Have to be able to introspect interpreter frames that do no interpretation

ie due to reoptimizing the function and then calling into the compiled code.
We could handle this specific case by not treating these as "real" frames
that need to be introspected, but we have these kinds of situations in other
places (reopt from compiled code, osr from either kind) so I'd rather
find a more general improvement.

src/codegen/ast_interpreter.cpp

@@ -141,6 +141,13 @@ static std::unordered_map<void*, ASTInterpreter*> s_interpreterMap;
 
 Box* astInterpretFunction(CompiledFunction* cf, int nargs, Box* closure, Box* generator, Box* arg1, Box* arg2,
                           Box* arg3, Box** args) {
+    // The ASTInterpreter ctor registers this frame so that we can introspect it.
+    // Currently we have to do that even if we are about to immediately reopt
+    // and we won't actually end up interpreting anything.
+    // TODO don't make "astInterpretFunction() == interpreter-is-running"?
+    void* frame_addr = __builtin_frame_address(0);
+    ASTInterpreter interpreter(cf, frame_addr);
+
     if (unlikely(cf->times_called > REOPT_THRESHOLD)) {
         CompiledFunction* optimized = reoptCompiledFuncInternal(cf);
         if (closure && generator)
@@ -155,9 +162,6 @@ Box* astInterpretFunction(CompiledFunction* cf, int nargs, Box* closure, Box* ge
 
     ++cf->times_called;
 
-    void* frame_addr = __builtin_frame_address(0);
-    ASTInterpreter interpreter(cf, frame_addr);
-
     interpreter.initArguments(nargs, (BoxedClosure*)closure, (BoxedGenerator*)generator, arg1, arg2, arg3, args);
     Value v = interpreter.execute();
 
@@ -171,6 +175,9 @@ const LineInfo* getLineInfoForInterpretedFrame(void* frame_ptr) {
 }
 
 LineInfo* ASTInterpreter::getCurrentLineInfo() {
+    if (!current_inst)
+        return NULL;
+
     LineInfo* line_info = new LineInfo(current_inst->lineno, current_inst->col_offset, source_info->parent_module->fn,
                                        source_info->getName());
     return line_info;

src/codegen/unwinding.cpp

@@ -384,7 +384,9 @@ std::vector<const LineInfo*> getTracebackEntries() {
     std::vector<const LineInfo*> entries;
 
     for (auto& frame_info : unwindPythonFrames()) {
-        entries.push_back(lineInfoForFrame(frame_info));
+        const LineInfo* line_info = lineInfoForFrame(frame_info);
+        if (line_info)
+            entries.push_back(line_info);
     }
 
     std::reverse(entries.begin(), entries.end());