diff -r 4752fafb579d Lib/test/regrtest.py
--- a/Lib/test/regrtest.py	Wed Jul 11 19:21:31 2012 +0200
+++ b/Lib/test/regrtest.py	Wed Jul 11 17:44:54 2012 -0700
@@ -166,6 +166,7 @@
 """
 
 import builtins
+import collections
 import faulthandler
 import getopt
 import io
@@ -587,20 +588,27 @@
             resource_denieds.append(test)
 
     if forever:
-        def test_forever(tests=list(selected)):
-            while True:
-                for test in tests:
-                    yield test
-                    if bad:
-                        return
-        tests = test_forever()
         test_count = ''
         test_count_width = 3
     else:
-        tests = iter(selected)
         test_count = '/{}'.format(len(selected))
         test_count_width = len(test_count) - 1
 
+    pending = collections.deque(selected)  # thread-safe pops and appends.
+
+    def next_test():
+        while True:
+            if interrupted or (forever and bad):
+                break
+            try:
+                return pending.popleft()
+            except IndexError:
+                if forever and selected:  # don't loop forever if no tests selected.
+                    pending.extend(selected)
+                    continue
+                break
+        raise StopIteration('no more tests')
+
     if use_mp:
         try:
             from threading import Thread
@@ -611,17 +619,6 @@
         from subprocess import Popen, PIPE
         debug_output_pat = re.compile(r"\[\d+ refs\]$")
         output = Queue()
-        def tests_and_args():
-            for test in tests:
-                args_tuple = (
-                    (test, verbose, quiet),
-                    dict(huntrleaks=huntrleaks, use_resources=use_resources,
-                         debug=debug, output_on_failure=verbose3,
-                         timeout=timeout, failfast=failfast,
-                         match_tests=match_tests)
-                )
-                yield (test, args_tuple)
-        pending = tests_and_args()
         opt_args = support.args_from_interpreter_flags()
         base_cmd = [sys.executable] + opt_args + ['-m', 'test.regrtest']
         def work():
@@ -629,10 +626,17 @@
             try:
                 while True:
                     try:
-                        test, args_tuple = next(pending)
+                        test = next_test()
                     except StopIteration:
                         output.put((None, None, None, None))
                         return
+                    args_tuple = (
+                        (test, verbose, quiet),
+                        dict(huntrleaks=huntrleaks, use_resources=use_resources,
+                             debug=debug, output_on_failure=verbose3,
+                             timeout=timeout, failfast=failfast,
+                             match_tests=match_tests)
+                    )
                     # -E is needed by some tests, e.g. test_import
                     # Running the child from the same working directory ensures
                     # that TEMPDIR for the child is the same when
@@ -691,11 +695,16 @@
                 test_index += 1
         except KeyboardInterrupt:
             interrupted = True
-            pending.close()
         for worker in workers:
             worker.join()
     else:
-        for test_index, test in enumerate(tests, 1):
+        test_index = 0
+        while True:
+            try:
+                test = next_test()
+            except StopIteration:
+                break
+            test_index += 1
             if not quiet:
                 fmt = "[{1:{0}}{2}/{3}] {4}" if bad else "[{1:{0}}{2}] {4}"
                 print(fmt.format(
diff -r 4752fafb579d Misc/NEWS
--- a/Misc/NEWS	Wed Jul 11 19:21:31 2012 +0200
+++ b/Misc/NEWS	Wed Jul 11 17:44:54 2012 -0700
@@ -121,6 +121,9 @@
 Tests
 -----
 
+- Issue #15320: Make iterating the list of tests thread-safe when running
+  tests in multiprocess mode. Patch by Chris Jerdonek.
+
 - Issue #15300: Ensure the temporary test working directories are in the same
   parent folder when running tests in multiprocess mode from a Python build.
   Patch by Chris Jerdonek.