subscription: fix setCompletionCallback

This was not being tested, and it was really broken. Now it is being
tested and it is no longer broken.

src/subscription.zig

@@ -174,12 +174,12 @@ pub const Subscription = opaque {
         self: *Subscription,
         comptime T: type,
         comptime callback: *const thunk.SimpleCallbackThunkSignature(T),
-        userdata: *T,
+        userdata: T,
     ) Error!void {
         return Status.fromInt(nats_c.natsSubscription_SetOnCompleteCB(
             @ptrCast(self),
-            thunk.makeSimpleCallbackThunk(callback),
-            @constCast(userdata),
+            thunk.makeSimpleCallbackThunk(T, callback),
+            @constCast(@ptrCast(userdata)),
         )).raise();
     }
 };

src/thunk.zig

@@ -57,7 +57,7 @@ pub fn makeSimpleCallbackThunk(
     comptime checkUserDataType(T);
     return struct {
         fn thunk(userdata: ?*anyopaque) callconv(.C) void {
-            const data: *T = if (userdata) |u| @alignCast(@ptrCast(u)) else unreachable;
+            const data: T = if (userdata) |u| @alignCast(@ptrCast(u)) else unreachable;
             callback(data);
         }
     }.thunk;

tests/subscription.zig

@@ -93,6 +93,10 @@ fn onMessage(
     } else @panic("HOW");
 }
 
+fn onClose(userdata: *[]const u8) void {
+    userdata.* = "closed";
+}
+
 test "nats.Subscription (async)" {
     var server = try util.TestServer.launch(.{});
     defer server.stop();
@@ -110,17 +114,26 @@ test "nats.Subscription (async)" {
     const message = try nats.Message.create(message_subject, message_reply, message_data);
     defer message.destroy();
 
+    {
+        var closed: []const u8 = "test";
         {
             const count: u32 = 0;
             const subscription = try connection.subscribe(*const u32, message_subject, onMessage, &count);
             defer subscription.destroy();
 
+            try subscription.setCompletionCallback(*[]const u8, onClose, &closed);
+
             const response = try connection.requestMessage(message, 1000);
             try std.testing.expectEqualStrings(
                 "greetings",
                 response.getData() orelse return error.TestUnexpectedResult,
             );
         }
+        // we have to sleep to allow the close callback to run. I am worried this may
+        // still end up being flaky, however.
+        nats.sleep(1);
+        try std.testing.expectEqualStrings("closed", closed);
+    }
 
     {
         const count: u32 = 0;