelper(&tick_sample, true);
+  CopyTickSampleToSample(&tick_sample, sample);
+  return sample;
+}
+
 #endif  // USE_SIGNALS