Implement support for PEP517 backend-path setting

Resolves tox-dev#1575

src/tox/helper/build_isolated.py

@@ -1,8 +1,19 @@
+"""PEP 517 build backend invocation script.
+
+It accepts externally parsed build configuration from `[build-system]`
+in `pyproject.toml` and invokes an API endpoint for building an sdist
+tarball.
+"""
+
+import os
 import sys
 
 dist_folder = sys.argv[1]
 backend_spec = sys.argv[2]
 backend_obj = sys.argv[3] if len(sys.argv) >= 4 else None
+backend_paths = sys.argv[4].split(os.path.pathsep) if sys.argv[4] else []
+
+sys.path[:0] = backend_paths
 
 backend = __import__(backend_spec, fromlist=["_trash"])
 if backend_obj:

src/tox/package/builder/isolated.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import json
+import os
 from collections import namedtuple
 
 import six
@@ -11,7 +12,9 @@
 from tox.config import DepConfig, get_py_project_toml
 from tox.constants import BUILD_ISOLATED, BUILD_REQUIRE_SCRIPT
 
-BuildInfo = namedtuple("BuildInfo", ["requires", "backend_module", "backend_object"])
+BuildInfo = namedtuple(
+    "BuildInfo", ["requires", "backend_module", "backend_object", "backend_paths"],
+)
 
 
 def build(config, session):
@@ -47,6 +50,17 @@ def build(config, session):
     return perform_isolated_build(build_info, package_venv, config.distdir, config.setupdir)
 
 
+def _ensure_importable_in_path(python_path, importable):
+    """Figure out if the importable exists in the given path."""
+    mod_chunks = importable.split(".")
+    pkg_path = python_path.join(*mod_chunks[:-1])
+    if not pkg_path.exists():
+        return False
+    if pkg_path.join(mod_chunks[-1], "__init__.py").exists():
+        return True
+    return pkg_path.join(mod_chunks[-1] + ".py").exists()
+
+
 def get_build_info(folder):
     toml_file = folder.join("pyproject.toml")
 
@@ -84,7 +98,15 @@ def abort(message):
     module = args[0]
     obj = args[1] if len(args) > 1 else ""
 
-    return BuildInfo(requires, module, obj)
+    backend_paths = build_system.get("backend-path", [])
+    if not isinstance(backend_paths, list):
+        abort("backend-path key at build-system section must be a list, if specified")
+    backend_paths = [folder.join(p) for p in backend_paths]
+
+    if backend_paths and not any(_ensure_importable_in_path(p, module) for p in backend_paths):
+        abort("build-backend must exist in one of the paths specified by backend-path")
+
+    return BuildInfo(requires, module, obj, backend_paths)
 
 
 def perform_isolated_build(build_info, package_venv, dist_dir, setup_dir):
@@ -103,6 +125,7 @@ def perform_isolated_build(build_info, package_venv, dist_dir, setup_dir):
                 str(dist_dir),
                 build_info.backend_module,
                 build_info.backend_object,
+                os.path.pathsep.join(str(p) for p in build_info.backend_paths),
             ],
             returnout=True,
             action=action,