Respect env markers when checking resolves.

Previously we failed to do this and would erroneously fail resolves as
a result.

Fixes pex-tool#851

pex/distribution_target.py

@@ -38,6 +38,21 @@ def get_interpreter(self):
   def get_platform(self):
     return self._platform or Platform.current()
 
+  def requirement_applies(self, requirement):
+    """Determines if the given requirement applies to this distribution target.
+
+    :param requirement: The requirement to evaluate.
+    :type requirement: :class:`pex.third_party.pkg_resources.Requirement`
+    :rtype: bool
+    """
+    if not requirement.marker:
+      return True
+
+    if self._interpreter is None:
+      return True
+
+    return requirement.marker.evaluate(self._interpreter.identity.env_markers)
+
   @property
   def id(self):
     """A unique id for a resolve target suitable as a path name component.

pex/resolver.py

@@ -33,13 +33,15 @@ class Unsatisfiable(Exception):
   pass
 
 
-class ResolvedDistribution(namedtuple('ResolvedDistribution', ['requirement', 'distribution'])):
-  """A requirement and the resolved distribution that satisfies it."""
+class ResolvedDistribution(namedtuple('ResolvedDistribution',
+                                      ['target', 'requirement', 'distribution'])):
+  """A distribution target, requirement and the resolved distribution that satisfies them both."""
 
-  def __new__(cls, requirement, distribution):
+  def __new__(cls, target, requirement, distribution):
+    assert isinstance(target, DistributionTarget)
     assert isinstance(requirement, Requirement)
     assert isinstance(distribution, Distribution)
-    return super(ResolvedDistribution, cls).__new__(cls, requirement, distribution)
+    return super(ResolvedDistribution, cls).__new__(cls, target, requirement, distribution)
 
 
 class DistributionRequirements(object):
@@ -565,6 +567,7 @@ def add_requirements_requests(install_result):
       for distribution in requirements_request.distributions:
         resolved_distributions.add(
           ResolvedDistribution(
+            target=requirements_request.target,
             requirement=distribution_requirements.to_requirement(distribution),
             distribution=distribution
           )
@@ -575,21 +578,26 @@ def add_requirements_requests(install_result):
     return resolved_distributions
 
   def _check_resolve(self, resolved_distributions):
-    dist_by_key = OrderedDict(
-      (resolved_distribution.requirement.key, resolved_distribution.distribution)
+    resolved_distribution_by_key = OrderedDict(
+      (resolved_distribution.requirement.key, resolved_distribution)
       for resolved_distribution in resolved_distributions
     )
 
     unsatisfied = []
-    for dist in dist_by_key.values():
+    for resolved_distribution in resolved_distribution_by_key.values():
+      dist = resolved_distribution.distribution
+      target = resolved_distribution.target
       for requirement in dist.requires():
-        resolved_dist = dist_by_key.get(requirement.key)
+        if not target.requirement_applies(requirement):
+          continue
+
+        resolved_dist = resolved_distribution_by_key.get(requirement.key)
         if not resolved_dist or resolved_dist not in requirement:
           unsatisfied.append(
             '{dist} requires {requirement} but {resolved_dist} was resolved'.format(
               dist=dist.as_requirement(),
               requirement=requirement,
-              resolved_dist=resolved_dist.as_requirement() if resolved_dist else None
+              resolved_dist=resolved_dist.distribution.as_requirement() if resolved_dist else None
             )
           )