Fix corner case in cached dependency resolution

  Ref: #178

  The error is as follows:

  1. Two resolvables, one pinned, the other not.
  2. First pex run with clean cache works.
  3. Change the pinned version.
  4. Second pex run fails with Unsatisfiable.

  The problem is that the non-pinned resolvable locates the cached dep
  and only returns it. Since that version doesn't match the pinned
  version, we fail. This fixes that behavior by returning both the
  cached version and all uncached pypi packages for unpinned
  resolvables.

  We change the pinned version to test the use case where you have a
  cached version of a dep and then you transitively pull in another
  requirement that either has a tighter bound or a pin on a version not
  found in the cache.

pex/resolver.py

@@ -248,8 +248,12 @@ def package_iterator(self, resolvable, existing=None):
 
       if self.__cache_ttl:
         packages = self.filter_packages_by_ttl(packages, self.__cache_ttl)
-        if packages:
-          return packages
+        # Return both cached packages and the pypi packages. The edge case is if a inexact
+        # resolvable finds a cached version, it needs to return that cached package AND all pypi
+        # packages because the cached version might not match another resolvable which has a tighter
+        # bound or an exact version.
+        return packages + super(CachingResolver, self).package_iterator(resolvable,
+                                                                        existing=existing)
 
     return super(CachingResolver, self).package_iterator(resolvable, existing=existing)
 

tests/test_resolver.py

@@ -7,6 +7,7 @@
 from twitter.common.contextutil import temporary_dir
 
 from pex.common import safe_copy
+from pex.crawler import Crawler
 from pex.fetcher import Fetcher
 from pex.package import EggPackage, SourcePackage
 from pex.resolvable import ResolvableRequirement
@@ -48,6 +49,29 @@ def test_diamond_local_resolve_cached():
       assert len(dists) == 2
 
 
+def test_cached_dependency_pinned_unpinned_resolution_multi_run():
+  # This exercises the issue described here: https://github.com/pantsbuild/pex/issues/178
+  project1_0_0 = make_sdist(name='project', version='1.0.0')
+  project1_1_0 = make_sdist(name='project', version='1.1.0')
+
+  with temporary_dir() as td:
+    for sdist in (project1_0_0, project1_1_0):
+      safe_copy(sdist, os.path.join(td, os.path.basename(sdist)))
+    fetchers = [Fetcher([td])]
+    with temporary_dir() as cd:
+      # First run, pinning 1.0.0 in the cache
+      dists = resolve(['project', 'project==1.0.0'], fetchers=fetchers, cache=cd, cache_ttl=1000)
+      assert len(dists) == 1
+      assert dists[0].version == '1.0.0'
+      # This simulates separate invocations of pex but allows us to keep the same tmp cache dir
+      Crawler.reset_cache()
+      # Second, run, the unbounded 'project' req will find the 1.0.0 in the cache. But should also
+      # return SourcePackages found in td
+      dists = resolve(['project', 'project==1.1.0'], fetchers=fetchers, cache=cd, cache_ttl=1000)
+      assert len(dists) == 1
+      assert dists[0].version == '1.1.0'
+
+
 def test_resolve_prereleases():
   stable_dep = make_sdist(name='dep', version='2.0.0')
   prerelease_dep = make_sdist(name='dep', version='3.0.0rc3')