Sync: Keep installed deps marked as to_install; Let pip sort it out

Fixes #896

By including already-installed matching requirements in the to_install set,
dependencies of pinned packages will be installed,
after potentially being uninstalled,
regardless of environment state at time of sync.

The already-installed reqs will still not be reinstalled,
thanks to pip's own handling.

The resulting state is more consistent, predictable, and practical.

Modify tests to expect explicit requirements in the to_install set, to match.

piptools/sync.py

@@ -150,7 +150,6 @@ def diff(
     """
     requirements_lut = {diff_key_from_ireq(r): r for r in compiled_requirements}
 
-    satisfied = set()  # holds keys
     to_install = set()  # holds InstallRequirement objects
     to_uninstall = set()  # holds keys
 
@@ -159,11 +158,9 @@ def diff(
         key = key_from_req(dist)
         if key not in requirements_lut or not requirements_lut[key].match_markers():
             to_uninstall.add(key)
-        elif requirements_lut[key].specifier.contains(dist.version):
-            satisfied.add(key)
 
     for key, requirement in requirements_lut.items():
-        if key not in satisfied and requirement.match_markers():
+        if requirement.match_markers():
             to_install.add(requirement)
 
     # Make sure to not uninstall any packages that should be ignored

tests/test_sync.py

@@ -118,7 +118,7 @@ def test_diff_should_do_nothing():
     reqs = []  # no requirements
 
     to_install, to_uninstall = diff(reqs, installed)
-    assert to_install == set()
+    assert to_install == set(reqs)
     assert to_uninstall == set()
 
 
@@ -136,7 +136,7 @@ def test_diff_should_uninstall(fake_dist):
     reqs = []
 
     to_install, to_uninstall = diff(reqs, installed)
-    assert to_install == set()
+    assert to_install == set(reqs)
     assert to_uninstall == {"django"}  # no version spec when uninstalling
 
 
@@ -200,7 +200,7 @@ def test_diff_leave_packaging_packages_alone(fake_dist, from_line):
     reqs = [from_line("django==1.7")]
 
     to_install, to_uninstall = diff(reqs, installed)
-    assert to_install == set()
+    assert to_install == set(reqs)
     assert to_uninstall == {"first"}
 
 
@@ -221,7 +221,7 @@ def test_diff_leave_piptools_alone(fake_dist, from_line):
     reqs = [from_line("django==1.7")]
 
     to_install, to_uninstall = diff(reqs, installed)
-    assert to_install == set()
+    assert to_install == set(reqs)
     assert to_uninstall == {"foobar"}
 
 
@@ -242,12 +242,12 @@ def test_diff_with_editable(fake_dist, from_editable):
 
 
 def test_diff_with_matching_url_versions(fake_dist, from_line):
-    # if URL version is explicitly provided, use it to avoid reinstalling
+    # if URL version is explicitly provided, pip itself will avoid reinstalling
     installed = [fake_dist("example==1.0")]
     reqs = [from_line("file:///example.zip#egg=example==1.0")]
 
     to_install, to_uninstall = diff(reqs, installed)
-    assert to_install == set()
+    assert to_install == set(reqs)
     assert to_uninstall == set()