Fix clean up of downloaded modules when using go.mod

Projects that use go.mod will result in the build downloading the
modules to an temporary path. Golang introduced read only directories to
protect the generated module cache from accidental writes, resulting in
failures to build when exiting the temporary directory context.

Adapt the solution from pre-commit to make the directory and file
writable on removal and include a rudimentary test that exercises
download via a simple example module hosted by the golang org in github.

Fixes asottile#49

setuptools_golang.py

@@ -1,6 +1,7 @@
 import argparse
 import contextlib
 import copy
+import errno
 import os
 import shlex
 import shutil
@@ -10,6 +11,8 @@
 import tempfile
 from distutils.ccompiler import CCompiler
 from distutils.dist import Distribution
+from types import TracebackType
+from typing import Any
 from typing import Callable
 from typing import Dict
 from typing import Generator
@@ -22,22 +25,33 @@
 from setuptools.command.build_ext import build_ext as _build_ext
 
 
+def rmtree(path: str) -> None:
+    """On windows, rmtree fails for readonly dirs."""
+    def handle_remove_readonly(
+            func: Callable[..., Any],
+            path: str,
+            exc: Tuple[Type[OSError], OSError, TracebackType],
+    ) -> None:
+        excvalue = exc[1]
+        if (
+                func in (os.rmdir, os.remove, os.unlink) and
+                excvalue.errno == errno.EACCES
+        ):
+            for p in (path, os.path.dirname(path)):
+                os.chmod(p, os.stat(p).st_mode | stat.S_IWUSR)
+            func(path)
+        else:
+            raise
+    shutil.rmtree(path, ignore_errors=False, onerror=handle_remove_readonly)
+
+
 @contextlib.contextmanager
 def _tmpdir() -> Generator[str, None, None]:
     tempdir = tempfile.mkdtemp()
     try:
         yield tempdir
     finally:
-        def err(
-                action: Callable[[str], None],
-                name: str,
-                exc: Exception,
-        ) -> None:  # pragma: no cover (windows)
-            """windows: can't remove readonly files, make them writeable!"""
-            os.chmod(name, stat.S_IWRITE)
-            action(name)
-
-        shutil.rmtree(tempdir, onerror=err)
+        rmtree(tempdir)
 
 
 def _get_cflags(

testing/gomodules/go.mod

@@ -0,0 +1,5 @@
+module github.com/asottile/setuptools-golang/testing/gomodules
+
+go 1.14
+
+require github.com/golang/example v0.0.0-20170904185048-46695d81d1fa

testing/gomodules/go.sum

@@ -0,0 +1,2 @@
+github.com/golang/example v0.0.0-20170904185048-46695d81d1fa h1:iqCQC2Z53KkwGgTN9szyL4q0OQHmuNjeoNnMT6lk66k=
+github.com/golang/example v0.0.0-20170904185048-46695d81d1fa/go.mod h1:tO/5UvQ/uKigUjQBPqzstj6uxd3fUIjddi19DxGJeWg=

testing/gomodules/reversemsg.go

@@ -0,0 +1,19 @@
+package main
+
+// #include <Python.h>
+import "C"
+
+import (
+	"fmt"
+
+	"github.com/golang/example/stringutil"
+)
+
+//export reversemsg
+func reversemsg() *C.PyObject {
+	fmt.Print(stringutil.Reverse("elpmaxe tset"))
+
+	return C.Py_None
+}
+
+func main() {}

testing/gomodules/reversemsg_support.go

@@ -0,0 +1,29 @@
+package main
+
+// #include <Python.h>
+//
+// PyObject* reversemsg();
+//
+// static struct PyMethodDef methods[] = {
+//     {"reversemsg", (PyCFunction)reversemsg, METH_NOARGS},
+//     {NULL, NULL}
+// };
+//
+// #if PY_MAJOR_VERSION >= 3
+// static struct PyModuleDef module = {
+//     PyModuleDef_HEAD_INIT,
+//     "gomodules",
+//     NULL,
+//     -1,
+//     methods
+// };
+//
+// PyMODINIT_FUNC PyInit_gomodules(void) {
+//     return PyModule_Create(&module);
+// }
+// #else
+// PyMODINIT_FUNC initgomodules(void) {
+//     Py_InitModule3("gomodules", methods, NULL);
+// }
+// #endif
+import "C"

testing/gomodules/setup.py

@@ -0,0 +1,15 @@
+from setuptools import Extension
+from setuptools import setup
+
+
+setup(
+    name='gomod',
+    ext_modules=[Extension('gomodules', ['reversemsg.go'])],
+    build_golang={
+        'root': 'github.com/asottile/setuptools-golang/testing/gomodules',
+    },
+    # Would do this, but we're testing *our* implementation and this would
+    # install from pypi.  We can rely on setuptools-golang being already
+    # installed under test.
+    # setup_requires=['setuptools-golang'],
+)

tests/setuptools_golang_test.py

@@ -103,6 +103,15 @@ def test_integration_imports_gh(venv):
     assert out == '\x1b[31mohai\x1b[0m\n'
 
 
+GOMOD = 'import gomodules; gomodules.reversemsg()'
+
+
+def test_integration_gomodules(venv):
+    run(venv.pip, 'install', os.path.join('testing', 'gomodules'))
+    out = run_output(venv.python, '-c', GOMOD)
+    assert out == 'test example'
+
+
 def test_integration_notfound(venv):
     ret = run(
         venv.pip, 'install', os.path.join('testing', 'notfound'),