SL-16220: Add LL::ThreadPool class and a "General" instance.

ThreadPool bundles a WorkQueue with the specified number of worker threads to
service it. Each ThreadPool has a name that can be used to locate its
WorkQueue.

Each worker thread calls WorkQueue::runUntilClose().

ThreadPool listens on the "LLApp" LLEventPump for shutdown notification. On
receiving that, it closes its WorkQueue and then join()s each of its worker
threads for orderly shutdown.

Add a settings.xml entry "ThreadPoolSizes", the first LLSD-valued settings
entry to expect a map: pool name->size. The expectation is that usually code
instantiating a particular ThreadPool will have a default size in mind, but it
should check "ThreadPoolSizes" for a user override.

Make idle_startup()'s STATE_SEED_CAP_GRANTED state instantiate a "General"
ThreadPool. This is function-static for lazy initialization.

Eliminate LLMainLoopRepeater, which is completely unreferenced. Any potential
future use cases are better addressed by posting to the main loop's WorkQueue.

Eliminate llappviewer.cpp's private LLDeferredTaskList class, which
implemented LLAppViewer::addOnIdleCallback(). Make addOnIdleCallback() post
work to the main loop's WorkQueue instead.

1 files changed, 46 insertions, 0 deletions

diff --git a/indra/llcommon/threadpool.h b/indra/llcommon/threadpool.h
new file mode 100644
index 0000000000..8f3c8514b5
--- /dev/null
+++ b/indra/llcommon/threadpool.h
@@ -0,0 +1,46 @@
+/**
+ * @file   threadpool.h
+ * @author Nat Goodspeed
+ * @date   2021-10-21
+ * @brief  ThreadPool configures a WorkQueue along with a pool of threads to
+ *         service it.
+ * 
+ * $LicenseInfo:firstyear=2021&license=viewerlgpl$
+ * Copyright (c) 2021, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_THREADPOOL_H)
+#define LL_THREADPOOL_H
+
+#include "workqueue.h"
+#include <string>
+#include <thread>
+#include <utility>                  // std::pair
+#include <vector>
+
+namespace LL
+{
+
+    class ThreadPool
+    {
+    public:
+        /**
+         * Pass ThreadPool a string name. This can be used to look up the
+         * relevant WorkQueue.
+         */
+        ThreadPool(const std::string& name, size_t threads=1);
+        ~ThreadPool();
+        void close();
+
+    private:
+        void run(const std::string& name);
+
+        WorkQueue mQueue;
+        std::string mName;
+        std::vector<std::pair<std::string, std::thread>> mThreads;
+    };
+
+} // namespace LL
+
+#endif /* ! defined(LL_THREADPOOL_H) */