Add a MinimalUpgradeRepository class to implement the --no-upgrade flag.

If we have an existing pin that would satisfy a requirement, return
that as a best match; otherwise, ask the proxied repository for the
best match.

piptools/repositories/__init__.py

@@ -1,2 +1,3 @@
 # flake8: noqa
 from .pypi import PyPIRepository
+from .minimal_upgrade import MinimalUpgradeRepository

piptools/repositories/minimal_upgrade.py

@@ -0,0 +1,33 @@
+# coding: utf-8
+from __future__ import (absolute_import, division, print_function,
+                        unicode_literals)
+
+from .base import BaseRepository
+
+
+class MinimalUpgradeRepository(BaseRepository):
+    """
+    The MinimalUpgradeRepository uses a provided requirements file as a proxy
+    in front of a repository.  If a requirement can be satisfied with
+    a version pinned in the requirements file, we use that version as the best
+    match.  In all other cases, the proxied repository is used instead.
+    """
+    def __init__(self, existing_pins, proxied_repository):
+        self.repository = proxied_repository
+        self.existing_pins = existing_pins
+
+    def clear_caches(self):
+        self.repository.clear_caches()
+
+    def freshen_build_caches(self):
+        self.repository.freshen_build_caches()
+
+    def find_best_match(self, ireq, prereleases=None):
+        existing_pin = self.existing_pins.get(ireq.req.project_name.lower())
+        if existing_pin and existing_pin.req.specs[0][1] in ireq.req:
+            return existing_pin
+        else:
+            return self.repository.find_best_match(ireq, prereleases)
+
+    def get_dependencies(self, ireq):
+        return self.repository.get_dependencies(ireq)

tests/conftest.py

@@ -82,6 +82,11 @@ def resolver(depcache, repository):
     return partial(Resolver, repository=repository, cache=depcache)
 
 
+@fixture
+def minimal_resolver(depcache):
+    return partial(Resolver, cache=depcache)
+
+
 @fixture
 def from_line():
     return InstallRequirement.from_line

tests/fixtures/fake-index.json

@@ -84,6 +84,7 @@
     "3.2.2": {"": []}
   },
   "werkzeug": {
+    "0.6": {"": []},
     "0.10": {"": []},
     "0.10.4": {"": []}
   }

tests/test_minimal_upgrade.py

@@ -0,0 +1,38 @@
+import pytest
+
+from piptools.repositories import MinimalUpgradeRepository
+
+
+@pytest.mark.parametrize(
+    ('input', 'pins', 'expected'),
+
+    ((tup) for tup in [
+
+        # Add Flask to an existing requirements.in, using --no-upgrade
+        (['flask', 'jinja2', 'werkzeug'],
+         [
+            # The requirements.txt from a previous round
+            'jinja2==2.7.3',
+            'markupsafe==0.23',
+            'werkzeug==0.6'],
+         [
+            # Add flask and upgrade werkzeug from incompatible 0.6
+            'flask==0.10.1',
+            'itsdangerous==0.24',
+            'werkzeug==0.10.4',
+            # Other requirements are unchanged from the original requirements.txt
+            'jinja2==2.7.3',
+            'markupsafe==0.23']
+         ),
+    ])
+)
+def test_minimal_upgrade(minimal_resolver, repository, from_line, input, pins, expected):
+    input = [from_line(line) for line in input]
+    existing_pins = dict()
+    for line in pins:
+        ireq = from_line(line)
+        existing_pins[ireq.req.project_name] = ireq
+    minimal_repository = MinimalUpgradeRepository(existing_pins, repository)
+    output = minimal_resolver(input, prereleases=False, repository=minimal_repository).resolve()
+    output = {str(line) for line in output}
+    assert output == {str(line) for line in expected}