req: fix possible exception in install_req_from_req

* allow calling it with `comes_from=None` (easier for testing)
* handle the case where `comes_from.link is None` (which can happen
  if a package using a PEP 508 URL requirement is already installed)

news/5889.bugfix

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

src/pip/_internal/req/constructors.py

@@ -286,7 +286,8 @@ 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

@@ -1359,6 +1359,37 @@ 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):