Add an optional watchdog timer (set vm.watchdog and build with -DENABLE_WATCHDOG)

vm.c

@@ -2363,6 +2363,7 @@ void krk_initVM(int flags) {
 	krk_initTable(&vm.globals);
 	krk_initTable(&vm.strings);
 	memset(vm.specialMethodNames,0,sizeof(vm.specialMethodNames));
+	vm.watchdog = 0;
 
 	/* To make lookup faster, store these so we can don't have to keep boxing
 	 * and unboxing, copying/hashing etc. */
@@ -2968,6 +2969,20 @@ static KrkValue run() {
 		}
 #endif
 
+#ifdef ENABLE_WATCHDOG
+		if (vm.watchdog - 1 == 0) {
+			fprintf(stderr, "Watchdog timer tripped.\n\n");
+			krk_dumpTraceback();
+			krk_resetStack();
+			fprintf(stderr, "\n\n");
+			vm.watchdog = 0;
+			exit(0);
+			return NONE_VAL();
+		} else if (vm.watchdog > 0) {
+			vm.watchdog--;
+		}
+#endif
+
 		uint8_t opcode = READ_BYTE();
 
 		/* We split the instruction opcode table in half and use the top bit

vm.h

@@ -97,6 +97,7 @@ typedef struct {
 
 	KrkValue currentException;
 	int flags;
+	long watchdog;
 } KrkVM;
 
 #define KRK_ENABLE_TRACING      (1 << 0)