Check in test script for circular references

From #1936

diff --git a/maint/circlerefs/circlerefs.py b/maint/circlerefs/circlerefs.py
new file mode 100644 (file)
index 0000000..0fb59c6
--- /dev/null
+++ b/maint/circlerefs/circlerefs.py
@@ -0,0 +1,100 @@
+#!/usr/bin/env python
+"""Test script to find circular references.
+
+Circular references are not leaks per se, because they will eventually
+be GC'd. However, on CPython, they prevent the reference-counting fast
+path from being used and instead rely on the slower full GC. This
+increases memory footprint and CPU overhead, so we try to eliminate
+circular references created by normal operation.
+"""
+from __future__ import print_function
+
+import gc
+import traceback
+import types
+from tornado import web, ioloop, gen, httpclient
+
+
+def find_circular_references(garbage=None):
+    def inner(level):
+        for item in level:
+            item_id = id(item)
+            if item_id not in garbage_ids:
+                continue
+            if item_id in visited_ids:
+                continue
+            if item_id in stack_ids:
+                candidate = stack[stack.index(item):]
+                candidate.append(item)
+                found.append(candidate)
+                continue
+
+            stack.append(item)
+            stack_ids.add(item_id)
+            inner(gc.get_referents(item))
+            stack.pop()
+            stack_ids.remove(item_id)
+            visited_ids.add(item_id)
+
+    garbage = garbage or gc.garbage
+    found = []
+    stack = []
+    stack_ids = set()
+    garbage_ids = set(map(id, garbage))
+    visited_ids = set()
+
+    inner(garbage)
+    inner = None
+    return found
+
+
+class CollectHandler(web.RequestHandler):
+    @gen.coroutine
+    def get(self):
+        self.write("Collected: {}\n".format(gc.collect()))
+        self.write("Garbage: {}\n".format(len(gc.garbage)))
+        for circular in find_circular_references():
+            print('\n==========\n Circular \n==========')
+            for item in circular:
+                print('    ', repr(item))
+            for item in circular:
+                if isinstance(item, types.FrameType):
+                    print('\nLocals:', item.f_locals)
+                    print('\nTraceback:', repr(item))
+                    traceback.print_stack(item)
+
+
+class DummyHandler(web.RequestHandler):
+    @gen.coroutine
+    def get(self):
+        self.write('ok\n')
+
+
+application = web.Application([
+    (r'/dummy/', DummyHandler),
+    (r'/collect/', CollectHandler),
+], debug=True)
+
+
+@gen.coroutine
+def main():
+    gc.disable()
+    gc.collect()
+    gc.set_debug(gc.DEBUG_STATS | gc.DEBUG_SAVEALL)
+    print('GC disabled')
+
+    print("Start on 8888")
+    application.listen(8888, '127.0.0.1')
+
+    # Do a little work. Alternately, could leave this script running and
+    # poke at it with a browser.
+    client = httpclient.AsyncHTTPClient()
+    yield client.fetch('http://127.0.0.1:8888/dummy/')
+
+    # Now report on the results.
+    gc.collect()
+    resp = yield client.fetch('http://127.0.0.1:8888/collect/')
+    print(resp.body)
+
+if __name__ == "__main__":
+    ioloop.IOLoop.current().run_sync(main)