Add a test demonstrating #7966

tests/functional/test_new_resolver.py

@@ -2,7 +2,10 @@ import json
 
 import pytest
 
-from tests.lib import create_basic_wheel_for_package
+from tests.lib import (
+    create_basic_sdist_for_package,
+    create_basic_wheel_for_package,
+)
 
 
 def assert_installed(script, **kwargs):
@@ -253,3 +256,41 @@ def test_new_resolver_ignore_installed(script):
     assert script.site_packages / "base" in result.files_updated, (
         "base 0.1.0 not reinstalled"
     )
+
+
+def test_new_resolver_only_builds_sdists_when_needed(script):
+    create_basic_wheel_for_package(
+        script,
+        "base",
+        "0.1.0",
+        depends=["dep"],
+    )
+    create_basic_sdist_for_package(
+        script,
+        "dep",
+        "0.1.0",
+        # Replace setup.py with something that fails
+        extra_files={"setup.py": "assert False"},
+    )
+    create_basic_sdist_for_package(
+        script,
+        "dep",
+        "0.2.0",
+    )
+    # We only ever need to check dep 0.2.0 as it's the latest version
+    script.pip(
+        "install", "--unstable-feature=resolver",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "base"
+    )
+    assert_installed(script, base="0.1.0", dep="0.2.0")
+
+    # We merge criteria here, as we have two "dep" requirements
+    script.pip(
+        "install", "--unstable-feature=resolver",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "base", "dep"
+    )
+    assert_installed(script, base="0.1.0", dep="0.2.0")

tests/lib/__init__.py

@@ -15,7 +15,7 @@ from textwrap import dedent
 from zipfile import ZipFile
 
 import pytest
-from pip._vendor.six import PY2
+from pip._vendor.six import PY2, ensure_binary, text_type
 from scripttest import FoundDir, TestFileEnvironment
 
 from pip._internal.index.collector import LinkCollector
@@ -1036,6 +1036,56 @@ def create_basic_wheel_for_package(
     return archive_path
 
 
+def create_basic_sdist_for_package(
+    script, name, version, extra_files=None
+):
+    files = {
+        "setup.py": """
+            from setuptools import find_packages, setup
+            setup(name={name!r}, version={version!r})
+        """,
+    }
+
+    # Some useful shorthands
+    archive_name = "{name}-{version}.tar.gz".format(
+        name=name, version=version
+    )
+
+    # Replace key-values with formatted values
+    for key, value in list(files.items()):
+        del files[key]
+        key = key.format(name=name)
+        files[key] = textwrap.dedent(value).format(
+            name=name, version=version
+        ).strip()
+
+    # Add new files after formatting
+    if extra_files:
+        files.update(extra_files)
+
+    for fname in files:
+        path = script.temp_path / fname
+        path.parent.mkdir(exist_ok=True, parents=True)
+        path.write_bytes(ensure_binary(files[fname]))
+
+    # The base_dir cast is required to make `shutil.make_archive()` use
+    # Unicode paths on Python 2, making it able to properly archive
+    # files with non-ASCII names.
+    retval = script.scratch_path / archive_name
+    generated = shutil.make_archive(
+        retval,
+        'gztar',
+        root_dir=script.temp_path,
+        base_dir=text_type(os.curdir),
+    )
+    shutil.move(generated, retval)
+
+    shutil.rmtree(script.temp_path)
+    script.temp_path.mkdir()
+
+    return retval
+
+
 def need_executable(name, check_cmd):
     def wrapper(fn):
         try: