kernel: Treat aborting by main() as a fatal system error

An application-supplied main() routine is now considered to be
essential to system operation. Thus, if main() experiences an
error that aborts the main thread a fatal system error is raised.

Note: If main() completes its work and does a standard return-
to-caller the main thread terminates normally.

Change-Id: Icc9499f13578299244a856a246ad2a7d34a72f54
Signed-off-by: Allan Stephens <[email protected]>

doc/kernel_v2/threads/system_threads.rst

@@ -17,20 +17,18 @@ The kernel spawns the following system threads.
 
 **Main thread**
     This thread performs kernel initialization, then calls the application's
-    :cpp:func:`main()` function. If the application does not supply a
-    :cpp:func:`main()` function, the main thread terminates once initialization
-    is complete.
+    :cpp:func:`main()` function (if one is defined).
 
     By default, the main thread uses the highest configured preemptible thread
     priority (i.e. 0). If the kernel is not configured to support preemptible
     threads, the main thread uses the lowest configured cooperative thread
     priority (i.e. -1).
 
     The main thread is an essential thread while it is performing kernel
-    initialization, which means a fatal system error is raised if the thread
-    aborts. Once initialization is complete the thread is considered
-    non-essential, so the termination or aborting of a :cpp:func:`main()`
-    function supplied by the application does not cause a fatal system error.
+    initialization or executing the application's :cpp:func:`main()` function;
+    this means a fatal system error is raised if the thread aborts. If
+    :cpp:func:`main()` is not defined, or if it executes and then does a normal
+    return, the main thread terminates normally and no error is raised.
 
 **Idle thread**
     This thread executes when there is no other work for the system to do.

kernel/unified/init.c

@@ -200,8 +200,6 @@ static void _main(void *unused1, void *unused2, void *unused3)
 
 	_init_static_threads();
 
-	_main_thread->flags &= ~K_ESSENTIAL;
-
 #ifdef CONFIG_BOOT_TIME_MEASUREMENT
 	/* record timestamp for kernel's _main() function */
 	extern uint64_t __main_tsc;
@@ -219,6 +217,9 @@ static void _main(void *unused1, void *unused2, void *unused3)
 	k_thread_priority_set(_main_thread, MDEF_MAIN_THREAD_PRIORITY);
 #endif
 	main();
+
+	/* Terminate thread normally since it has no more work to do */
+	_main_thread->flags &= ~K_ESSENTIAL;
 }
 
 void __weak main(void)