bpo-36279: Ensure os.wait3() rusage is initialized

Lib/test/test_os.py

@@ -2472,6 +2472,22 @@ def test_waitpid(self):
         status = os.waitpid(pid, 0)
         self.assertEqual(status, (pid, 0))
 
+    @unittest.skipUnless(hasattr(os, 'wait3'), "os.wait3() is required")
+    def test_wait3_rusage_initialized(self):
+        # Ensure a successful wait3() call where no child was ready to report
+        # its exit status does not return uninitialized memory in the rusage
+        # structure. See bpo-36279.
+        args = [sys.executable, '-c', 'import sys; sys.stdin.read()']
+        proc = subprocess.Popen(args, stdin=subprocess.PIPE)
+        try:
+            pid, status, rusage = os.wait3(os.WNOHANG)
+            self.assertEqual(0, pid)
+            self.assertEqual(0, status)
+            self.assertEqual(0, sum(rusage))
+        finally:
+            proc.stdin.close()
+            proc.wait()
+
 
 class SpawnTests(unittest.TestCase):
     def create_args(self, *, with_env=False, use_bytes=False):

Misc/NEWS.d/next/Library/2019-03-13-03-56-36.bpo-36279.4CC01n.rst

@@ -0,0 +1,2 @@
+The :func:`os.wait3` function could return uninitialized memory if the system
+call succeeded, but no child process exit status was reported.

Modules/posixmodule.c

@@ -7354,6 +7354,12 @@ wait_helper(pid_t pid, int status, struct rusage *ru)
     if (pid == -1)
         return posix_error();
 
+    /* If wait succeeded but no child was ready to report status, ru will not
+     * have been populated. Clear it to avoid returning uninitialized stack to
+     * the caller. */
+    if (pid == 0)
+        memset(ru, 0, sizeof(*ru));
+
     if (struct_rusage == NULL) {
         PyObject *m = PyImport_ImportModuleNoBlock("resource");
         if (m == NULL)