fix(store): serialize concurrent installs, and make the lock reentrant

Second bug found while validating the previous commit on hardware.

install_plugin's new set-aside/restore had no lock. The web UI runs Flask
threaded, so a double-clicked Install button gives two threads the same
plugin_id; interleaved, one thread's restore deletes the other's freshly
installed copy. _reinstall_with_rollback already guards exactly this with a
per-plugin lock, and install_plugin needs the same one.

Taking that lock naively deadlocks. _reinstall_with_rollback holds it across
its call to install_plugin, and threading.Lock is not reentrant -- so the
request thread hangs forever on the standard monorepo update path
(update_plugin -> _reinstall_with_rollback -> install_plugin), which is to say
on every plugin update. Verified by reverting to a plain Lock: the regression
test times out after 10s instead of passing.

The per-plugin locks are now RLocks, and install_plugin holds one for its
whole set-aside/install/restore sequence.

Verified on devpi (Pi, Python 3.13.5, real registry and network):
- update_plugin on an up-to-date plugin: True in 5.4s
- update_plugin forced through the full reinstall-with-rollback path:
  True in 13.1s, correct version restored, old copy replaced, no backup
  directories left behind
- install -> reinstall-over-existing -> failed-reinstall-restores: all pass
  against real downloads
- 22 plugins load, no tracebacks, web API and UI 200, steady-state journal
  50 lines/min

791 core unit tests pass, including 2 new concurrency tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

src/plugin_system/store_manager.py

@@ -149,18 +149,27 @@ class PluginStoreManager:
         # loser can end up renaming the winner's in-progress install aside
         # mid-download, stealing its own rollback safety net. Keyed by
         # plugin_id so unrelated plugins still update concurrently.
-        self._reinstall_locks: Dict[str, threading.Lock] = {}
+        # Reentrant: install_plugin takes this lock, and _reinstall_with_rollback
+        # holds it across its call to install_plugin. A plain Lock would
+        # self-deadlock on that nesting.
+        self._reinstall_locks: Dict[str, "threading.RLock"] = {}
         self._reinstall_locks_guard = threading.Lock()
 
         # Ensure plugins directory exists
         self.plugins_dir.mkdir(exist_ok=True)
 
-    def _get_reinstall_lock(self, plugin_id: str) -> threading.Lock:
+    def _get_reinstall_lock(self, plugin_id: str):
-        """Lazily create (or fetch) the per-plugin reinstall lock."""
+        """Lazily create (or fetch) the per-plugin reinstall lock.
+
+        Reentrant by necessity: `install_plugin` acquires it to protect its
+        set-aside/restore, and `_reinstall_with_rollback` holds it across its
+        own call to `install_plugin`. With a plain `Lock` that nesting
+        deadlocks the request thread.
+        """
         with self._reinstall_locks_guard:
             lock = self._reinstall_locks.get(plugin_id)
             if lock is None:
-                lock = threading.Lock()
+                lock = threading.RLock()
                 self._reinstall_locks[plugin_id] = lock
             return lock
 
@@ -1208,7 +1217,15 @@ class PluginStoreManager:
         The aside name embeds '.standalone-backup-' so plugin discovery
         (`plugin_manager._scan_directory_for_plugins`) skips it even though it
         still holds a manifest.json.
+
+        Held under the per-plugin reinstall lock for the same reason
+        `_reinstall_with_rollback` is: the web UI runs Flask with
+        threaded=True, so a double-clicked Install button gives two threads the
+        same plugin_id. Interleaved, one thread's restore would delete the
+        other's freshly installed copy. The lock is reentrant because the
+        rollback path already holds it when it calls in here.
         """
+        with self._get_reinstall_lock(plugin_id):
             plugin_path = self.plugins_dir / plugin_id
             if not plugin_path.exists():
                 return self._install_plugin_impl(plugin_id, branch)
@@ -1216,8 +1233,9 @@ class PluginStoreManager:
             backup_path = plugin_path.with_name(
                 f"{plugin_path.name}.standalone-backup-preinstall")
             if backup_path.exists() and not self._safe_remove_directory(backup_path):
-            # Can't stage a safety net. Better to attempt the install than to
+                # Can't stage a safety net. Better to attempt the install than
-            # refuse outright, which is what the caller got before this existed.
+                # to refuse outright, which is what callers got before this
+                # existed.
                 self.logger.warning(
                     "Could not clear stale pre-install backup for %s at %s; "
                     "installing without a rollback net", plugin_id, backup_path)
@@ -1240,9 +1258,9 @@ class PluginStoreManager:
             if installed:
                 if not self._safe_remove_directory(backup_path):
                     self.logger.warning(
-                    "Install of %s succeeded but the previous copy at %s could "
+                        "Install of %s succeeded but the previous copy at %s "
-                    "not be removed; it will be cleared on the next install",
+                        "could not be removed; it will be cleared on the next "
-                    plugin_id, backup_path)
+                        "install", plugin_id, backup_path)
                 return True
 
             self._restore_preinstall_backup(plugin_id, plugin_path, backup_path)

test/test_install_preserves_existing.py

@@ -150,3 +150,73 @@ class TestUpdatePathStillWorks:
         assert observed["dirs"] == ["hockey-scoreboard.standalone-backup-migrating"]
         # And the user still has their plugin.
         assert (plugins_dir / "hockey-scoreboard" / "marker.txt").read_text() == "the-original"
+
+
+class TestConcurrency:
+    """The web UI runs Flask threaded, so a double-clicked Install button puts
+    two threads on the same plugin_id. `_reinstall_with_rollback` already
+    guarded against this; the install wrapper has to as well, or one thread's
+    restore deletes the other's freshly installed copy."""
+
+    def test_rollback_calling_install_does_not_deadlock(self, store, monkeypatch):
+        """The rollback path holds the per-plugin lock across its call to
+        install_plugin. A non-reentrant lock would hang the request thread
+        forever — this test would time out rather than fail."""
+        import threading
+
+        mgr, plugins_dir = store
+        path = _existing_install(plugins_dir, "hockey-scoreboard", "the-original")
+        monkeypatch.setattr(
+            mgr, "_install_plugin_impl",
+            lambda pid, branch=None: bool(_existing_install(plugins_dir, pid, "new")))
+
+        done = threading.Event()
+        result = {}
+
+        def run():
+            result["ok"] = mgr._reinstall_with_rollback("hockey-scoreboard", path)
+            done.set()
+
+        t = threading.Thread(target=run, daemon=True)
+        t.start()
+        assert done.wait(timeout=10), (
+            "install_plugin deadlocked when called from _reinstall_with_rollback "
+            "— the per-plugin lock must be reentrant"
+        )
+        assert result["ok"] is True
+
+    def test_concurrent_installs_serialize(self, store, monkeypatch):
+        """Two threads installing the same plugin must not interleave their
+        set-aside/restore, and the survivor must be a complete install."""
+        import threading
+
+        mgr, plugins_dir = store
+        _existing_install(plugins_dir, "hockey-scoreboard", "the-original")
+
+        in_flight = []
+        overlap = []
+
+        def slow_impl(plugin_id, branch=None):
+            in_flight.append(1)
+            if len(in_flight) > 1:
+                overlap.append(1)
+            threading.Event().wait(0.05)
+            _existing_install(plugins_dir, plugin_id, "installed")
+            in_flight.pop()
+            return True
+
+        monkeypatch.setattr(mgr, "_install_plugin_impl", slow_impl)
+
+        threads = [threading.Thread(target=mgr.install_plugin,
+                                    args=("hockey-scoreboard",), daemon=True)
+                   for _ in range(2)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join(timeout=10)
+            assert not t.is_alive(), "concurrent install hung"
+
+        assert not overlap, "two installs of the same plugin ran concurrently"
+        assert (plugins_dir / "hockey-scoreboard" / "marker.txt").exists()
+        leftovers = [p.name for p in plugins_dir.iterdir() if "backup" in p.name]
+        assert not leftovers, f"backup left behind: {leftovers}"