greendns: full comment lines were not skipped; Thanks to nat-goodspeed

`greendns.HostsResolver.LINES_RE` doesn't admit the possibility of a
comment starting at the beginning of a line. This has been ignored since `is_ipv4_addr()` and `is_ipv6_addr()` catch `dns.exception.SyntaxError` and return `False`. But in a runtime environment that encounters eventlet#413, `dns.exception.SyntaxError` is not caught, and the
import fails.

Changing '+' to '*' allows `HostsResolver._readlines()` to recognize and skip
comment lines.

greendns.HostsResolver._readlines(), a purely internal method, now returns an
itertools generator rather than a list. Change relevant asserts to build a
list before comparing.

eventlet/support/greendns.py

@@ -158,7 +158,7 @@ class HostsResolver(object):
 
     LINES_RE = re.compile(r"""
         \s*  # Leading space
-        ([^\r\n#]+?)  # The actual match, non-greedy so as not to include trailing space
+        ([^\r\n#]*?)  # The actual match, non-greedy so as not to include trailing space
         \s*  # Trailing space
         (?:[#][^\r\n]+)?  # Comments
         (?:$|[\r\n]+)  # EOF or newline
@@ -196,7 +196,7 @@ def _readlines(self):
 
         udata = fdata.decode(errors='ignore')
 
-        return self.LINES_RE.findall(udata)
+        return six.moves.filter(None, self.LINES_RE.findall(udata))
 
     def _load(self):
         """Load hosts file

tests/greendns_test.py

@@ -35,31 +35,37 @@ def test_readlines_lines(self):
         hr = _make_host_resolver()
         hr.hosts.write(b'line0\n')
         hr.hosts.flush()
-        assert hr._readlines() == ['line0']
+        assert list(hr._readlines()) == ['line0']
         hr._last_stat = 0
         hr.hosts.write(b'line1\n')
         hr.hosts.flush()
-        assert hr._readlines() == ['line0', 'line1']
+        assert list(hr._readlines()) == ['line0', 'line1']
         # Test reading of varied newline styles
         hr._last_stat = 0
         hr.hosts.seek(0)
         hr.hosts.truncate()
         hr.hosts.write(b'\naa\r\nbb\r  cc  \n\n\tdd ee')
         hr.hosts.flush()
-        assert hr._readlines() == ['aa', 'bb', 'cc', 'dd ee']
+        assert list(hr._readlines()) == ['aa', 'bb', 'cc', 'dd ee']
         # Test comments, including inline comments
         hr._last_stat = 0
         hr.hosts.seek(0)
         hr.hosts.truncate()
-        hr.hosts.write(b' line1\n#comment\nline2 # inline comment\n')
+        hr.hosts.write(b'''\
+# First couple lines
+# are comments.
+line1
+#comment
+line2 # inline comment
+''')
         hr.hosts.flush()
-        assert hr._readlines() == ['line1', 'line2']
+        assert list(hr._readlines()) == ['line1', 'line2']
 
     def test_readlines_missing_file(self):
         hr = _make_host_resolver()
         hr.hosts.close()
         hr._last_stat = 0
-        assert hr._readlines() == []
+        assert list(hr._readlines()) == []
 
     def test_load_no_contents(self):
         hr = _make_host_resolver()