api: transfer metadata context via fd

Metadata information can easily become very big, like in the case
of the package metadata of the org.osbuild.rpm stage, quite likely
exceeding the configured maximum package length of the underlying
socket. To avoid potential issues here, transfer the actual data
by writing it to a temporary file and sending a open fd over.

osbuild/api.py

@@ -177,8 +177,11 @@ class API(BaseAPI):
         self._output_data.write(data)
         self.monitor.log(data)
 
-    def _set_metadata(self, message):
-        self.metadata.update(message["metadata"])
+    def _set_metadata(self, message, fds):
+        fd = message["metadata"]
+        with os.fdopen(fds.steal(fd), encoding="utf-8") as f:
+            data = json.load(f)
+        self.metadata.update(data)
 
     def _get_arguments(self, sock):
         with self._prepare_input() as data:
@@ -194,7 +197,7 @@ class API(BaseAPI):
 
     def _message(self, msg, fds, sock):
         if msg["method"] == 'add-metadata':
-            self._set_metadata(msg)
+            self._set_metadata(msg, fds)
         elif msg["method"] == 'exception':
             self._get_exception(msg)
         elif msg["method"] == 'get-arguments':
@@ -250,9 +253,16 @@ def arguments(path="/run/osbuild/api/osbuild"):
 
 def metadata(data: Dict, path="/run/osbuild/api/osbuild"):
     """Update metadata for the current module"""
-    with jsoncomm.Socket.new_client(path) as client:
+
+    def data_to_file():
+        with tempfile.TemporaryFile() as f:
+            f.write(json.dumps(data).encode('utf-8'))
+            # re-open the file to get a read-only file descriptor
+            return open(f"/proc/self/fd/{f.fileno()}", "r")
+
+    with jsoncomm.Socket.new_client(path) as client, data_to_file() as f:
         msg = {
             "method": "add-metadata",
-            "metadata": data
+            "metadata": 0
         }
-        client.send(msg)
+        client.send(msg, fds=[f.fileno()])