req: fix possible exception in install_req_from_req

news/5889.bugfix

@@ -0,0 +1 @@
+Fix exception when installing a package depending on an already installed dependency using a PEP 508 requirement.

news/5892.bugfix

@@ -0,0 +1 @@
+Improve handling of file URIs: correctly handle `file://localhost/...` and don't try to use UNC paths on Unix.

src/pip/_internal/download.py

@@ -470,9 +470,18 @@ def url_to_path(url):
 
     _, netloc, path, _, _ = urllib_parse.urlsplit(url)
 
-    # if we have a UNC path, prepend UNC share notation
     if netloc:
-        netloc = '\\\\' + netloc
+        if netloc == 'localhost':
+            # According to RFC 8089, same as empty authority.
+            netloc = ''
+        elif sys.platform == 'win32':
+            # If we have a UNC path, prepend UNC share notation.
+            netloc = '\\\\' + netloc
+        else:
+            raise ValueError(
+                'non-local file URIs are not supported on this platform: %r'
+                % url
+            )
 
     path = urllib_request.url2pathname(netloc + path)
     return path

src/pip/_internal/req/constructors.py

@@ -285,7 +285,8 @@ def install_req_from_req(
         PyPI.file_storage_domain,
         TestPyPI.file_storage_domain,
     ]
-    if req.url and comes_from.link.netloc in domains_not_allowed:
+    if req.url and comes_from and comes_from.link \
+       and comes_from.link.netloc in domains_not_allowed:
         # Explicitly disallow pypi packages that depend on external urls
         raise InstallationError(
             "Packages installed from PyPI cannot depend on packages "

tests/functional/test_install.py

@@ -12,9 +12,9 @@
 from pip._internal.models.index import PyPI, TestPyPI
 from pip._internal.utils.misc import rmtree
 from tests.lib import (
-    _create_svn_repo, _create_test_package, create_test_package_with_setup,
-    need_bzr, need_mercurial, path_to_url, pyversion, pyversion_tuple,
-    requirements_file,
+    _create_svn_repo, _create_test_package, create_basic_wheel_for_package,
+    create_test_package_with_setup, need_bzr, need_mercurial, path_to_url,
+    pyversion, pyversion_tuple, requirements_file,
 )
 from tests.lib.local_repos import local_checkout
 from tests.lib.path import Path
@@ -1366,6 +1366,37 @@ def test_install_pep508_with_url_in_install_requires(script):
     assert "Successfully installed packaging-15.3" in str(res), str(res)
 
 
+def test_pep508_url_already_installed(script):
+    pkgA_path = create_basic_wheel_for_package(
+        script,
+        name='pkgA', version='1.0',
+        depends=[], extras={},
+    )
+    pkgA_file_url = path_to_url(pkgA_path)
+    assert pkgA_file_url.startswith('file:///')
+    pkgA_file_url = 'file://localhost/' + pkgA_file_url[8:]
+    pkgB_path = create_basic_wheel_for_package(
+        script,
+        name='pkgB', version='2.0',
+        depends=['pkgA @ %s' % pkgA_file_url],
+        extras={},
+    )
+    pkgC_path = create_basic_wheel_for_package(
+        script,
+        name='pkgC', version='3.0',
+        depends=['pkgB'],
+        extras={},
+    )
+    r = script.pip_install_local(
+        '-v', pkgB_path
+    )
+    assert "Successfully installed pkgA-1.0 pkgB-2.0" in r.stdout, str(r)
+    r = script.pip_install_local(
+        '-v', pkgC_path
+    )
+    assert "Successfully installed pkgC-3.0" in r.stdout, str(r)
+
+
 @pytest.mark.network
 @pytest.mark.parametrize('index', (PyPI.simple_url, TestPyPI.simple_url))
 def test_install_from_test_pypi_with_ext_url_dep_is_blocked(script, index):

tests/unit/test_download.py

@@ -127,7 +127,12 @@ def test_path_to_url_unix():
 
 @pytest.mark.skipif("sys.platform == 'win32'")
 def test_url_to_path_unix():
+    assert url_to_path('file:tmp') == 'tmp'
     assert url_to_path('file:///tmp/file') == '/tmp/file'
+    assert url_to_path('file:/path/to/file') == '/path/to/file'
+    assert url_to_path('file://localhost/tmp/file') == '/tmp/file'
+    with pytest.raises(ValueError):
+        url_to_path('file://somehost/tmp/file')
 
 
 @pytest.mark.skipif("sys.platform != 'win32'")
@@ -141,8 +146,11 @@ def test_path_to_url_win():
 
 @pytest.mark.skipif("sys.platform != 'win32'")
 def test_url_to_path_win():
-    assert url_to_path('file:///c:/tmp/file') == 'C:\\tmp\\file'
+    assert url_to_path('file:tmp') == 'tmp'
+    assert url_to_path('file:///c:/tmp/file') == r'C:\tmp\file'
     assert url_to_path('file://unc/as/path') == r'\\unc\as\path'
+    assert url_to_path('file:c:/path/to/file') == r'C:\path\to\file'
+    assert url_to_path('file://localhost/c:/tmp/file') == r'C:\tmp\file'
 
 
 @pytest.mark.skipif("sys.platform != 'win32'")