Fix building packages with backend-path in pyproject.toml

Closes gh-6599

news/6599.bugfix

@@ -0,0 +1 @@
+Fix building packages which specify ``backend-path`` in pyproject.toml.

src/pip/_internal/pyproject.py

@@ -39,7 +39,7 @@ def load_pyproject_toml(
     setup_py,  # type: str
     req_name  # type: str
 ):
-    # type: (...) -> Optional[Tuple[List[str], str, List[str]]]
+    # type: (...) -> Optional[Tuple[List[str], str, List[str], List[str]]]
     """Load the pyproject.toml file.
 
     Parameters:
@@ -57,6 +57,8 @@ def load_pyproject_toml(
             name of PEP 517 backend,
             requirements we should check are installed after setting
                 up the build environment
+            directory paths to import the backend from (backend-path),
+                relative to the project root.
         )
     """
     has_pyproject = os.path.isfile(pyproject_toml)
@@ -167,6 +169,7 @@ def load_pyproject_toml(
             )
 
     backend = build_system.get("build-backend")
+    backend_path = build_system.get("backend-path", [])
     check = []  # type: List[str]
     if backend is None:
         # If the user didn't specify a backend, we assume they want to use
@@ -184,4 +187,4 @@ def load_pyproject_toml(
         backend = "setuptools.build_meta:__legacy__"
         check = ["setuptools>=40.8.0", "wheel"]
 
-    return (requires, backend, check)
+    return (requires, backend, check, backend_path)

src/pip/_internal/req/req_install.py

@@ -601,11 +601,11 @@ class InstallRequirement(object):
             return
 
         self.use_pep517 = True
-        requires, backend, check = pyproject_toml_data
+        requires, backend, check, backend_path = pyproject_toml_data
         self.requirements_to_check = check
         self.pyproject_requires = requires
         self.pep517_backend = Pep517HookCaller(
-            self.unpacked_source_directory, backend
+            self.unpacked_source_directory, backend, backend_path=backend_path,
         )
 
     def prepare_metadata(self):

tests/functional/test_pep517.py

@@ -6,12 +6,14 @@ from pip._internal.req import InstallRequirement
 from tests.lib import make_test_finder, path_to_url
 
 
-def make_project(tmpdir, requires=[], backend=None):
+def make_project(tmpdir, requires=[], backend=None, backend_path=[]):
     project_dir = tmpdir / 'project'
     project_dir.mkdir()
     buildsys = {'requires': requires}
     if backend:
         buildsys['build-backend'] = backend
+    if backend_path:
+        buildsys['backend-path'] = backend_path
     data = pytoml.dumps({'build-system': buildsys})
     project_dir.joinpath('pyproject.toml').write_text(data)
     return project_dir
@@ -32,6 +34,34 @@ def test_backend(tmpdir, data):
         assert req.pep517_backend.build_wheel("dir") == "Backend called"
 
 
+dummy_backend_code = """\
+def build_wheel(
+    wheel_directory,
+    config_settings=None,
+    metadata_directory=None
+):
+    return "Backend called"
+"""
+
+
+def test_backend_path(tmpdir, data):
+    """Check we can call a backend inside the project"""
+    project_dir = make_project(
+        tmpdir, backend="dummy_backend", backend_path=['.']
+    )
+    (project_dir / 'dummy_backend.py').write_text(dummy_backend_code)
+    print(project_dir)
+    import os
+    print(os.listdir(project_dir))
+    req = InstallRequirement(None, None, source_dir=project_dir)
+    req.load_pyproject_toml()
+
+    env = BuildEnvironment()
+    assert hasattr(req.pep517_backend, 'build_wheel')
+    with env:
+        assert req.pep517_backend.build_wheel("dir") == "Backend called"
+
+
 def test_pep517_install(script, tmpdir, data):
     """Check we can build with a custom backend"""
     project_dir = make_project(