Add |joinable| to Thread::Options

base/threading/thread.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_THREADING_THREAD_H_
 #define BASE_THREADING_THREAD_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 56 matching lines @@
     // MessageLoop::Type to TYPE_CUSTOM.
     MessagePumpFactory message_pump_factory;
 
     // Specifies the maximum stack size that the thread is allowed to use.
     // This does not necessarily correspond to the thread's initial stack size.
     // A value of 0 indicates that the default maximum should be used.
     size_t stack_size = 0;
 
     // Specifies the initial thread priority.
     ThreadPriority priority = ThreadPriority::NORMAL;
+
+    // If false, the thread will not be joined on destruction. This is intended
+    // for threads that want TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN
+    // semantics. Stop() will not be synchronous and will instead merely have
+    // StopSoon() semantics on such threads.
+    bool joinable = true;
   };
 
   // Constructor.
   // name is a display string to identify the thread.
   explicit Thread(const std::string& name);
 
   // Destroys the thread, stopping it if necessary.
   //
   // NOTE: ALL SUBCLASSES OF Thread MUST CALL Stop() IN THEIR DESTRUCTORS (or
   // guarantee Stop() is explicitly called before the subclass is destroyed).
@@ skipping 37 matching lines @@
   // WaitUntilThreadStarted().
   // Note that using this (instead of Start() or StartWithOptions() causes
   // jank on the calling thread, should be used only in testing code.
   bool StartAndWaitForTesting();
 
   // Blocks until the thread starts running. Called within StartAndWait().
   // Note that calling this causes jank on the calling thread, must be used
   // carefully for production code.
   bool WaitUntilThreadStarted() const;
 
-  // Signals the thread to exit and returns once the thread has exited.  After
-  // this method returns, the Thread object is completely reset and may be used
-  // as if it were newly constructed (i.e., Start may be called again).
+  // Signals the thread to exit and returns once the thread has exited (or right
+  // away if the thread is non-joinable).  For joinable threads only: after this
+  // method returns, the Thread object is completely reset and may be used as if
+  // it were newly constructed (i.e., Start may be called again) -- non-joinable
+  // threads are not re-usable.

[danakj, 2016/07/25 18:32:53]

Is there some DCHECK that will fire if you try to reuse it after calling Stop?

[gab, 2016/07/25 19:18:29]

Yes, stopping_ is never reset when not joining (remains |true| forever) and
there's a new DCHECK in StartWithOptions() to catch that.

I also just added a death test for this to confirm it works (and prevent
regressions).

   //
   // Stop may be called multiple times and is simply ignored if the thread is
-  // already stopped.
+  // already stopped or currently stopping.
   //
   // NOTE: If you are a consumer of Thread, it is not necessary to call this
   // before deleting your Thread objects, as the destructor will do it.
   // IF YOU ARE A SUBCLASS OF Thread, YOU MUST CALL THIS IN YOUR DESTRUCTOR.
   void Stop();
 
   // Signals the thread to exit in the near future.
   //
   // WARNING: This function is not meant to be commonly used. Use at your own
   // risk. Calling this function will cause message_loop() to become invalid in
@@ skipping 138 matching lines @@
   // This class is not thread-safe, use this to verify access from the owning
   // sequence of the Thread.
   SequenceChecker owning_sequence_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_H_