host: add support for emitting signals

Add support for emitting signals to host.Service which can be used to
transmit data back to the client during an ongoing method call. This
provides the possibility for the services to send information to their
client counterpart while running. The signal can take file descriptors
as extra parameters to send data on separate files.

osbuild/host.py

@@ -48,7 +48,7 @@ import sys
 import threading
 import traceback
 from collections import OrderedDict
-from typing import Any, Dict, List, Optional, Tuple
+from typing import Any, Dict, List, Optional, Tuple, Callable
 
 from osbuild.util.jsoncomm import FdSet, Socket
 
@@ -131,6 +131,16 @@ class ServiceProtocol:
         # method call, which can also be `None`
         return data
 
+    @staticmethod
+    def encode_signal(sig: Any):
+        msg = {
+            "type": "signal",
+            "data": {
+                "reply": sig
+            }
+        }
+        return msg
+
     @staticmethod
     def encode_exception(value, tb):
         backtrace = "".join(traceback.format_tb(tb))
@@ -293,6 +303,10 @@ class Service(abc.ABC):
 
         return msg, fds
 
+    def emit_signal(self, data: Any, fds: Optional[list] = None):
+        self._check_fds(fds)
+        self.sock.send(self.protocol.encode_signal(data), fds=fds)
+
     @staticmethod
     def _close_all(fds: Optional[List[int]]):
         if not fds:
@@ -336,7 +350,8 @@ class ServiceClient:
 
     def call_with_fds(self, method: str,
                       args: Optional[Any] = None,
-                      fds: Optional[List] = None) -> Tuple[Any, FdSet]:
+                      fds: Optional[List] = None,
+                      on_signal: Callable[[Any, FdSet], None] = None) -> Tuple[Any, FdSet]:
         """
         Remotely call a method and return the result, including file
         descriptors.
@@ -344,16 +359,20 @@ class ServiceClient:
 
         msg = self.protocol.encode_method(method, args)
 
-        ret, fds, _ = self.sock.send_and_recv(msg, fds=fds)
+        self.sock.send(msg, fds=fds)
 
-        kind, data = self.protocol.decode_message(ret)
-
-        if kind == "reply":
-            ret = self.protocol.decode_reply(data)
-            return ret, fds
-        if kind == "exception":
-            error = self.protocol.decode_exception(data)
-            raise error
+        while True:
+            ret, fds, _ = self.sock.recv()
+            kind, data = self.protocol.decode_message(ret)
+            if kind == "signal":
+                ret = self.protocol.decode_reply(data)
+                on_signal(ret, fds)
+            if kind == "reply":
+                ret = self.protocol.decode_reply(data)
+                return ret, fds
+            if kind == "exception":
+                error = self.protocol.decode_exception(data)
+                raise error
 
         raise ProtocolError(f"unknown message type: {kind}")