objectstore: add api server and client

The StoreServer and corresponding Client provide access to small
subset of the store methods to other process than the main osbuild one.
Currently it can be used to read trees of objects given their id and
create  temporary directories within the store's tmp path.
The lifetime of the result of both operations are bound to the Server.

osbuild/objectstore.py

@@ -7,7 +7,8 @@ import tempfile
 from typing import Optional
 
 from osbuild.util.types import PathLike
-from osbuild.util import ctx, rmrf
+from osbuild.util import ctx, jsoncomm, rmrf
+from . import api
 from . import treesum
 
 
@@ -360,3 +361,79 @@ class ObjectStore(contextlib.AbstractContextManager):
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         self.cleanup()
+
+
+class StoreServer(api.BaseAPI):
+
+    endpoint = "store"
+
+    def __init__(self, store: ObjectStore, *, socket_address=None):
+        super().__init__(socket_address)
+        self.store = store
+        self.tmproot = store.tempdir(prefix="store-server-")
+        self._stack = contextlib.ExitStack()
+
+    def _cleanup(self):
+        self.tmproot.cleanup()
+        self.tmproot = None
+        self._stack.close()
+        self._stack = None
+
+    def _read_tree(self, msg, sock):
+        object_id = msg["object-id"]
+        obj = self.store.get(object_id)
+        if not obj:
+            sock.send({"path": None})
+            return
+
+        reader = obj.read()
+        path = self._stack.enter_context(reader)
+        sock.send({"path": path})
+
+    def _mkdtemp(self, msg, sock):
+        args = {
+            "suffix": msg.get("suffix"),
+            "prefix": msg.get("prefix"),
+            "dir": self.tmproot.name
+        }
+
+        path = tempfile.mkdtemp(**args)
+        sock.send({"path": path})
+
+    def _message(self, msg, _fds, sock):
+        if msg["method"] == "read-tree":
+            self._read_tree(msg, sock)
+        if msg["method"] == "mkdtemp":
+            self._mkdtemp(msg, sock)
+
+
+class StoreClient:
+    def __init__(self, connect_to="/run/osbuild/api/store"):
+        self.client = jsoncomm.Socket.new_client(connect_to)
+
+    def __del__(self):
+        if self.client is not None:
+            self.client.close()
+
+    def mkdtemp(self, suffix=None, prefix=None):
+        msg = {
+            "method": "mkdtemp",
+            "suffix": suffix,
+            "prefix": prefix
+        }
+
+        self.client.send(msg)
+        msg, _, _ = self.client.recv()
+
+        return msg["path"]
+
+    def read_tree(self, object_id: str):
+        msg = {
+            "method": "read-tree",
+            "object-id": object_id
+        }
+
+        self.client.send(msg)
+        msg, _, _ = self.client.recv()
+
+        return msg["path"]