DRTVWR-588: Move LLSingleton dependency on LLMainThreadTask to .cpp.

Introduce LLSingletonBase::getInstanceForSecondaryThread(), used both by
LLSingleton and LLParamSingleton. Because it's a method of the non-template
base class, because it's not itself a template method,
getInstanceForSecondaryThread()'s definition can live in llsingleton.cpp. This
is what calls LLMainThreadTask::dispatch().

To support LLParamSingleton, though, getInstanceForSecondaryThread() must be
capable of handling arguments. For that, it accepts a nullary std::function
returning the LLSingletonBase* of interest.

Packing initParamSingleton() arguments into a nullary std::function to pass to
getInstanceForSecondaryThread() sounds like a job for std::bind().
Unfortunately std::bind() has trouble forwarding int and string literals to a
function that infers its argument types. To work around that, use
boost::call_traits::param_type and a lambda with an explicit tuple.

2 files changed, 70 insertions, 33 deletions

diff --git a/indra/llcommon/llsingleton.cpp b/indra/llcommon/llsingleton.cpp
index 6b1986d0e9..5c99048123 100644
--- a/indra/llcommon/llsingleton.cpp
+++ b/indra/llcommon/llsingleton.cpp
@@ -27,11 +27,12 @@
 #include "linden_common.h"
 #include "llsingleton.h"
 
+#include "llcoros.h"
+#include "lldependencies.h"
 #include "llerror.h"
 #include "llerrorcontrol.h"
-#include "lldependencies.h"
 #include "llexception.h"
-#include "llcoros.h"
+#include "llmainthreadtask.h"
 #include <boost/foreach.hpp>
 #include <algorithm>
 #include <iostream>                 // std::cerr in dire emergency
@@ -486,3 +487,29 @@ std::string LLSingletonBase::demangle(const char* mangled)
 {
     return LLError::Log::demangle(mangled);
 }
+
+LLSingletonBase* LLSingletonBase::getInstanceForSecondaryThread(
+    const std::string& name,
+    const std::string& method,
+    const std::function<LLSingletonBase*()>& getInstance)
+{
+    // Normally it would be the height of folly to reference-bind args into a
+    // lambda to be executed on some other thread! By the time that thread
+    // executed the lambda, the references would all be dangling, and Bad
+    // Things would result. But LLMainThreadTask::dispatch() promises to block
+    // the calling thread until the passed task has completed. So in this case
+    // we know the references will remain valid until the lambda has run, so
+    // we dare to bind references.
+    return LLMainThreadTask::dispatch(
+        [&name, &method, &getInstance](){
+            // VERY IMPORTANT to call getInstance() on the main thread,
+            // rather than going straight to constructSingleton()!
+            // During the time window before mInitState is INITIALIZED,
+            // multiple requests might be queued. It's essential that, as
+            // the main thread processes them, only the FIRST such request
+            // actually constructs the instance -- every subsequent one
+            // simply returns the existing instance.
+            loginfos({name, "::", method, " on main thread"});
+            return getInstance();
+        });
+}
diff --git a/indra/llcommon/llsingleton.h b/indra/llcommon/llsingleton.h
index 51ef514cf7..caecc3594f 100644
--- a/indra/llcommon/llsingleton.h
+++ b/indra/llcommon/llsingleton.h
@@ -25,16 +25,18 @@
 #ifndef LLSINGLETON_H
 #define LLSINGLETON_H
 
+#include <boost/call_traits.hpp>
 #include <boost/noncopyable.hpp>
 #include <boost/unordered_set.hpp>
+#include <functional>
 #include <initializer_list>
 #include <list>
 #include <typeinfo>
 #include <vector>
 #include "mutex.h"
 #include "lockstatic.h"
+#include "apply.h"
 #include "llthread.h"               // on_main_thread()
-#include "llmainthreadtask.h"
 
 class LLSingletonBase: private boost::noncopyable
 {
@@ -134,6 +136,17 @@ protected:
     // internal wrapper around calls to cleanupSingleton()
     void cleanup_();
 
+    // This method is where we dispatch to LLMainThreadTask to acquire the
+    // subclass LLSingleton instance when the first getInstance() call is from
+    // a secondary thread. We delegate to the .cpp file to untangle header
+    // circularity. It accepts a std::function referencing the subclass
+    // getInstance() method -- which can't be virtual because it's static; we
+    // don't yet have an instance! For messaging, it also accepts the name of
+    // the subclass and the subclass method.
+    static LLSingletonBase* getInstanceForSecondaryThread(
+        const std::string& name, const std::string& method,
+        const std::function<LLSingletonBase*()>& getInstance);
+
     // deleteSingleton() isn't -- and shouldn't be -- a virtual method. It's a
     // class static. However, given only Foo*, deleteAll() does need to be
     // able to reach Foo::deleteSingleton(). Make LLSingleton (which declares
@@ -555,19 +568,11 @@ public:
         // Per the comment block above, dispatch to the main thread.
         loginfos({classname<DERIVED_TYPE>(),
                  "::getInstance() dispatching to main thread"});
-        auto instance = LLMainThreadTask::dispatch(
-            [](){
-                // VERY IMPORTANT to call getInstance() on the main thread,
-                // rather than going straight to constructSingleton()!
-                // During the time window before mInitState is INITIALIZED,
-                // multiple requests might be queued. It's essential that, as
-                // the main thread processes them, only the FIRST such request
-                // actually constructs the instance -- every subsequent one
-                // simply returns the existing instance.
-                loginfos({classname<DERIVED_TYPE>(),
-                         "::getInstance() on main thread"});
-                return getInstance();
-            });
+        auto instance = static_cast<DERIVED_TYPE*>(
+            getInstanceForSecondaryThread(
+                classname<DERIVED_TYPE>(),
+                "getInstance()",
+                getInstance));
         // record the dependency chain tracked on THIS thread, not the main
         // thread (consider a getInstance() overload with a tag param that
         // suppresses dep tracking when dispatched to the main thread)
@@ -632,8 +637,14 @@ private:
 
     // Passes arguments to DERIVED_TYPE's constructor and sets appropriate
     // states, returning a pointer to the new instance.
+    // If we just let initParamSingleton_() infer its argument types, the
+    // compiler has trouble passing int and string literals. Use
+    // boost::call_traits::param_type to smooth parameter passing. This
+    // construction requires, though, that each invocation of this method
+    // explicitly specify template arguments, instead of inferring them.
     template <typename... Args>
-    static DERIVED_TYPE* initParamSingleton_(Args&&... args)
+    static LLSingletonBase* initParamSingleton_(
+        typename boost::call_traits<Args>::param_type... args)
     {
         // In case racing threads both call initParamSingleton() at the same
         // time, serialize them. One should initialize; the other should see
@@ -652,7 +663,7 @@ private:
             // on the main thread, simply construct instance while holding lock
             super::logdebugs({super::template classname<DERIVED_TYPE>(),
                              "::initParamSingleton()"});
-            super::constructSingleton(lk, std::forward<Args>(args)...);
+            super::constructSingleton(lk, args...);
             return lk->mInstance;
         }
         else
@@ -665,20 +676,19 @@ private:
             lk.unlock();
             super::loginfos({super::template classname<DERIVED_TYPE>(),
                             "::initParamSingleton() dispatching to main thread"});
-            // Normally it would be the height of folly to reference-bind
-            // 'args' into a lambda to be executed on some other thread! By
-            // the time that thread executed the lambda, the references would
-            // all be dangling, and Bad Things would result. But
-            // LLMainThreadTask::dispatch() promises to block until the passed
-            // task has completed. So in this case we know the references will
-            // remain valid until the lambda has run, so we dare to bind
-            // references.
-            auto instance = LLMainThreadTask::dispatch(
-                [&](){
-                    super::loginfos({super::template classname<DERIVED_TYPE>(),
-                                    "::initParamSingleton() on main thread"});
-                    return initParamSingleton_(std::forward<Args>(args)...);
-                });
+            auto instance = static_cast<DERIVED_TYPE*>(
+                super::getInstanceForSecondaryThread(
+                    super::template classname<DERIVED_TYPE>(),
+                    "initParamSingleton()",
+                    // This lambda does what std::bind() is supposed to do --
+                    // but when the actual parameter is (e.g.) a string
+                    // literal, type inference makes it fail. Apply param_type
+                    // to each incoming type to make it work.
+                    [args=std::tuple<typename boost::call_traits<Args>::param_type...>(args...)]
+                    ()
+                    {
+                        return LL::apply(initParamSingleton_<Args...>, args);
+                    }));
             super::loginfos({super::template classname<DERIVED_TYPE>(),
                             "::initParamSingleton() returning on requesting thread"});
             return instance;
@@ -695,7 +705,7 @@ public:
     template <typename... Args>
     static DERIVED_TYPE& initParamSingleton(Args&&... args)
     {
-        return *initParamSingleton_(std::forward<Args>(args)...);
+        return *static_cast<DERIVED_TYPE*>(initParamSingleton_<Args...>(args...));
     }
 
     static DERIVED_TYPE* getInstance()