SL-16024: Add LL::WorkQueue for passing work items between threads.

A typical WorkQueue has a string name, which can be used to find it to post
work to it. "Work" is a nullary callable.

WorkQueue is a multi-producer, multi-consumer thread-safe queue: multiple
threads can service the WorkQueue, multiple threads can post work to it.

Work can be scheduled in the future by submitting with a timestamp. In
addition, a given work item can be scheduled to run on a recurring basis.

A requesting thread servicing a WorkQueue of its own, such as the viewer's
main thread, can submit work to another WorkQueue along with a callback to be
passed the result (of arbitrary type) of the first work item. The callback is
posted to the originating WorkQueue, permitting safe data exchange between
participating threads.

Methods are provided for different kinds of servicing threads. runUntilClose()
is useful for a simple worker thread. runFor(duration) devotes no more than a
specified time slice to that WorkQueue, e.g. for use by the main thread.

1 files changed, 114 insertions, 0 deletions

diff --git a/indra/llcommon/workqueue.cpp b/indra/llcommon/workqueue.cpp
new file mode 100644
index 0000000000..15e292fb43
--- /dev/null
+++ b/indra/llcommon/workqueue.cpp
@@ -0,0 +1,114 @@
+/**
+ * @file   workqueue.cpp
+ * @author Nat Goodspeed
+ * @date   2021-10-06
+ * @brief  Implementation for WorkQueue.
+ * 
+ * $LicenseInfo:firstyear=2021&license=viewerlgpl$
+ * Copyright (c) 2021, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+// Precompiled header
+#include "linden_common.h"
+// associated header
+#include "workqueue.h"
+// STL headers
+// std headers
+// external library headers
+// other Linden headers
+#include "llerror.h"
+#include "llexception.h"
+#include "stringize.h"
+
+LL::WorkQueue::WorkQueue(const std::string& name):
+    super(makeName(name))
+{
+    // TODO: register for "LLApp" events so we can implicitly close() on
+    // viewer shutdown.
+}
+
+void LL::WorkQueue::close()
+{
+    mQueue.close();
+}
+
+void LL::WorkQueue::runUntilClose()
+{
+    try
+    {
+        for (;;)
+        {
+            callWork(mQueue.pop());
+        }
+    }
+    catch (const Queue::Closed&)
+    {
+    }
+}
+
+bool LL::WorkQueue::runPending()
+{
+    for (Work work; mQueue.tryPop(work); )
+    {
+        callWork(work);
+    }
+    return ! mQueue.done();
+}
+
+bool LL::WorkQueue::runOne()
+{
+    Work work;
+    if (mQueue.tryPop(work))
+    {
+        callWork(work);
+    }
+    return ! mQueue.done();
+}
+
+bool LL::WorkQueue::runUntil(const TimePoint& until)
+{
+    // Should we subtract some slop to allow for typical Work execution time?
+    // How much slop?
+    Work work;
+    while (TimePoint::clock::now() < until && mQueue.tryPopUntil(until, work))
+    {
+        callWork(work);
+    }
+    return ! mQueue.done();
+}
+
+std::string LL::WorkQueue::makeName(const std::string& name)
+{
+    if (! name.empty())
+        return name;
+
+    static thread_local U32 discriminator = 0;
+    return STRINGIZE("WorkQueue" << discriminator++);
+}
+
+void LL::WorkQueue::callWork(const Queue::DataTuple& work)
+{
+    // ThreadSafeSchedule::pop() always delivers a tuple, even when
+    // there's only one data field per item, as for us.
+    callWork(std::get<0>(work));
+}
+
+void LL::WorkQueue::callWork(const Work& work)
+{
+    try
+    {
+        work();
+    }
+    catch (...)
+    {
+        // No matter what goes wrong with any individual work item, the worker
+        // thread must go on! Log our own instance name with the exception.
+        LOG_UNHANDLED_EXCEPTION(getKey());
+    }
+}
+
+void LL::WorkQueue::error(const std::string& msg)
+{
+    LL_ERRS("WorkQueue") << msg << LL_ENDL;
+}