PlatformThread: add support for detached threads.

The change introduces support for detachable PlatformThreads, for which
the Stop() call doesn't wait until the thread has finished executing.

The change also introduces rtc::ThreadAttributes that carries priority
and detachability thread attributes. It additionally refactors all
known use to use the new semantics.

Bug: b:181572711, webrtc:12659
Change-Id: Id96e87c2a0dafabc8047767d241fd5da4505d14c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/214704
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33796}
diff --git a/rtc_base/platform_thread_unittest.cc b/rtc_base/platform_thread_unittest.cc
index a52e4cd..d09772f 100644
--- a/rtc_base/platform_thread_unittest.cc
+++ b/rtc_base/platform_thread_unittest.cc
@@ -10,7 +10,9 @@
 
 #include "rtc_base/platform_thread.h"
 
-#include "test/gtest.h"
+#include "rtc_base/event.h"
+#include "system_wrappers/include/sleep.h"
+#include "test/gmock.h"
 
 namespace rtc {
 namespace {
@@ -23,6 +25,11 @@
   *obj_as_bool = true;
 }
 
+void StdFunctionRunFunction(void* obj) {
+  std::function<void()>* fun = static_cast<std::function<void()>*>(obj);
+  (*fun)();
+}
+
 }  // namespace
 
 TEST(PlatformThreadTest, StartStop) {
@@ -58,4 +65,41 @@
   EXPECT_TRUE(flag);
 }
 
+TEST(PlatformThreadTest, JoinsThread) {
+  // This test flakes if there are problems with the join implementation.
+  EXPECT_TRUE(ThreadAttributes().joinable);
+  rtc::Event event;
+  std::function<void()> thread_function = [&] { event.Set(); };
+  PlatformThread thread(&StdFunctionRunFunction, &thread_function, "T");
+  thread.Start();
+  thread.Stop();
+  EXPECT_TRUE(event.Wait(/*give_up_after_ms=*/0));
+}
+
+TEST(PlatformThreadTest, StopsBeforeDetachedThreadExits) {
+  // This test flakes if there are problems with the detached thread
+  // implementation.
+  bool flag = false;
+  rtc::Event thread_started;
+  rtc::Event thread_continue;
+  rtc::Event thread_exiting;
+  std::function<void()> thread_function = [&] {
+    thread_started.Set();
+    thread_continue.Wait(Event::kForever);
+    flag = true;
+    thread_exiting.Set();
+  };
+  {
+    PlatformThread thread(&StdFunctionRunFunction, &thread_function, "T",
+                          ThreadAttributes().SetDetached());
+    thread.Start();
+    thread.Stop();
+  }
+  thread_started.Wait(Event::kForever);
+  EXPECT_FALSE(flag);
+  thread_continue.Set();
+  thread_exiting.Wait(Event::kForever);
+  EXPECT_TRUE(flag);
+}
+
 }  // namespace rtc