app: add spdk_app_start_shutdown()

This enables using SPDK within a larger process that
is SPDK-centric.  In this case the process may start
SPDK and then wish to stop it explicitly (without a
signal).

While here, remove an incorrect comment - DPDK mempools
can be used from non-DPDK threads.  Also set the
g_shutdown_event to NULL after it is called.  After the
event executes, the event is freed and is no longer valid.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ie4f07bee7d05fae683c72f6680cb3bcce2d4a119

include/spdk/event.h

@@ -167,6 +167,13 @@ int spdk_app_fini(void);
 */
 int spdk_app_start(spdk_event_fn start_fn, void *arg1, void *arg2);
 
+/**
+ * \brief Start shutting down the framework.  Typically this function is not called directly, and
+ * the shutdown process is started implicitly by a process signal.  But in applications that are
+ * using SPDK for a subset of its process threads, this function can be called in lieu of a signal.
+ */
+void spdk_app_start_shutdown(void);
+
 /**
  * \brief Stop the framework. This does not wait for all threads to exit. Instead, it kicks off
  * the shutdown process and returns. Once the shutdown process is complete, \ref spdk_app_start will return.

lib/event/app.c

@@ -189,20 +189,21 @@ spdk_get_log_facility(struct spdk_conf *config)
 	return logfacility;
 }
 
-static void
-__shutdown_signal(int signo)
+void
+spdk_app_start_shutdown(void)
 {
-	/*
-	 * Call pre-allocated shutdown event.  Note that it is not
-	 *  safe to allocate the event within the signal handlers
-	 *  context, since that context is not a DPDK thread so
-	 *  buffer allocation is not permitted.
-	 */
 	if (g_shutdown_event != NULL) {
 		spdk_event_call(g_shutdown_event);
+		g_shutdown_event = NULL;
 	}
 }
 
+static void
+__shutdown_signal(int signo)
+{
+	spdk_app_start_shutdown();
+}
+
 static void
 __shutdown_event_cb(spdk_event_t event)
 {