firmware: Fix kernel RPC handling of zero-size values (e.g. empty arrays)

artiq/firmware/runtime/session.rs

@@ -277,6 +277,11 @@ fn process_host_message(io: &Io,
                 }
             })?;
             rpc::recv_return(stream, &tag, slot, &|size| -> Result<_, Error<SchedError>> {
+                if size == 0 {
+                    // Don't try to allocate zero-length values, as RpcRecvReply(0) is
+                    // used to terminate the kernel-side receive loop.
+                    return Ok(0 as *mut ())
+                }
                 kern_send(io, &kern::RpcRecvReply(Ok(size)))?;
                 Ok(kern_recv(io, |reply| {
                     match reply {

artiq/test/coredevice/test_embedding.py

@@ -363,9 +363,27 @@ class _NestedTupleList(EnvExperiment):
         if a != self.data:
             raise ValueError
 
+
+class _EmptyList(EnvExperiment):
+    def build(self):
+        self.setattr_device("core")
+
+    def get_empty(self) -> TList(TInt32):
+        return []
+
+    @kernel
+    def run(self):
+        a = self.get_empty()
+        if a != []:
+            raise ValueError
+
+
 class ListTupleTest(ExperimentCase):
     def test_list_tuple(self):
         self.create(_ListTuple).run()
 
     def test_nested_tuple_list(self):
         self.create(_NestedTupleList).run()
+
+    def test_empty_list(self):
+        self.create(_EmptyList).run()