Fix a deadlock in the plugin manager (#355)

* Fix a deadlock in the plugin manager

I'm not even sure why the deadlock is there, but it happens on the
server. Removing the global pluginmanager is a good idea anyway,
at a small cost of performance we remove ugly manual locking.

src/app.py

@@ -1,6 +1,5 @@
 from lib.ursadb import UrsaDb
 import os
-from threading import Lock
 
 import uvicorn  # type: ignore
 from config import app_config
@@ -11,7 +10,6 @@
     HTTPException,
     Depends,
     Header,
-    BackgroundTasks,
 )  # type: ignore
 from starlette.requests import Request  # type: ignore
 from starlette.responses import Response, FileResponse, StreamingResponse  # type: ignore
@@ -50,28 +48,16 @@
 
 db = Database(app_config.redis.host, app_config.redis.port)
 app = FastAPI()
-plugins = PluginManager(app_config.mquery.plugins, db)
-plugin_lock = Lock()
 
 
-def use_plugins(background: BackgroundTasks) -> None:
-    """Acquires a plugin_lock, and releases it after cleanup and returning a response.
-    This function should be called by every API endpoint that uses plugins.
+def with_plugins() -> Iterable[PluginManager]:
+    """Cleans up plugins after processing."""
 
-    This lock is necessary, because nothing in the plugins API makes it obvious that
-    they should be thread-safe - so we assume that they're not.
-    """
-
-    def release_and_cleanup():
-        try:
-            # Hopefully this won't crash...
-            plugins.cleanup()
-        finally:
-            # ...but just in case it does, we absolutely have to release the lock.
-            plugin_lock.release()
-
-    plugin_lock.acquire()
-    background.add_task(release_and_cleanup)
+    plugins = PluginManager(app_config.mquery.plugins, db)
+    try:
+        yield plugins
+    finally:
+        plugins.cleanup()
 
 
 class User:
@@ -326,9 +312,14 @@ def backend_status_datasets() -> BackendStatusDatasetsSchema:
 @app.get(
     "/api/download",
     tags=["stable"],
-    dependencies=[Depends(can_download_files), Depends(use_plugins)],
+    dependencies=[Depends(can_download_files)],
 )
-def download(job_id: str, ordinal: int, file_path: str) -> Response:
+def download(
+    job_id: str,
+    ordinal: int,
+    file_path: str,
+    plugins: PluginManager = Depends(with_plugins),
+) -> Response:
     """
     Sends a file from given `file_path`. This path should come from
     results of one of the previous searches.
@@ -366,7 +357,13 @@ def download_hashes(job_id: str) -> Response:
     return Response(hashes + "\n")
 
 
-def zip_files(matches: List[Dict[Any, Any]]) -> Iterable[bytes]:
+def zip_files(
+    plugins: PluginManager, matches: List[Dict[str, Any]]
+) -> Iterable[bytes]:
+    """Adds all the samples to a zip archive (replacing original filename
+    with sha256) and returns it as a stream of bytes."""
+    plugins = PluginManager(app_config.mquery.plugins, db)
+
     with tempfile.NamedTemporaryFile() as writer:
         with open(writer.name, "rb") as reader:
             with zipfile.ZipFile(writer, mode="w") as zipwriter:
@@ -385,9 +382,11 @@ def zip_files(matches: List[Dict[Any, Any]]) -> Iterable[bytes]:
     "/api/download/files/{job_id}",
     dependencies=[Depends(is_user), Depends(can_download_files)],
 )
-async def download_files(job_id: str) -> StreamingResponse:
+async def download_files(
+    job_id: str, plugins: PluginManager = Depends(with_plugins)
+) -> StreamingResponse:
     matches = db.get_job_matches(job_id).matches
-    return StreamingResponse(zip_files(matches))
+    return StreamingResponse(zip_files(plugins, matches))
 
 
 @app.post(