bpo-39855: Fix test_subprocess if nobody user doesn't exist (GH-18781)

test_subprocess.test_user() now skips the test on an user name if the
user name doesn't exist. For example, skip the test if the user
"nobody" doesn't exist on Linux.

Lib/test/test_subprocess.py

@@ -1791,7 +1791,12 @@ def test_user(self):
         name_uid = "nobody" if sys.platform != 'darwin' else "unknown"
 
         if pwd is not None:
-            test_users.append(name_uid)
+            try:
+                pwd.getpwnam(name_uid)
+                test_users.append(name_uid)
+            except KeyError:
+                # unknown user name
+                name_uid = None
 
         for user in test_users:
             # posix_spawn() may be used with close_fds=False
@@ -1819,7 +1824,7 @@ def test_user(self):
         with self.assertRaises(ValueError):
             subprocess.check_call(ZERO_RETURN_CMD, user=-1)
 
-        if pwd is None:
+        if pwd is None and name_uid is not None:
             with self.assertRaises(ValueError):
                 subprocess.check_call(ZERO_RETURN_CMD, user=name_uid)
 

Misc/NEWS.d/next/Tests/2020-03-04-23-03-01.bpo-39855.Ql5xv8.rst

@@ -0,0 +1,3 @@
+test_subprocess.test_user() now skips the test on an user name if the user
+name doesn't exist. For example, skip the test if the user "nobody" doesn't
+exist on Linux.