py/scheduler: Implement VM abort flag and mp_sched_vm_abort().

This is intended to be used by the very outer caller of the VM/runtime.  It
allows setting a top-level NLR handler that can be jumped to directly, in
order to forcefully abort the VM/runtime.

Enable using:

    #define MICROPY_ENABLE_VM_ABORT (1)

Set up the handler at the top level using:

    nlr_buf_t nlr;
    nlr.ret_val = NULL;
    if (nlr_push(&nlr) == 0) {
        nlr_set_abort(&nlr);
        // call into the VM/runtime
        ...
        nlr_pop();
    } else {
        if (nlr.ret_val == NULL) {
            // handle abort
            ...
        } else {
            // handle other exception that propagated to the top level
            ...
        }
    }
    nlr_set_abort(NULL);

Schedule an abort, eg from an interrupt handler, using:

    mp_sched_vm_abort();

Signed-off-by: Damien George <damien@micropython.org>

py/mpconfig.h

@@ -909,6 +909,11 @@ typedef double mp_float_t;
 #define MICROPY_INTERNAL_PRINTF_PRINTER (&mp_plat_print)
 #endif
 
+// Whether to support mp_sched_vm_abort to asynchronously abort to the top level.
+#ifndef MICROPY_ENABLE_VM_ABORT
+#define MICROPY_ENABLE_VM_ABORT (0)
+#endif
+
 // Support for internal scheduler
 #ifndef MICROPY_ENABLE_SCHEDULER
 #define MICROPY_ENABLE_SCHEDULER (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)
@@ -1740,6 +1745,10 @@ typedef double mp_float_t;
 #define MICROPY_WRAP_MP_SCHED_SCHEDULE(f) f
 #endif
 
+#ifndef MICROPY_WRAP_MP_SCHED_VM_ABORT
+#define MICROPY_WRAP_MP_SCHED_VM_ABORT(f) f
+#endif
+
 /*****************************************************************************/
 /* Miscellaneous settings                                                    */
 

py/mpstate.h

@@ -226,6 +226,11 @@ typedef struct _mp_state_vm_t {
     uint8_t sched_idx;
     #endif
 
+    #if MICROPY_ENABLE_VM_ABORT
+    bool vm_abort;
+    nlr_buf_t *nlr_abort;
+    #endif
+
     #if MICROPY_PY_THREAD_GIL
     // This is a global mutex used to make the VM/runtime thread-safe.
     mp_thread_mutex_t gil_mutex;

py/nlr.c

@@ -49,3 +49,10 @@ void nlr_pop(void) {
     nlr_buf_t **top = &MP_STATE_THREAD(nlr_top);
     *top = (*top)->prev;
 }
+
+#if MICROPY_ENABLE_VM_ABORT
+NORETURN void nlr_jump_abort(void) {
+    MP_STATE_THREAD(nlr_top) = MP_STATE_VM(nlr_abort);
+    nlr_jump(NULL);
+}
+#endif

py/nlr.h

@@ -101,9 +101,16 @@
 
 typedef struct _nlr_buf_t nlr_buf_t;
 struct _nlr_buf_t {
-    // the entries here must all be machine word size
+    // The entries in this struct must all be machine word size.
+
+    // Pointer to the previous nlr_buf_t in the chain.
+    // Or NULL if it's the top-level one.
     nlr_buf_t *prev;
-    void *ret_val; // always a concrete object (an exception instance)
+
+    // The exception that is being raised:
+    // - NULL means the jump is because of a VM abort (only if MICROPY_ENABLE_VM_ABORT enabled)
+    // - otherwise it's always a concrete object (an exception instance)
+    void *ret_val;
 
     #if MICROPY_NLR_SETJMP
     jmp_buf jmpbuf;
@@ -149,6 +156,12 @@ unsigned int nlr_push_tail(nlr_buf_t *top);
 void nlr_pop(void);
 NORETURN void nlr_jump(void *val);
 
+#if MICROPY_ENABLE_VM_ABORT
+#define nlr_set_abort(buf) MP_STATE_VM(nlr_abort) = buf
+#define nlr_get_abort() MP_STATE_VM(nlr_abort)
+NORETURN void nlr_jump_abort(void);
+#endif
+
 // This must be implemented by a port.  It's called by nlr_jump
 // if no nlr buf has been pushed.  It must not return, but rather
 // should bail out with a fatal error.

py/runtime.h

@@ -75,6 +75,9 @@ void mp_deinit(void);
 
 void mp_sched_exception(mp_obj_t exc);
 void mp_sched_keyboard_interrupt(void);
+#if MICROPY_ENABLE_VM_ABORT
+void mp_sched_vm_abort(void);
+#endif
 void mp_handle_pending(bool raise_exc);
 
 #if MICROPY_ENABLE_SCHEDULER

py/scheduler.c

@@ -51,6 +51,12 @@ void MICROPY_WRAP_MP_SCHED_KEYBOARD_INTERRUPT(mp_sched_keyboard_interrupt)(void)
 }
 #endif
 
+#if MICROPY_ENABLE_VM_ABORT
+void MICROPY_WRAP_MP_SCHED_VM_ABORT(mp_sched_vm_abort)(void) {
+    MP_STATE_VM(vm_abort) = true;
+}
+#endif
+
 #if MICROPY_ENABLE_SCHEDULER
 
 #define IDX_MASK(i) ((i) & (MICROPY_SCHEDULER_DEPTH - 1))
@@ -203,6 +209,17 @@ MP_REGISTER_ROOT_POINTER(mp_sched_item_t sched_queue[MICROPY_SCHEDULER_DEPTH]);
 // Called periodically from the VM or from "waiting" code (e.g. sleep) to
 // process background tasks and pending exceptions (e.g. KeyboardInterrupt).
 void mp_handle_pending(bool raise_exc) {
+    // Handle pending VM abort.
+    #if MICROPY_ENABLE_VM_ABORT
+    if (MP_STATE_VM(vm_abort) && mp_thread_is_main_thread()) {
+        MP_STATE_VM(vm_abort) = false;
+        if (raise_exc && nlr_get_abort() != NULL) {
+            nlr_jump_abort();
+        }
+    }
+    #endif
+
+    // Handle any pending exception.
     if (MP_STATE_THREAD(mp_pending_exception) != MP_OBJ_NULL) {
         mp_uint_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
         mp_obj_t obj = MP_STATE_THREAD(mp_pending_exception);
@@ -215,6 +232,8 @@ void mp_handle_pending(bool raise_exc) {
         }
         MICROPY_END_ATOMIC_SECTION(atomic_state);
     }
+
+    // Handle any pending callbacks.
     #if MICROPY_ENABLE_SCHEDULER
     if (MP_STATE_VM(sched_state) == MP_SCHED_PENDING) {
         mp_sched_run_pending();

py/vm.c

@@ -1333,6 +1333,10 @@ pending_exception_check:
                     // No scheduler: Just check pending exception.
                     MP_STATE_THREAD(mp_pending_exception) != MP_OBJ_NULL
                 #endif
+                #if MICROPY_ENABLE_VM_ABORT
+                    // Check if the VM should abort execution.
+                    || MP_STATE_VM(vm_abort)
+                #endif
                 ) {
                     MARK_EXC_IP_SELECTIVE();
                     mp_handle_pending(true);