Importing eventlet shouldn't read DNS configuration

[nedbat]

(Note: this stack trace is because I tried to use eventlet 0.22.0 on Python 2.6, which may be unsupported. This isn't about 2.6 support)

I have import eventlet in my code, and that one line resulted in reading host configurations on the off chance that I will eventually need to perform some DNS operations. It would be better if importing eventlet didn't try to do so much on import.

tests/test_concurrency.py:30: in <module>
    import eventlet
.tox/py26/lib/python2.6/site-packages/eventlet/__init__.py:10: in <module>
    from eventlet import convenience
.tox/py26/lib/python2.6/site-packages/eventlet/convenience.py:7: in <module>
    from eventlet.green import socket
.tox/py26/lib/python2.6/site-packages/eventlet/green/socket.py:21: in <module>
    from eventlet.support import greendns
.tox/py26/lib/python2.6/site-packages/eventlet/support/greendns.py:399: in <module>
    resolver = ResolverProxy(hosts_resolver=HostsResolver())
.tox/py26/lib/python2.6/site-packages/eventlet/support/greendns.py:181: in __init__
    self._load()
.tox/py26/lib/python2.6/site-packages/eventlet/support/greendns.py:207: in _load
    lines = self._readlines()
.tox/py26/lib/python2.6/site-packages/eventlet/support/greendns.py:197: in _readlines
    udata = fdata.decode(errors='ignore')
E   TypeError: decode() takes no keyword arguments

[nedbat]

Actually, I can't find a statement about what Python versions are supported. Perhaps you mean to support 2.6, and this needs to be fixed?

[temoto]

Completely agree, Eventlet is doing too much on import. And it should be easy to make resolver defer reading configuration until first query.

If you don't use DNS resolver, this workaround helps: EVENTLET_NO_GREENDNS=yes python2.6 blabla.py

Green DNS resolution will not be supported for 2.6.

setup.py classifiers still say we support 2.6. But it's long been in the deprecation stage, for example, 2.6 tests are listed as "allowed to fail" on Travis CI.

[dtantsur]

Still a problem with recent eventlet (presumably 0.32.0, don't know 100%). We see dns.resolver.NoResolverConfiguration on importing eventlet on a machine without DNS.

[musicinmybrain]

We see dns.resolver.NoResolverConfiguration on importing eventlet on a machine without DNS.

This showed up downstream in Fedora Linux when the python-eventlet package was updated to 0.32.0: https://bugzilla.redhat.com/show_bug.cgi?id=2017933. That aspect of this problem is, I think, a new regression.

[dtantsur]

I've split away the DNS failure into #736 since this issue is a feature request.

[temoto]

Fixed by #737 thank you @dtantsur

[Fat-Zer]

Hi, on my system without a DNS resolver the test still fails with the following backtrace. If you may, could you skip the tests if there is no resolver configuration on a system?

test_clear (tests.greendns_test.TestProxyResolver) ... ERROR

======================================================================
ERROR: test_clear (tests.greendns_test.TestProxyResolver)
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Traceback (most recent call last):
  File "/var/tmp/portage/dev-python/eventlet-0.33.0/work/eventlet-0.33.0/tests/greendns_test.py", line 301, in test_clear
    resolver = rp._resolver
  File "/var/tmp/portage/dev-python/eventlet-0.33.0/work/eventlet-0.33.0/eventlet/support/greendns.py", line 334, in _resolver
    self.clear()
  File "/var/tmp/portage/dev-python/eventlet-0.33.0/work/eventlet-0.33.0/eventlet/support/greendns.py", line 342, in clear
    self._resolver = dns.resolver.Resolver(filename=self._filename)
  File "/usr/lib/python3.8/site-packages/dns/resolver.py", line 734, in __init__
    self.read_resolv_conf(filename)
  File "/usr/lib/python3.8/site-packages/dns/resolver.py", line 826, in read_resolv_conf
    raise NoResolverConfiguration
dns.resolver.NoResolverConfiguration: Resolver configuration could not be read or specified no nameservers.

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