fix(git-gate): make the gateway access-hook executable regardless of copy transport

Cloning/fetching from the git-gate on the Apple-container backend failed with
"empty reply from server" (curl exit 52). Root cause: the git-http handler
crashed on every upload-pack with

    PermissionError: [Errno 13] Permission denied: '/etc/git-gate/access-hook'

The access-hook is exec'd directly, so it needs the x bit. prepare() stages it
0o700 and trusted the gateway copy to carry that mode. `docker cp` does; the
Apple `container cp` (AppleGatewayTransport) does not, landing the hook 0o644 →
EACCES. The unhandled exception killed the handler thread, closing the socket
with no HTTP response — which the client sees as the opaque empty reply.

- provision_git_gate now `chmod +x`es the access-hook on the gateway side after
  the copy, so it's executable under every transport (docker/apple/firecracker).
- git-http handler wraps the access-hook subprocess.run: an OSError /
  SubprocessError (un-execable, timed out) now fails closed with a 503 instead
  of crashing the thread into an empty reply — a gate that can't run its hook
  should deny, visibly.
- Updates the now-misleading "docker cp preserves source mode" comment in
  git_gate.prepare().

Regression tests: provisioning applies +x to the access-hook; the handler
returns 503 (not an empty reply) when the hook can't be exec'd.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/docker/gateway_provision.py

@@ -94,6 +94,12 @@ def provision_git_gate(
     transport.exec(["mkdir", "-p", "/etc/git-gate"])
     transport.cp_into(str(plan.hook_script), "/etc/git-gate/pre-receive")
     transport.cp_into(str(plan.access_hook_script), "/etc/git-gate/access-hook")
+    # The access-hook is exec'd directly (not via `sh`), so it needs the x bit.
+    # Set it here rather than trusting the copy to carry the staged 0o700:
+    # `docker cp` preserves source mode, but the Apple `container cp` does not,
+    # landing the hook 0o644 → EACCES when the git-http handler tries to exec it.
+    # chmod on the gateway side is backend-neutral and fixes every transport.
+    transport.exec(["chmod", "+x", "/etc/git-gate/access-hook"])
     creds = _creds_dir(bottle_id)
     transport.exec(["mkdir", "-p", creds])
     for u in plan.upstreams:

bot_bottle/git_gate.py

@@ -112,8 +112,10 @@ class GitGate(ABC):
         access_hook = stage_dir / "git_gate_access_hook.sh"
         access_hook.write_text(git_gate_render_access_hook())
         # 0o700 (not 0o600): git daemon execs --access-hook directly,
-        # not via `sh`, so the script needs the x bit. docker cp
-        # preserves source mode into the container.
+        # not via `sh`, so the script needs the x bit. The gateway copy
+        # does not necessarily preserve this mode (`docker cp` does, the
+        # Apple `container cp` does not), so provision_git_gate re-applies
+        # +x on the gateway side — see backend/docker/gateway_provision.py.
         access_hook.chmod(0o700)
         upstreams_with_files: list[GitGateUpstream] = []
         for u in upstreams:

bot_bottle/git_http_backend.py

@@ -148,12 +148,24 @@ class GitHttpHandler(BaseHTTPRequestHandler):
                 "GIT_GATE_ACCESS_HOOK", "/etc/git-gate/access-hook",
             )
             peer = self.client_address[0]
-            hook = subprocess.run(
-                [hook_path, "upload-pack", str(repo_dir), peer, peer],
-                capture_output=True,
-                check=False,
-                timeout=GIT_GATE_TIMEOUT_SECS,
-            )
+            try:
+                hook = subprocess.run(
+                    [hook_path, "upload-pack", str(repo_dir), peer, peer],
+                    capture_output=True,
+                    check=False,
+                    timeout=GIT_GATE_TIMEOUT_SECS,
+                )
+            except (OSError, subprocess.SubprocessError) as exc:
+                # The access-hook couldn't be run (missing, not executable,
+                # timed out, …). Fail closed with a real HTTP error rather
+                # than letting the exception kill the handler thread — an
+                # unhandled exception closes the socket with no response, which
+                # the client sees as an opaque "empty reply from server".
+                self.log_message(
+                    "access-hook could not run for %s: %s", parsed.path, exc,
+                )
+                self.send_error(503, "git-gate access-hook unavailable")
+                return
             if hook.returncode != 0:
                 detail = (hook.stderr or hook.stdout).decode(
                     "utf-8", errors="replace",

tests/unit/test_gateway_provision.py

@@ -69,6 +69,18 @@ class TestProvisionGitGate(unittest.TestCase):
         self.assertEqual(1, len(exec_scripts))
         self.assertIn("repo=/git/bottle1/${name}.git", exec_scripts[0][-1])
 
+    def test_makes_access_hook_executable_on_the_gateway(self) -> None:
+        # Regression: the access-hook is exec'd directly, so it needs the x
+        # bit. The copy alone can't be trusted to carry the staged 0o700
+        # (`docker cp` preserves mode, the Apple `container cp` does not),
+        # so provisioning must re-apply +x on the gateway side.
+        calls: list[list[str]] = []
+        with patch(_RUN, side_effect=_recorder(calls)):
+            provision_git_gate(DockerGatewayTransport("gw"), "bottle1", _plan(_up("foo")))
+        self.assertIn(
+            ["docker", "exec", "gw", "chmod", "+x", "/etc/git-gate/access-hook"], calls,
+        )
+
     def test_omits_known_hosts_copy_when_absent(self) -> None:
         calls: list[list[str]] = []
         with patch(_RUN, side_effect=_recorder(calls)):

tests/unit/test_git_http_backend.py

@@ -364,6 +364,48 @@ class TestGitHttpBackend(unittest.TestCase):
                 self.assertIn("access-hook denied", logged)
                 self.assertIn("exit=2", logged)
 
+    def test_access_hook_that_cannot_run_fails_closed_503(self):
+        """Regression: when the access-hook can't be exec'd (missing / not
+        executable — a PermissionError from subprocess.run), the handler must
+        fail closed with a real HTTP status instead of letting the exception
+        kill the thread, which closes the socket with no response and the
+        client sees an opaque "empty reply from server"."""
+        from http.server import ThreadingHTTPServer
+        import io
+        import sys
+
+        with tempfile.TemporaryDirectory() as tmp:
+            root = Path(tmp)
+            (root / "repo.git").mkdir()
+            old_root = os.environ.get("GIT_PROJECT_ROOT")
+            os.environ["GIT_PROJECT_ROOT"] = str(root)
+            self.addCleanup(self._restore_env, old_root)
+
+            server = ThreadingHTTPServer(("127.0.0.1", 0), GitHttpHandler)
+            thread = threading.Thread(target=server.serve_forever, daemon=True)
+            thread.start()
+            self.addCleanup(server.shutdown)
+            self.addCleanup(server.server_close)
+
+            with mock.patch(
+                "bot_bottle.git_http_backend.subprocess.run",
+                side_effect=PermissionError(13, "Permission denied"),
+            ):
+                buf = io.StringIO()
+                with mock.patch.object(sys, "stdout", buf):
+                    req = urllib.request.Request(
+                        f"http://127.0.0.1:{server.server_port}"
+                        "/repo.git/info/refs?service=git-upload-pack",
+                        method="GET",
+                    )
+                    try:
+                        urllib.request.urlopen(req, timeout=5)
+                        self.fail("expected HTTPError 503")
+                    except urllib.error.HTTPError as e:  # type: ignore
+                        self.assertEqual(503, e.code)
+
+                self.assertIn("access-hook could not run", buf.getvalue())
+
     @staticmethod
     def _restore_env(value: str | None) -> None:
         if value is None: