Filter out undefined variables from locals()

You can imagine what happens if the variable is undefined and we
try to return it.

src/codegen/unwinding.cpp

@@ -28,6 +28,7 @@
 #include "codegen/compvars.h"
 #include "codegen/irgen/hooks.h"
 #include "codegen/stackmaps.h"
+#include "core/util.h"
 #include "runtime/types.h"
 
 
@@ -526,10 +527,37 @@ BoxedDict* getLocals(bool only_user_visible) {
             unsigned offset = ip - cf->code_start;
 
             assert(cf->location_map);
+
+            // We have to detect + ignore any entries for variables that
+            // could have been defined (so they have entries) but aren't (so the
+            // entries point to uninitialized memory).
+            std::unordered_set<std::string> is_undefined;
+
+            for (const auto& p : cf->location_map->names) {
+                if (!startswith(p.first, "!is_defined_"))
+                    continue;
+
+                for (const LocationMap::LocationTable::LocationEntry& e : p.second.locations) {
+                    if (e.offset < offset && offset <= e.offset + e.length) {
+                        const auto& locs = e.locations;
+
+                        assert(locs.size() == 1);
+                        uint64_t v = frame_info.readLocation(locs[0]);
+                        if ((v & 1) == 0)
+                            is_undefined.insert(p.first.substr(12));
+
+                        break;
+                    }
+                }
+            }
+
             for (const auto& p : cf->location_map->names) {
                 if (only_user_visible && (p.first[0] == '#' || p.first[0] == '!'))
                     continue;
 
+                if (is_undefined.count(p.first))
+                    continue;
+
                 for (const LocationMap::LocationTable::LocationEntry& e : p.second.locations) {
                     if (e.offset < offset && offset <= e.offset + e.length) {
                         const auto& locs = e.locations;
@@ -552,9 +580,8 @@ BoxedDict* getLocals(bool only_user_visible) {
             return d;
         } else if (frame_info.getId().type == PythonFrameId::INTERPRETED) {
             return localsForInterpretedFrame((void*)frame_info.getId().bp, only_user_visible);
-        } else {
-            abort();
         }
+        abort();
     }
     RELEASE_ASSERT(0, "Internal error: unable to find any python frames");
 }

test/tests/locals_test.py

@@ -31,11 +31,12 @@ def f3():
     print sorted(locals().items())
 f3()
 
-def f4():
+def f4(t):
     """testing synthetic 'is_defined' variables"""
-    if 0:
+    if t:
         x = 1
     else:
         y = 2
     print sorted(locals().items())
-# f4()
+f4(0)
+f4(1)