Fix the profiler for threads spawned from the Linux eventhandler

runtime/vm/os_thread_linux.cc

Index: runtime/vm/os_thread_linux.cc
diff --git a/runtime/vm/os_thread_linux.cc b/runtime/vm/os_thread_linux.cc
index a0e68f1870e5b3222039a3b5467aea05d29cd214..ffcd2e67368d4c7bc4dc53acc956b07a8250fc5b 100644
--- a/runtime/vm/os_thread_linux.cc
+++ b/runtime/vm/os_thread_linux.cc
@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"  // NOLINT
+#include "platform/signal_blocker.h"  // NOLINT
+
 #if defined(TARGET_OS_LINUX)
 
 #include "vm/os_thread.h"
@@ -85,6 +87,21 @@ class ThreadStartData {
 };
 
 
+// Spawned threads inherit their spawner's signal mask. We sometimes spawn
+// threads for running Dart code from a thread that is blocking SIGPROF.
+// This function explicitly unblocks SIGPROF so the profiler continues to
+// sample this thread.
+static void UnblockSIGPROF() {
+  sigset_t set;
+  sigemptyset(&set);
+  sigaddset(&set, SIGPROF);
+  int r = pthread_sigmask(SIG_UNBLOCK, &set, NULL);
+  USE(r);
+  ASSERT(r == 0);
+  ASSERT(!CHECK_IS_BLOCKING(SIGPROF));
+}
+
+
 // Dispatch to the thread start function provided by the caller. This trampoline
 // is used to ensure that the thread is properly destroyed if the thread just
 // exits.
@@ -101,7 +118,7 @@ static void* ThreadStart(void* data_ptr) {
   if (thread != NULL) {
     OSThread::SetCurrent(thread);
     thread->set_name(name);
-
+    UnblockSIGPROF();
     // Call the supplied thread start function handing it its parameters.
     function(parameter);
   }