py: Add mp_pending_exception global variable, for VM soft interrupt.

This allows to implement KeyboardInterrupt on unix, and a much safer
ctrl-C in stmhal port.  First ctrl-C is a soft one, with hope that VM
will notice it; second ctrl-C is a hard one that kills anything (for
both unix and stmhal).

One needs to check for a pending exception in the VM only for jump
opcodes.  Others can't produce an infinite loop (infinite recursion is
caught by stack check).

3 files changed, 17 insertions, 7 deletions

diff --git a/stmhal/pendsv.c b/stmhal/pendsv.c
index a0eff7e5d..e71836d01 100644
--- a/stmhal/pendsv.c
+++ b/stmhal/pendsv.c
@@ -31,6 +31,7 @@
 #include "misc.h"
 #include "qstr.h"
 #include "obj.h"
+#include "runtime.h"
 #include "pendsv.h"
 
 static void *pendsv_object = NULL;
@@ -40,12 +41,22 @@ void pendsv_init(void) {
     HAL_NVIC_SetPriority(PendSV_IRQn, 0xf, 0xf);
 }
 
-// call this function to raise a pending exception during an interrupt
-// it will wait until all interrupts are finished then raise the given
-// exception object using nlr_jump in the context of the top-level thread
+// Call this function to raise a pending exception during an interrupt.
+// It will first try to raise the exception "softly" by setting the
+// mp_pending_exception variable and hoping that the VM will notice it.
+// If this function is called a second time (ie with the mp_pending_exception
+// variable already set) then it will force the exception by using the hardware
+// PENDSV feature.  This will wait until all interrupts are finished then raise
+// the given exception object using nlr_jump in the context of the top-level
+// thread.
 void pendsv_nlr_jump(void *o) {
-    pendsv_object = o;
-    SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
+    if (mp_pending_exception == MP_OBJ_NULL) {
+        mp_pending_exception = o;
+    } else {
+        mp_pending_exception = MP_OBJ_NULL;
+        pendsv_object = o;
+        SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
+    }
 }
 
 // since we play tricks with the stack, the compiler must not generate a
diff --git a/stmhal/usb.c b/stmhal/usb.c
index c522b6443..4211c8829 100644
--- a/stmhal/usb.c
+++ b/stmhal/usb.c
@@ -50,7 +50,7 @@ STATIC mp_obj_t mp_const_vcp_interrupt = MP_OBJ_NULL;
 
 void pyb_usb_init0(void) {
     // create an exception object for interrupting by VCP
-    mp_const_vcp_interrupt = mp_obj_new_exception_msg(&mp_type_OSError, "VCPInterrupt");
+    mp_const_vcp_interrupt = mp_obj_new_exception(&mp_type_KeyboardInterrupt);
     USBD_CDC_SetInterrupt(VCP_CHAR_NONE, mp_const_vcp_interrupt);
 }
 
diff --git a/stmhal/usbd_cdc_interface.c b/stmhal/usbd_cdc_interface.c
index d43795550..28f06067b 100644
--- a/stmhal/usbd_cdc_interface.c
+++ b/stmhal/usbd_cdc_interface.c
@@ -374,7 +374,6 @@ static int8_t CDC_Itf_Receive(uint8_t* Buf, uint32_t *Len) {
 
         if (char_found) {
             // raise exception when interrupts are finished
-            user_interrupt_char = VCP_CHAR_NONE;
             pendsv_nlr_jump(user_interrupt_data);
         }