fix races in CrashGenerator::CreateChildCrash

The current CreateChildCrash logic is racy when it comes to creating a
crash dump for two reasons:

The main thread that calls kill() on a different thread is guaranteed
the signal will be *queued* when it returns, but not *delivered*.  If
the kernel doesn't automatically schedule the receiving thread, but
instead lets the main thread run to the exit() call, then the signal
never triggers a coredump and the whole process simply exits.

The main thread is using kill() to try to deliver a signal to a
specific thread, but that function is for sending signals to a
process.  That means the kernel is free to deliver the signal to
any thread in the process and not just the one requested.  This
manifests itself as the pr_pid in the coredump not being the one
expected.  Instead, we must use tkill() with the tid (which we
already took care of gathering) to deliver to a specific thread.

These are a lot easier to see on a UMP system as contention is heavier.

BUG=chromium:207918
TEST=`dumper_unittest` still passes, and doesn't flake out in a UMP system
TEST=`linux_client_unittest` still passes
R=benchan@chromium.org

Review URL: https://breakpad.appspot.com/1304005

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1299 4c0a9323-5329-0410-9bdc-e9ce6186880e

1 files changed, 2 insertions, 8 deletions

diff --git a/src/common/linux/elf_core_dump_unittest.cc b/src/common/linux/elf_core_dump_unittest.cc
index 45aa5364..7c122aa6 100644
--- a/src/common/linux/elf_core_dump_unittest.cc
+++ b/src/common/linux/elf_core_dump_unittest.cc
@@ -138,14 +138,8 @@ TEST(ElfCoreDumpTest, ValidCoreFile) {
   const unsigned kNumOfThreads = 3;
   const unsigned kCrashThread = 1;
   const int kCrashSignal = SIGABRT;
-  // TODO(benchan): Revert to use ASSERT_TRUE once the flakiness in
-  // CrashGenerator is identified and fixed.
-  if (!crash_generator.CreateChildCrash(kNumOfThreads, kCrashThread,
-                                        kCrashSignal, NULL)) {
-    fprintf(stderr, "ElfCoreDumpTest.ValidCoreFile test is skipped "
-            "due to no core dump generated");
-    return;
-  }
+  ASSERT_TRUE(crash_generator.CreateChildCrash(kNumOfThreads, kCrashThread,
+                                               kCrashSignal, NULL));
   pid_t expected_crash_thread_id = crash_generator.GetThreadId(kCrashThread);
   set<pid_t> expected_thread_ids;
   for (unsigned i = 0; i < kNumOfThreads; ++i) {