gdb: Be more cautious when search for containing closure

gdb/ghc_gdb/find_refs.py

@@ -153,28 +153,28 @@ def find_containing_closure(inferior: gdb.Inferior,
         bs = None
         try:
             bs = inferior.read_memory(start.addr(), word_size)
+            addr = Ptr.unpack(bs)
+            sym = find_symbol_name(addr)
+            # Is this an info table pointer?
+            if sym is not None and sym.endswith('_info'): # and sym.value == addr:
+                info = ghc_heap.get_itbl(gdb.parse_and_eval('(StgClosure *) %d' % start.addr()))
+                if int(info['type']) in closure.stack_frame_types:
+                    return find_containing_stack(inferior, addr)
+
+                nptrs = int(info['layout']['payload']['ptrs'])
+                #print(ptr, i, info, nptrs)
+                if int(info['type']) == closure.ClosureType.IND_STATIC:
+                    if ptr.addr() - start.addr() > 8:
+                        # Only trace the indirectee of IND_STATICs
+                        return None
+                elif i <= nptrs + 5: # A bit of fudge for the headers
+                    return start
+                else:
+                    print('suspicious info table: too far (field=%s, info@%s=%s, nptrs=%d, i=%d)' % (ptr, start, sym, nptrs, i))
+                    return None
         except gdb.MemoryError:
             continue
 
-        addr = Ptr.unpack(bs)
-        sym = find_symbol_name(addr)
-        # Is this an info table pointer?
-        if sym is not None and sym.endswith('_info'): # and sym.value == addr:
-            info = ghc_heap.get_itbl(gdb.parse_and_eval('(StgClosure *) %d' % start.addr()))
-            if int(info['type']) in closure.stack_frame_types:
-                return find_containing_stack(inferior, addr)
-
-            nptrs = int(info['layout']['payload']['ptrs'])
-            #print(ptr, i, info, nptrs)
-            if int(info['type']) == closure.ClosureType.IND_STATIC:
-                if ptr.addr() - start.addr() > 8:
-                    # Only trace the indirectee of IND_STATICs
-                    return None
-            elif i <= nptrs + 5: # A bit of fudge for the headers
-                return start
-            else:
-                print('suspicious info table: too far (field=%s, info@%s=%s, nptrs=%d, i=%d)' % (ptr, start, sym, nptrs, i))
-                return None
 
     return None