Potential fix for Ambiguous Resolvable bug in transitive dependency resolution

pex/resolver.py

@@ -200,10 +200,8 @@ def resolve(self, resolvables, resolvable_set=None):
         assert len(packages) > 0, 'ResolvableSet.packages(%s) should not be empty' % resolvable
         package = next(iter(packages))
         if resolvable.name in processed_packages:
-          # TODO implement backtracking?
-          if package != processed_packages[resolvable.name]:
-            raise self.Error('Ambiguous resolvable: %s' % resolvable)
-          continue
+          if package == processed_packages[resolvable.name]:
+            continue
         if package not in distributions:
           dist = self.build(package, resolvable.options)
           built_package = Package.from_href(dist.location)
@@ -219,7 +217,22 @@ def resolve(self, resolvables, resolvable_set=None):
             distribution.requires(extras=resolvable_set.extras(resolvable.name)))
       resolvable_set = resolvable_set.replace_built(built_packages)
 
-    return list(distributions.values())
+    # We may have built multiple distributions depending upon if we found transitive dependencies
+    # for the same package. But ultimately, resolvable_set.packages() contains the correct version
+    # for all packages. So loop through it and only return the package version in
+    # resolvable_set.packages() that is found in distributions.
+    dists = []
+    # No point in proceeding if distributions is empty
+    if not distributions:
+      return dists
+
+    for resolvable, packages, parent, constraint_only in resolvable_set.packages():
+      if constraint_only:
+        continue
+      assert len(packages) > 0, 'ResolvableSet.packages(%s) should not be empty' % resolvable
+      package = next(iter(packages))
+      dists.append(distributions[package])
+    return dists
 
 
 class CachingResolver(Resolver):

tests/test_resolver.py

@@ -72,6 +72,24 @@ def test_cached_dependency_pinned_unpinned_resolution_multi_run():
       assert dists[0].version == '1.1.0'
 
 
+def test_ambiguous_transitive_resolvable():
+  # If an unbounded or larger bounded resolvable is resolved first, and a
+  # transitive resolvable is resolved later in another round, Error(Ambiguous resolvable) can be
+  # raised because foo pulls in foo-2.0.0 and bar->foo==1.0.0 pulls in foo-1.0.0.
+  foo1_0 = make_sdist(name='foo', version='1.0.0')
+  foo2_0 = make_sdist(name='foo', version='2.0.0')
+  bar1_0 = make_sdist(name='bar', version='1.0.0', install_reqs=['foo==1.0.0'])
+  with temporary_dir() as td:
+    for sdist in (foo1_0, foo2_0, bar1_0):
+      safe_copy(sdist, os.path.join(td, os.path.basename(sdist)))
+    fetchers = [Fetcher([td])]
+    with temporary_dir() as cd:
+      dists = resolve(['foo', 'bar'], fetchers=fetchers, cache=cd,
+                      cache_ttl=1000)
+      assert len(dists) == 2
+      assert dists[0].version == '1.0.0'
+
+
 def test_resolve_prereleases():
   stable_dep = make_sdist(name='dep', version='2.0.0')
   prerelease_dep = make_sdist(name='dep', version='3.0.0rc3')