Unblock SIGPROF on flutter_tester start (#12813)

Fixes https://github.com/flutter/flutter/issues/35140

shell/testing/tester_main.cc

@@ -22,6 +22,10 @@
 #include "third_party/dart/runtime/include/bin/dart_io_api.h"
 #include "third_party/dart/runtime/include/dart_api.h"
 
+#if defined(OS_POSIX)
+#include <signal.h>
+#endif  // defined(OS_POSIX)
+
 namespace flutter {
 
 // Checks whether the engine's main Dart isolate has no pending work.  If so,
@@ -71,9 +75,31 @@ class ScriptCompletionTaskObserver {
   FML_DISALLOW_COPY_AND_ASSIGN(ScriptCompletionTaskObserver);
 };
 
+// Processes spawned via dart:io inherit their signal handling from the parent
+// process. As part of spawning, the spawner blocks signals temporarily, so we
+// need to explicitly unblock the signals we care about in the new process. In
+// particular, we need to unblock SIGPROF for CPU profiling to work on the
+// mutator thread in the main isolate in this process (threads spawned by the VM
+// know about this limitation and automatically have this signal unblocked).
+static void UnblockSIGPROF() {
+#if defined(OS_POSIX)
+  sigset_t set;
+  sigemptyset(&set);
+  sigaddset(&set, SIGPROF);
+  pthread_sigmask(SIG_UNBLOCK, &set, NULL);
+#endif  // defined(OS_POSIX)
+}
+
 int RunTester(const flutter::Settings& settings, bool run_forever) {
   const auto thread_label = "io.flutter.test";
 
+  // Necessary if we want to use the CPU profiler on the main isolate's mutator
+  // thread.
+  //
+  // OSX WARNING: avoid spawning additional threads before this call due to a
+  // kernel bug that may enable SIGPROF on an unintended thread in the process.
+  UnblockSIGPROF();
+
   fml::MessageLoop::EnsureInitializedForCurrentThread();
 
   auto current_task_runner = fml::MessageLoop::GetCurrent().GetTaskRunner();