From 85cf1d514b84dc9a4bcb40e20a12e1d82ff19f20 Mon Sep 17 00:00:00 2001
From: "Gregory P. Smith" <gps@google.com>
Date: Wed, 4 Mar 2020 16:45:22 -0800
Subject: [PATCH] bpo-13487: Use sys.modules.copy() in inspect.getmodule() for
 thread safety. (GH-18786)

`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.
---
 Lib/inspect.py                                                 | 2 +-
 .../next/Library/2020-03-04-16-10-59.bpo-13487.gqe4Fb.rst      | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-03-04-16-10-59.bpo-13487.gqe4Fb.rst

diff --git a/Lib/inspect.py b/Lib/inspect.py
index bb82f96fdf3..125bd456a1c 100644
--- 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
new file mode 100644
index 00000000000..5a1f02a7bdf
--- /dev/null
+++ 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`.