diff --git a/callback.c b/callback.c
index 84ea0c80..3273002a 100644
--- a/callback.c
+++ b/callback.c
@@ -26,6 +26,21 @@ void request_callback_notifications(toplevel_callback_notify_fn_t fn,
     frontend = fr;
 }
 
+static void run_idempotent_callback(void *ctx)
+{
+    struct IdempotentCallback *ic = (struct IdempotentCallback *)ctx;
+    ic->queued = FALSE;
+    ic->fn(ic->ctx);
+}
+
+void queue_idempotent_callback(struct IdempotentCallback *ic)
+{
+    if (ic->queued)
+        return;
+    ic->queued = TRUE;
+    queue_toplevel_callback(run_idempotent_callback, ic);
+}
+
 void delete_callbacks_for_context(void *ctx)
 {
     struct callback *newhead, *newtail;
@@ -34,7 +49,9 @@ void delete_callbacks_for_context(void *ctx)
     while (cbhead) {
         struct callback *cb = cbhead;
         cbhead = cbhead->next;
-        if (cb->ctx == ctx) {
+        if (cb->ctx == ctx ||
+            (cb->fn == run_idempotent_callback &&
+             ((struct IdempotentCallback *)cb->ctx)->ctx == ctx)) {
             sfree(cb);
         } else {
             if (!newhead)
diff --git a/putty.h b/putty.h
index 590370f7..a96230f2 100644
--- a/putty.h
+++ b/putty.h
@@ -1603,6 +1603,22 @@ void run_toplevel_callbacks(void);
 int toplevel_callback_pending(void);
 void delete_callbacks_for_context(void *ctx);
 
+/*
+ * Another facility in callback.c deals with 'idempotent' callbacks,
+ * defined as those which never need to be scheduled again if they are
+ * already scheduled and have not yet run. (An example would be one
+ * which, when called, empties a queue of data completely: when data
+ * is added to the queue, you must ensure a run of the queue-consuming
+ * function has been scheduled, but if one is already pending, you
+ * don't need to schedule a second one.)
+ */
+struct IdempotentCallback {
+    toplevel_callback_fn_t fn;
+    void *ctx;
+    int queued;
+};
+void queue_idempotent_callback(struct IdempotentCallback *ic);
+
 typedef void (*toplevel_callback_notify_fn_t)(void *frontend);
 void request_callback_notifications(toplevel_callback_notify_fn_t notify,
                                     void *frontend);