Create a DNS resolver lazily rather than on import (fixes #736)

Creating a DNS resolver on import results in a failure in environments
where DNS is not available (containers, service ramdisks, etc).

2 files changed, 18 insertions, 1 deletions

diff --git a/eventlet/support/greendns.py b/eventlet/support/greendns.py
index 76545c7..b2f77ab 100644
--- a/eventlet/support/greendns.py
+++ b/eventlet/support/greendns.py
@@ -322,7 +322,21 @@ class ResolverProxy(object):
         """
         self._hosts = hosts_resolver
         self._filename = filename
-        self.clear()
+        # NOTE(dtantsur): we cannot create a resolver here since this code is
+        # executed on eventlet import. In an environment without DNS, creating
+        # a Resolver will fail making eventlet unusable at all. See
+        # https://github.com/eventlet/eventlet/issues/736 for details.
+        self._cached_resolver = None
+
+    @property
+    def _resolver(self):
+        if self._cached_resolver is None:
+            self.clear()
+        return self._cached_resolver
+
+    @_resolver.setter
+    def _resolver(self, value):
+        self._cached_resolver = value
 
     def clear(self):
         self._resolver = dns.resolver.Resolver(filename=self._filename)
diff --git a/tests/greendns_test.py b/tests/greendns_test.py
index 7419b35..feab2b6 100644
--- a/tests/greendns_test.py
+++ b/tests/greendns_test.py
@@ -297,8 +297,11 @@ class TestProxyResolver(tests.LimitedTestCase):
 
     def test_clear(self):
         rp = greendns.ResolverProxy()
+        assert rp._cached_resolver is None
         resolver = rp._resolver
+        assert resolver is not None
         rp.clear()
+        assert rp._resolver is not None
         assert rp._resolver != resolver
 
     def _make_mock_hostsresolver(self):