bpo-28009: Fix uuid SkipUnless logic to be based on platform programs capable of introspection (GH-12777)

uuid could try fallback methods that had no chance of working on a particular
platform, and this could cause spurious test failures, as well as degraded
performance as fallback options were tried and failed.

This fixes both the uuid module and its test's SkipUnless logic to use a
prefiltered list of techniques that may at least potentially work on that platform.

Patch by Michael Felt (aixtools).
(cherry picked from commit 3a1d50e7e573efb577714146bed5c03b9c95f466)

Co-authored-by: Michael Felt <aixtools@users.noreply.github.com>

3 files changed, 47 insertions, 19 deletions

diff --git a/Lib/test/test_uuid.py b/Lib/test/test_uuid.py
index 992ef0c..92642d2 100644
--- a/Lib/test/test_uuid.py
+++ b/Lib/test/test_uuid.py
@@ -462,8 +462,7 @@ class BaseTestUUID:
         with unittest.mock.patch.multiple(
             self.uuid,
             _node=None,  # Ignore any cached node value.
-            _NODE_GETTERS_WIN32=[too_large_getter],
-            _NODE_GETTERS_UNIX=[too_large_getter],
+            _GETTERS=[too_large_getter],
         ):
             node = self.uuid.getnode()
         self.assertTrue(0 < node < (1 << 48), '%012x' % node)
@@ -673,7 +672,7 @@ class TestUUIDWithExtModule(BaseTestUUID, unittest.TestCase):
 
 
 class BaseTestInternals:
-    uuid = None
+    _uuid = py_uuid
 
     @unittest.skipUnless(os.name == 'posix', 'requires Posix')
     def test_find_mac(self):
@@ -708,27 +707,32 @@ eth0      Link encap:Ethernet  HWaddr 12:34:56:78:90:ab
         self.assertTrue(0 < node < (1 << 48),
                         "%s is not an RFC 4122 node ID" % hex)
 
-    @unittest.skipUnless(os.name == 'posix', 'requires Posix')
+    @unittest.skipUnless(_uuid._ifconfig_getnode in _uuid._GETTERS,
+        "ifconfig is not used for introspection on this platform")
     def test_ifconfig_getnode(self):
         node = self.uuid._ifconfig_getnode()
         self.check_node(node, 'ifconfig')
 
-    @unittest.skipUnless(os.name == 'posix', 'requires Posix')
+    @unittest.skipUnless(_uuid._ip_getnode in _uuid._GETTERS,
+        "ip is not used for introspection on this platform")
     def test_ip_getnode(self):
         node = self.uuid._ip_getnode()
         self.check_node(node, 'ip')
 
-    @unittest.skipUnless(os.name == 'posix', 'requires Posix')
+    @unittest.skipUnless(_uuid._arp_getnode in _uuid._GETTERS,
+        "arp is not used for introspection on this platform")
     def test_arp_getnode(self):
         node = self.uuid._arp_getnode()
         self.check_node(node, 'arp')
 
-    @unittest.skipUnless(os.name == 'posix', 'requires Posix')
+    @unittest.skipUnless(_uuid._lanscan_getnode in _uuid._GETTERS,
+        "lanscan is not used for introspection on this platform")
     def test_lanscan_getnode(self):
         node = self.uuid._lanscan_getnode()
         self.check_node(node, 'lanscan')
 
-    @unittest.skipUnless(os.name == 'posix', 'requires Posix')
+    @unittest.skipUnless(_uuid._netstat_getnode in _uuid._GETTERS,
+        "netstat is not used for introspection on this platform")
     def test_netstat_getnode(self):
         node = self.uuid._netstat_getnode()
         self.check_node(node, 'netstat')
diff --git a/Lib/uuid.py b/Lib/uuid.py
index ddc63cc..7aa01bb 100644
--- a/Lib/uuid.py
+++ b/Lib/uuid.py
@@ -45,6 +45,7 @@ Typical usage:
 """
 
 import os
+import platform
 import sys
 
 from enum import Enum
@@ -52,6 +53,12 @@ from enum import Enum
 
 __author__ = 'Ka-Ping Yee <ping@zesty.ca>'
 
+# The recognized platforms - known behaviors
+_AIX     = platform.system() == 'AIX'
+_DARWIN  = platform.system() == 'Darwin'
+_LINUX   = platform.system() == 'Linux'
+_WINDOWS = platform.system() == 'Windows'
+
 RESERVED_NCS, RFC_4122, RESERVED_MICROSOFT, RESERVED_FUTURE = [
     'reserved for NCS compatibility', 'specified in RFC 4122',
     'reserved for Microsoft compatibility', 'reserved for future definition']
@@ -673,12 +680,31 @@ def _random_getnode():
     return random.getrandbits(48) | (1 << 40)
 
 
-_node = None
-
-_NODE_GETTERS_WIN32 = [_windll_getnode, _netbios_getnode, _ipconfig_getnode]
+# _OS_GETTERS, when known, are targetted for a specific OS or platform.
+# The order is by 'common practice' on the specified platform.
+# Note: 'posix' and 'windows' _OS_GETTERS are prefixed by a dll/dlload() method
+# which, when successful, means none of these "external" methods are called.
+# _GETTERS is (also) used by test_uuid.py to SkipUnless(), e.g.,
+#     @unittest.skipUnless(_uuid._ifconfig_getnode in _uuid._GETTERS, ...)
+if _LINUX:
+    _OS_GETTERS = [_ip_getnode, _ifconfig_getnode]
+elif _DARWIN:
+    _OS_GETTERS = [_ifconfig_getnode, _arp_getnode, _netstat_getnode]
+elif _WINDOWS:
+    _OS_GETTERS = [_netbios_getnode, _ipconfig_getnode]
+elif _AIX:
+    _OS_GETTERS = [_netstat_getnode]
+else:
+    _OS_GETTERS = [_ifconfig_getnode, _ip_getnode, _arp_getnode,
+                   _netstat_getnode, _lanscan_getnode]
+if os.name == 'posix':
+    _GETTERS = [_unix_getnode] + _OS_GETTERS
+elif os.name == 'nt':
+    _GETTERS = [_windll_getnode] + _OS_GETTERS
+else:
+    _GETTERS = _OS_GETTERS
 
-_NODE_GETTERS_UNIX = [_unix_getnode, _ifconfig_getnode, _ip_getnode,
-                      _arp_getnode, _lanscan_getnode, _netstat_getnode]
+_node = None
 
 def getnode(*, getters=None):
     """Get the hardware address as a 48-bit positive integer.
@@ -692,12 +718,7 @@ def getnode(*, getters=None):
     if _node is not None:
         return _node
 
-    if sys.platform == 'win32':
-        getters = _NODE_GETTERS_WIN32
-    else:
-        getters = _NODE_GETTERS_UNIX
-
-    for getter in getters + [_random_getnode]:
+    for getter in _GETTERS + [_random_getnode]:
         try:
             _node = getter()
         except:
diff --git a/Misc/NEWS.d/next/Tests/2019-04-11-07-59-43.bpo-28009.s85urF.rst b/Misc/NEWS.d/next/Tests/2019-04-11-07-59-43.bpo-28009.s85urF.rst
new file mode 100644
index 0000000..2336407
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2019-04-11-07-59-43.bpo-28009.s85urF.rst
@@ -0,0 +1,3 @@
+Modify the test_uuid logic to test when a program is available
+AND can be used to obtain a MACADDR as basis for an UUID.
+Patch by M. Felt