Bug 1598992 - Stop Base Profiler before starting Gecko Profiler - r=gregtatum

This ensures that no more Base Profiler (BP) activity can happen when Gecko
Profiler (GP) starts.
In particular on Linux this is needed because the BP sampler sends signals to
stop threads, and the just-starting GP could receive this signal before it is
fully ready to handle it with its own sampler.

Differential Revision: https://phabricator.services.mozilla.com/D54443

--HG--
extra : moz-landing-system : lando

tools/profiler/core/platform.cpp

@@ -4025,6 +4025,33 @@ static void locked_profiler_start(PSLockRef aLock, PowerOfTwo32 aCapacity,
 
   MOZ_RELEASE_ASSERT(CorePS::Exists() && !ActivePS::Exists(aLock));
 
+#ifdef MOZ_BASE_PROFILER
+  UniquePtr<char[]> baseprofile;
+  if (baseprofiler::profiler_is_active()) {
+    // Note that we still hold the lock, so the sampler cannot run yet and
+    // interact negatively with the still-active BaseProfiler sampler.
+    // Assume that Base Profiler is active because of MOZ_BASE_PROFILER_STARTUP.
+    // Capture the Base Profiler startup profile threads (if any).
+    baseprofile = baseprofiler::profiler_get_profile(
+        /* aSinceTime */ 0, /* aIsShuttingDown */ false,
+        /* aOnlyThreads */ true);
+
+    // Now stop Base Profiler (BP), as further recording will be ignored anyway,
+    // and so that it won't clash with Gecko Profiler (GP) sampling starting
+    // after the lock is dropped.
+    // On Linux this is especially important to do before creating the GP
+    // sampler, because the BP sampler may send a signal (to stop threads to be
+    // sampled), which the GP would intercept before its own initialization is
+    // complete and ready to handle such signals.
+    // Note that even though `profiler_stop()` doesn't immediately destroy and
+    // join the sampler thread, it safely deactivates it in such a way that the
+    // thread will soon exit without doing any actual work.
+    // TODO: Allow non-sampling profiling to continue.
+    // TODO: Re-start BP after GP shutdown, to capture post-XPCOM shutdown.
+    baseprofiler::profiler_stop();
+  }
+#endif
+
 #if defined(GP_PLAT_amd64_windows)
   InitializeWin64ProfilerHooks();
 #endif
@@ -4049,31 +4076,13 @@ static void locked_profiler_start(PSLockRef aLock, PowerOfTwo32 aCapacity,
   MOZ_ASSERT(ActivePS::Exists(aLock));
 
 #ifdef MOZ_BASE_PROFILER
-  if (baseprofiler::profiler_is_active()) {
-    // Note that we still hold the lock, so the sampler cannot run yet and
-    // interact negatively with the still-active BaseProfiler sampler.
-    // Assume that BaseProfiler is active because of MOZ_BASE_PROFILER_STARTUP.
-    // Capture the BaseProfiler startup profile threads (if any).
-    UniquePtr<char[]> baseprofile = baseprofiler::profiler_get_profile(
-        /* aSinceTime */ 0, /* aIsShuttingDown */ false,
-        /* aOnlyThreads */ true);
-
-    // Now stop BaseProfiler, as further recording will be ignored anyway, and
-    // so that it won't clash with Gecko Profiler sampling starting after the
-    // lock is dropped.
-    // TODO: Allow non-sampling profiling to continue.
-    // TODO: Re-start BaseProfiler after Gecko Profiler shutdown, to capture
-    // post-XPCOM shutdown.
-    baseprofiler::profiler_stop();
-
-    // An "empty" profile string may in fact contain 1 character (a newline),
-    // so we want at least 2 characters to register a profile.
-    if (HasMinimumLength(baseprofile.get(), 2)) {
-      // The BaseProfiler startup profile will be stored as a separate "process"
-      // in the Gecko Profiler profile, and shown as a new track under the
-      // corresponding Gecko Profiler thread.
-      ActivePS::AddBaseProfileThreads(aLock, std::move(baseprofile));
-    }
+  // An "empty" profile string may in fact contain 1 character (a newline), so
+  // we want at least 2 characters to register a profile.
+  if (HasMinimumLength(baseprofile.get(), 2)) {
+    // The BaseProfiler startup profile will be stored as a separate "process"
+    // in the Gecko Profiler profile, and shown as a new track under the
+    // corresponding Gecko Profiler thread.
+    ActivePS::AddBaseProfileThreads(aLock, std::move(baseprofile));
   }
 #endif