Keep install options in requirements.txt from leaking

The list of install options passed into the setup routine is mutable,
passed by reference, so adding items for "this package" to that list
mutates the options for all subsequent packages.

Isolate the lists before mutating them.

Includes a functional test, which has been confirmed to fail without
this fix.

Fixes #3763
Fixes #4453
Fixes #5089

news/3763.bugfix

@@ -0,0 +1 @@
+Keep install options in requirements.txt from leaking.

src/pip/_internal/req/req_install.py

@@ -771,11 +771,13 @@ class InstallRequirement(object):
         # Options specified in requirements file override those
         # specified on the command line, since the last option given
         # to setup.py is the one that is used.
-        global_options += self.options.get('global_options', [])
-        install_options += self.options.get('install_options', [])
+        global_options = list(global_options) + \
+            self.options.get('global_options', [])
+        install_options = list(install_options) + \
+            self.options.get('install_options', [])
 
         if self.isolated:
-            global_options = list(global_options) + ["--no-user-cfg"]
+            global_options = global_options + ["--no-user-cfg"]
 
         with TempDirectory(kind="record") as temp_dir:
             record_filename = os.path.join(temp_dir.path, 'install-record.txt')

tests/functional/test_install_reqs.py

@@ -513,3 +513,37 @@ def test_install_unsupported_wheel_file(script, data):
     assert ("simple.dist-0.1-py1-none-invalid.whl is not a supported " +
             "wheel on this platform" in result.stderr)
     assert len(result.files_created) == 0
+
+
+def test_install_options_local_to_package(script, data, virtualenv):
+    """Make sure --install-options does not leak across packages.
+
+    A requirements.txt file can have per-package --install-options; these
+    should be isolated to just the package instead of leaking to subsequent
+    packages.  This needs to be a functional test because the bug was around
+    cross-contamination at install time.
+    """
+    home_simple = script.scratch_path.join("for-simple")
+    test_simple = script.scratch.join("for-simple")
+    home_simple.mkdir()
+    reqs_file = script.scratch_path.join("reqs.txt")
+    reqs_file.write(
+          textwrap.dedent("""
+              simple --install-option='--home=%s'
+              INITools
+              """ % home_simple))
+    result = script.pip(
+            'install',
+            '--no-index', '-f', data.find_links,
+            '-r', reqs_file,
+            expect_error=True,
+    )
+
+    simple = test_simple / 'lib' / 'python' / 'simple'
+    bad = test_simple / 'lib' / 'python' / 'initools'
+    good = script.site_packages / 'initools'
+    assert simple in result.files_created
+    assert result.files_created[simple].dir
+    assert bad not in result.files_created
+    assert good in result.files_created
+    assert result.files_created[good].dir