1 files changed, 99 insertions, 0 deletions
diff --git a/indra/llcommon/llmainthreadtask.h b/indra/llcommon/llmainthreadtask.h
new file mode 100644
index 0000000000..d509b687c0
--- /dev/null
+++ b/indra/llcommon/llmainthreadtask.h
@@ -0,0 +1,99 @@
+/**
+ * @file   llmainthreadtask.h
+ * @author Nat Goodspeed
+ * @date   2019-12-04
+ * @brief  LLMainThreadTask dispatches work to the main thread. When invoked on
+ *         the main thread, it performs the work inline.
+ * 
+ * $LicenseInfo:firstyear=2019&license=viewerlgpl$
+ * Copyright (c) 2019, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_LLMAINTHREADTASK_H)
+#define LL_LLMAINTHREADTASK_H
+
+#include "lleventtimer.h"
+#include "llthread.h"
+#include "llmake.h"
+#include <future>
+#include <type_traits>              // std::result_of
+
+/**
+ * LLMainThreadTask provides a way to perform some task specifically on the
+ * main thread, waiting for it to complete. A task consists of a C++ nullary
+ * invocable (i.e. any callable that requires no arguments) with arbitrary
+ * return type.
+ *
+ * Instead of instantiating LLMainThreadTask, pass your invocable to its
+ * static dispatch() method. dispatch() returns the result of calling your
+ * task. (Or, if your task throws an exception, dispatch() throws that
+ * exception. See std::packaged_task.)
+ *
+ * When you call dispatch() on the main thread (as determined by
+ * on_main_thread() in llthread.h), it simply calls your task and returns the
+ * result.
+ *
+ * When you call dispatch() on a secondary thread, it instantiates an
+ * LLEventTimer subclass scheduled immediately. Next time the main loop calls
+ * LLEventTimer::updateClass(), your task will be run, and LLMainThreadTask
+ * will fulfill a future with its result. Meanwhile the requesting thread
+ * blocks on that future. As soon as it is set, the requesting thread wakes up
+ * with the task result.
+ */
+class LLMainThreadTask
+{
+private:
+    // Don't instantiate this class -- use dispatch() instead.
+    LLMainThreadTask() {}
+
+public:
+    /// dispatch() is the only way to invoke this functionality.
+    template <typename CALLABLE>
+    static auto dispatch(CALLABLE&& callable) -> decltype(callable())
+    {
+        if (on_main_thread())
+        {
+            // we're already running on the main thread, perfect
+            return callable();
+        }
+        else
+        {
+            // It's essential to construct LLEventTimer subclass instances on
+            // the heap because, on completion, LLEventTimer deletes them.
+            // Once we enable C++17, we can use Class Template Argument
+            // Deduction. Until then, use llmake_heap().
+            auto* task = llmake_heap<Task>(std::forward<CALLABLE>(callable));
+            auto future = task->mTask.get_future();
+            // Now simply block on the future.
+            return future.get();
+        }
+    }
+
+private:
+    template <typename CALLABLE>
+    struct Task: public LLEventTimer
+    {
+        Task(CALLABLE&& callable):
+            // no wait time: call tick() next chance we get
+            LLEventTimer(0),
+            mTask(std::forward<CALLABLE>(callable))
+        {}
+        BOOL tick() override
+        {
+            // run the task on the main thread, will populate the future
+            // obtained by get_future()
+            mTask();
+            // tell LLEventTimer we're done (one shot)
+            return TRUE;
+        }
+        // Given arbitrary CALLABLE, which might be a lambda, how are we
+        // supposed to obtain its signature for std::packaged_task? It seems
+        // redundant to have to add an argument list to engage result_of, then
+        // add the argument list again to complete the signature. At least we
+        // only support a nullary CALLABLE.
+        std::packaged_task<typename std::result_of<CALLABLE()>::type()> mTask;
+    };
+};
+
+#endif /* ! defined(LL_LLMAINTHREADTASK_H) */