Produce a better environment string for USE_RESOURCES, and actually pass the...

Produce a better environment string for USE_RESOURCES, and actually pass the right version to the environment variable so we're not just adding on.

src/gevent/testing/resources.py

@@ -121,10 +121,39 @@ def parse_resources(resource_str=None):
             if requested_resource[1:] in resources:
                 resources.remove(requested_resource[1:])
         else:
+            # TODO: Produce a warning if it's an unknown resource?
             resources.append(requested_resource)
 
     return resources
 
+def unparse_resources(resources):
+    """
+    Given a list of enabled resources, produce the correct environment variable
+    setting to enable (only) that list.
+    """
+    # By default, we assume all resources are enabled, so explicitly
+    # listing them here doesn't actually disable anything. To do that, we want to
+    # list the ones that are disabled. This is usually shorter than starting with
+    # 'none', and manually adding them back in one by one.
+    #
+    # 'none' must be special cased because an empty option string
+    # means 'all'. Still, we're explicit about that.
+    #
+    # TODO: Make this produce the minimal output; sometimes 'none' and
+    # adding would be shorter.
+
+    all_resources = set(get_ALL_RESOURCES())
+    enabled = set(resources)
+
+    if enabled == all_resources:
+        result = 'all'
+    elif resources:
+        explicitly_disabled = all_resources - enabled
+        result = ''.join(sorted('-' + x for x in explicitly_disabled))
+    else:
+        result = 'none'
+    return result
+
 
 def setup_resources(resources=None):
     """

src/gevent/testing/testrunner.py

@@ -14,7 +14,7 @@ from multiprocessing import cpu_count
 from . import util
 from .resources import parse_resources
 from .resources import setup_resources
-from .resources import get_ALL_RESOURCES
+from .resources import unparse_resources
 from .sysinfo import RUNNING_ON_CI
 from .sysinfo import PYPY
 from .sysinfo import PY2
@@ -522,15 +522,8 @@ def main():
 
     options.use = list(set(options.use))
     # Whether or not it came from the environment, put it in the
-    # environment now. 'none' must be special cased because an empty
-    # option string means 'all'. Still, we're explicit about that.
-    if set(options.use) == set(get_ALL_RESOURCES()):
-        option_str = 'all'
-    elif options.use:
-        option_str = ','.join(options.use)
-    else:
-        option_str = 'none'
-    os.environ['GEVENTTEST_USE_RESOURCES'] = option_str
+    # environment now.
+    os.environ['GEVENTTEST_USE_RESOURCES'] = unparse_resources(options.use)
     setup_resources(options.use)
 
 

src/gevent/tests/test__socket_dns.py

@@ -486,6 +486,7 @@ class SanitizedHostsFile(HostsFile):
                 continue
             yield name, addr
 
+
 @greentest.skipIf(greentest.RUNNING_ON_CI,
                   "This sometimes randomly fails on Travis with ares and on appveyor, beginning Feb 13, 2018")
 # Probably due to round-robin DNS,