Use bash to open extensionless hooks on windows

[idbrii]

Fix #971. Partly resolve #703.

If the hook doesn't have a file extension, then Windows won't know how
to run it and you'll get "[WinError 193] %1 is not a valid Win32
application". It's very likely that it's a shell script of some kind, so
use bash.exe (commonly installed via Windows Subsystem for Linux). We
don't want to run all hooks with bash because they could be .bat files.

test_index.py passes (two are skipped).

[Byron]

Thanks a lot for the contribution, much appreciated.

Doing this makes a lot of sense and will generally make life easier on windows. If there is any demand, one could imagine making the default interpreter/runner binary configurable, and using bash.exe for now seems like a decent default.

[idbrii]

I wrote a test for a python hook to prove to myself it would fail:

diff --git a/test/test_index.py b/test/test_index.py
index 233a4c64..d6373250 100644
--- a/test/test_index.py
+++ b/test/test_index.py
@@ -56,14 +56,14 @@ HOOKS_SHEBANG = "#!/usr/bin/env sh\n"
 is_win_without_bash = is_win and not shutil.which('bash.exe')
 
 
-def _make_hook(git_dir, name, content, make_exec=True):
+def _make_hook(git_dir, name, content, make_exec=True, shebang=HOOKS_SHEBANG):
     """A helper to create a hook"""
     hp = hook_path(name, git_dir)
     hpd = osp.dirname(hp)
     if not osp.isdir(hpd):
         os.mkdir(hpd)
     with open(hp, "wt") as fp:
-        fp.write(HOOKS_SHEBANG + content)
+        fp.write(shebang + content)
     if make_exec:
         os.chmod(hp, 0o744)
     return hp
@@ -912,6 +912,33 @@ class TestIndex(TestBase):
         new_commit = index.commit(commit_message)
         self.assertEqual(new_commit.message, "{} {}".format(commit_message, from_hook_message))
 
+    @with_rw_repo('HEAD', bare=True)
+    def test_commit_msg_hook_success_in_py(self, rw_repo):
+        index = rw_repo.index
+        hp = _make_hook(
+            index.repo.git_dir,
+            'commit-msg',
+            "import sys; print('stdout'); print('stderr', file=sys.stderr); sys.exit(1)",
+            shebang = "#!/usr/bin/env python3\n"
+        )
+        try:
+            index.commit("This should fail")
+        except HookExecutionError as err:
+            if is_win_without_bash:
+                self.assertIsInstance(err.status, OSError)
+                self.assertEqual(err.command, [hp])
+                self.assertEqual(err.stdout, '')
+                self.assertEqual(err.stderr, '')
+                assert str(err)
+            else:
+                self.assertEqual(err.status, 1)
+                self.assertEqual(err.command, [hp])
+                self.assertEqual(err.stdout, "\n  stdout: 'stdout\n'")
+                self.assertEqual(err.stderr, "\n  stderr: 'stderr\n'")
+                assert str(err)
+        else:
+            raise AssertionError("Should have cought a HookExecutionError")
+
     @with_rw_repo('HEAD', bare=True)
     def test_commit_msg_hook_fail(self, rw_repo):
         index = rw_repo.index

And I was thinking instead of calling bash.exe with the hook, we could call it with a call_from_unix script like this so bash will figure out the right program to use:

#! /bin/sh
$1

And use:

        cmd = ["bash.exe", "call_from_unix.sh", relative_hp]

I'm currently not sure if this is a good idea or terrible idea.

[Byron]

Sorry, actually I forgot to merge yesterday and didn't mean to instill doubt.

I'd say this PR is already an improvement due to the added chance for success.

If there are any issues that the new suggestions with the 'proxy script' (for lack of a better name) would solve that can be implemented, too, but I'd wait. The current solution won't add another program invocation which might be unnecessary, but always costly.