code coverage: Fix writing profiles from child processes.

Notable changes:
. Introduce CLANG_COVERAGE define if use_clang_coverage gn arg is set.
. For child processes to be able to correctly write the coverage profile
  during teardown, it is necessary to allow them to shut-down cleanly.
  So avoid terminating the processes from ~ChildProcessLauncher() when
  CLANG_COVERAGE is set.
. Child processes can do fast-shutdown by calling _exit(0). So force the
  profile file to be written from the child process before calling that.

With both changes, it is still necessary to run with --no-sandbox, since
otherwise the child processes are not able to write to the profile file.

BUG=838582, 834781

Change-Id: I742521e756a7dead983f40462798f7c4dac2ac02
Reviewed-on: https://chromium-review.googlesource.com/1041271Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556236}

build/config/BUILD.gn

@@ -6,6 +6,7 @@ import("//build/config/allocator.gni")
 import("//build/config/c++/c++.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
+import("//build/config/coverage/coverage.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
@@ -118,6 +119,9 @@ config("feature_flags") {
   if (is_ubsan || is_ubsan_null || is_ubsan_vptr || is_ubsan_security) {
     defines += [ "UNDEFINED_SANITIZER" ]
   }
+  if (use_clang_coverage) {
+    defines += [ "CLANG_COVERAGE" ]
+  }
   if (safe_browsing_mode == 1) {
     defines += [ "FULL_SAFE_BROWSING" ]
     defines += [ "SAFE_BROWSING_CSD" ]

content/browser/child_process_launcher.cc

@@ -35,7 +35,7 @@ ChildProcessLauncher::ChildProcessLauncher(
       start_time_(base::TimeTicks::Now()),
 #if defined(ADDRESS_SANITIZER) || defined(LEAK_SANITIZER) ||  \
     defined(MEMORY_SANITIZER) || defined(THREAD_SANITIZER) || \
-    defined(UNDEFINED_SANITIZER)
+    defined(UNDEFINED_SANITIZER) || defined(CLANG_COVERAGE)
       terminate_child_on_shutdown_(false),
 #else
       terminate_child_on_shutdown_(terminate_on_shutdown),

content/child/child_thread_impl.cc

@@ -74,9 +74,19 @@
 #include "content/public/common/content_descriptors.h"
 #endif
 
+#if defined(CLANG_COVERAGE)
+extern "C" int __llvm_profile_write_file(void);
+#endif
+
 namespace content {
 namespace {
 
+void WriteClangCoverageProfile() {
+#if defined(CLANG_COVERAGE)
+  __llvm_profile_write_file();
+#endif
+}
+
 // How long to wait for a connection to the browser process before giving up.
 const int kConnectionTimeoutS = 15;
 
@@ -100,6 +110,7 @@ class WaitAndExitDelegate : public base::PlatformThread::Delegate {
 
   void ThreadMain() override {
     base::PlatformThread::Sleep(duration_);
+    WriteClangCoverageProfile();
     _exit(0);
   }
 
@@ -160,6 +171,7 @@ class SuicideOnChannelErrorFilter : public IPC::MessageFilter {
     __lsan_do_leak_check();
 #endif
 #else
+    WriteClangCoverageProfile();
     _exit(0);
 #endif
   }
@@ -620,8 +632,10 @@ ChildThreadImpl::~ChildThreadImpl() {
 }
 
 void ChildThreadImpl::Shutdown() {
-  // Delete objects that hold references to blink so derived classes can
-  // safely shutdown blink in their Shutdown implementation.
+  // The renderer process (and others) can to fast shutdown by calling _exit(0),
+  // in which case the clang-coverage profile does not get written to the file.
+  // So force write the profile here before shutting down.
+  WriteClangCoverageProfile();
 }
 
 bool ChildThreadImpl::ShouldBeDestroyed() {