fix: chunk store_load over RPC to avoid msgpack BufferFull (#8440)

src/borg/remote.py

@@ -161,6 +161,8 @@ class RepositoryServer:  # pragma: no cover
         "put_manifest",
         "store_list",
         "store_load",
+        "store_load_chunk",
+        "store_get_size",
         "store_store",
         "store_delete",
         "store_move",
@@ -1049,9 +1051,16 @@ def put_manifest(self, data):
     def store_list(self, name, *, deleted=False):
         """actual remoting is done via self.call in the @api decorator"""
 
-    @api(since=parse_version("2.0.0b8"))
     def store_load(self, name):
-        """actual remoting is done via self.call in the @api decorator"""
+        # chunked fetch to avoid msgpack BufferFull on large repositories
+        total_size = self.call("store_get_size", {"name": name})
+        data = bytearray()
+        offset = 0
+        while offset < total_size:
+            chunk = self.call("store_load_chunk", {"name": name, "offset": offset, "size": MAX_DATA_SIZE})
+            data += chunk
+            offset += len(chunk)
+        return bytes(data)
 
     @api(since=parse_version("2.0.0b8"))
     def store_store(self, name, value):

src/borg/repository.py

@@ -560,6 +560,16 @@ def store_store(self, name, value):
         self._lock_refresh()
         return self.store.store(name, value)
 
+    def store_get_size(self, name):
+        self._lock_refresh()
+        data = self.store.load(name)
+        return len(data)
+
+    def store_load_chunk(self, name, offset, size):
+        self._lock_refresh()
+        data = self.store.load(name)
+        return data[offset : offset + size]
+
     def store_delete(self, name, *, deleted=False):
         self._lock_refresh()
         return self.store.delete(name, deleted=deleted)