task: concurrency cleanups

Update thread_local task state pointers while locked.  This avoids
potential concurrent access of the pointers while making copies of them.

Verify that the queue is really empty after splicing lists, and the
current consumer is really gone after swapping the empty one.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/task.cc

@@ -212,6 +212,7 @@ namespace crucible {
 				TaskMasterState::push_front(tq_one);
 			}
 		}
+		assert(queue.empty());
 	}
 
 	TaskState::~TaskState()
@@ -292,23 +293,23 @@ namespace crucible {
 			--m_run_count;
 			m_is_running = true;
 		}
+
+		TaskStatePtr this_task = shared_from_this();
+		swap(this_task, tl_current_task);
 		lock.unlock();
 
 		char buf[24] = { 0 };
 		DIE_IF_MINUS_ERRNO(pthread_getname_np(pthread_self(), buf, sizeof(buf)));
 		DIE_IF_MINUS_ERRNO(pthread_setname_np(pthread_self(), m_title.c_str()));
 
-		TaskStatePtr this_task = shared_from_this();
-		swap(this_task, tl_current_task);
-
 		catch_all([&]() {
 			m_exec_fn();
 		});
 
-		swap(this_task, tl_current_task);
 		pthread_setname_np(pthread_self(), buf);
 
 		lock.lock();
+		swap(this_task, tl_current_task);
 		m_is_running = false;
 
 		// Splice task post_exec queue at front of local queue
@@ -755,6 +756,7 @@ namespace crucible {
 		// There is no longer a current consumer, but hold our own shared
 		// state so it's still there in the destructor
 		swap(this_consumer, tl_current_consumer);
+		assert(!tl_current_consumer);
 
 		// Release lock to rescue queue (may attempt to queue a new task at TaskMaster).
 		// rescue_queue normally sends tasks to the local queue of the current TaskConsumer thread,