bpo-13487: Use sys.modules.copy() in inspect.getmodule() for thread safety. (GH-18786)

gpshead · web-flow · commit 85cf1d514b84 · 2020-03-04T16:45:22.000-08:00
`list(sys.modules.items())` was apparently not immune to "dictionary
changed size during iteration" errors.

Tested internally using an integration test that has run into this a couple of times in the past two years.  With this patch applied, the test is no longer flaky.
diff --git a/Lib/inspect.py b/Lib/inspect.py
@@ -741,7 +741,7 @@ def getmodule(object, _filename=None):
         return sys.modules.get(modulesbyfile[file])
     # Update the filename to module name cache and check yet again
     # Copy sys.modules in order to cope with changes while iterating
-    for modname, module in list(sys.modules.items()):
+    for modname, module in sys.modules.copy().items():
         if ismodule(module) and hasattr(module, '__file__'):
             f = module.__file__
             if f == _filesbymodname.get(modname, None):
diff --git a/Misc/NEWS.d/next/Library/2020-03-04-16-10-59.bpo-13487.gqe4Fb.rst b/Misc/NEWS.d/next/Library/2020-03-04-16-10-59.bpo-13487.gqe4Fb.rst
@@ -0,0 +1,3 @@
+Avoid a possible *"RuntimeError: dictionary changed size during iteration"*
+from :func:`inspect.getmodule` when it tried to loop through
+:attr:`sys.modules`.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+Avoid a possible "RuntimeError: dictionary changed size during iteration"`
	`2`	+from :func:`inspect.getmodule` when it tried to loop through
	`3`	+:attr:`sys.modules`.