DRTVWR-418: Unify control flow through LLAppViewer across platforms.

The LLApp API used to consist of init(), mainLoop(), cleanup() methods. This
makes sense -- but on Mac that structure was being subverted. The method
called mainLoop() was in fact being called once per frame. There was
initialization code in the method, which (on Mac) needed to be skipped with an
already-initialized bool. There was a 'while' loop which (on Mac) needed to be
turned into an 'if' instead so the method would return after every frame.

Rename LLApp::mainLoop() to frame(). Propagate through subclasses LLAppViewer
and LLCrashLogger. Document the fact that frame() returns true to mean "done."
(This was always the case, but had to be inferred from the code.)

Rename the Mac Objective-C function mainLoop to oneFrame. Rename the C++ free
function it calls from runMainLoop() to pumpMainLoop(). Add comments to
llappdelegate-objc.mm explaining (inferred) control flow.

Change the Linux viewer main() and the Windows viewer WINMAIN() from a single
LLAppViewer::mainLoop() call to repeatedly call frame() until it returns true.

Move initialization code from the top of LLAppViewer::frame() to the init()
method, where it more properly belongs. Remove corresponding
mMainLoopInitialized flag (and all references) from LLAppViewer.

Remove 'while (! LLApp::isExiting())' (or on Mac, 'if (! LLApp::isExiting())')
from LLAppViewer::frame() -- thus unindenting the whole body of the 'while'
and causing many lines of apparent change. (Apologies to reviewers.)

There are four LLApp states: APP_STATUS_RUNNING, APP_STATUS_QUITTING,
APP_STATUS_STOPPED and APP_STATUS_ERROR. Change LLAppViewer::frame() return
value from (isExiting()) (QUITTING or ERROR) to (! isRunning()). I do not know
under what circumstances the state might transition to STOPPED during a
frame() call, but I'm quite sure that if it does, we don't want to call
frame() again. We only want a subsequent call if the state is RUNNING.

Also rename mainLoop() method in LLCrashLogger subclasses
LLCrashLoggerWindows, LLCrashLoggerMac, LLCrashLoggerLinux. Of course it's
completely up to the frame() method whether to yield control; none of those in
fact do. Honor protocol by returning true (frame() is done), even though each
one's main() caller ignores the return value.

In fact LLCrashLoggerWindows::mainLoop() wasn't using the return protocol
correctly anyway, returning wParam or 0 or 1 -- possibly because the return
protocol was never explicitly documented. It should always return true: "I'm
done, don't call me again."

20 files changed, 263 insertions, 281 deletions

diff --git a/indra/linux_crash_logger/linux_crash_logger.cpp b/indra/linux_crash_logger/linux_crash_logger.cpp
index 9d5ec33fed..63e5409876 100644
--- a/indra/linux_crash_logger/linux_crash_logger.cpp
+++ b/indra/linux_crash_logger/linux_crash_logger.cpp
@@ -51,7 +51,7 @@ int main(int argc, char **argv)
 		return 1;
 	}
 
-	app.mainLoop();
+	app.frame();
 	app.cleanup();
 	LL_INFOS() << "Crash reporter finished normally." << LL_ENDL;
 	return 0;
diff --git a/indra/linux_crash_logger/llcrashloggerlinux.cpp b/indra/linux_crash_logger/llcrashloggerlinux.cpp
index e2d2e7ff26..4092d43fc5 100644
--- a/indra/linux_crash_logger/llcrashloggerlinux.cpp
+++ b/indra/linux_crash_logger/llcrashloggerlinux.cpp
@@ -114,7 +114,7 @@ void LLCrashLoggerLinux::gatherPlatformSpecificFiles()
 {
 }
 
-bool LLCrashLoggerLinux::mainLoop()
+bool LLCrashLoggerLinux::frame()
 {
 	bool send_logs = true;
 	if(CRASH_BEHAVIOR_ASK == getCrashBehavior())
diff --git a/indra/linux_crash_logger/llcrashloggerlinux.h b/indra/linux_crash_logger/llcrashloggerlinux.h
index dae6c46651..789f6f03f5 100644
--- a/indra/linux_crash_logger/llcrashloggerlinux.h
+++ b/indra/linux_crash_logger/llcrashloggerlinux.h
@@ -36,7 +36,7 @@ class LLCrashLoggerLinux : public LLCrashLogger
 public:
 	LLCrashLoggerLinux(void);
 	~LLCrashLoggerLinux(void);
-	virtual bool mainLoop();
+	virtual bool frame();
 	virtual void updateApplication(const std::string& = LLStringUtil::null);
 	virtual void gatherPlatformSpecificFiles();
 	virtual bool cleanup();
diff --git a/indra/llcommon/llapp.h b/indra/llcommon/llapp.h
index d9933b3d36..ff9a92b45f 100644
--- a/indra/llcommon/llapp.h
+++ b/indra/llcommon/llapp.h
@@ -172,12 +172,12 @@ public:
 	virtual bool cleanup() = 0;			// Override to do application cleanup
 
 	//
-	// mainLoop()
+	// frame()
 	//
-	// Runs the application main loop.  It's assumed that when you exit
-	// this method, the application is in one of the cleanup states, either QUITTING or ERROR
+	// Pass control to the application for a single frame. Returns 'done'
+	// flag: if frame() returns false, it expects to be called again.
 	//
-	virtual bool mainLoop() = 0; // Override for the application main loop.  Needs to at least gracefully notice the QUITTING state and exit.
+	virtual bool frame() = 0; // Override for application body logic
 
 	//
 	// Crash logging
diff --git a/indra/llcrashlogger/llcrashlogger.h b/indra/llcrashlogger/llcrashlogger.h
index 8b4afae24a..56e26c23ba 100644
--- a/indra/llcrashlogger/llcrashlogger.h
+++ b/indra/llcrashlogger/llcrashlogger.h
@@ -57,7 +57,7 @@ public:
 	LLSD constructPostData();
 	virtual void updateApplication(const std::string& message = LLStringUtil::null);
 	virtual bool init();
-	virtual bool mainLoop() = 0;
+	virtual bool frame() = 0;
 	virtual bool cleanup() = 0;
 	void commonCleanup();
 	void setUserText(const std::string& text) { mCrashInfo["UserNotes"] = text; }
diff --git a/indra/llwindow/llappdelegate-objc.h b/indra/llwindow/llappdelegate-objc.h
index faa5d3abb7..6daf1ac55b 100644
--- a/indra/llwindow/llappdelegate-objc.h
+++ b/indra/llwindow/llappdelegate-objc.h
@@ -41,7 +41,7 @@
 
 @property (retain) NSString *currentInputLanguage;
 
-- (void) mainLoop;
+- (void) oneFrame;
 - (void) showInputWindow:(bool)show withEvent:(NSEvent*)textEvent;
 - (void) languageUpdated;
 - (bool) romanScript;
diff --git a/indra/llwindow/llwindowmacosx-objc.h b/indra/llwindow/llwindowmacosx-objc.h
index c22f3382fb..b06cd2c184 100644
--- a/indra/llwindow/llwindowmacosx-objc.h
+++ b/indra/llwindow/llwindowmacosx-objc.h
@@ -69,7 +69,7 @@ typedef const NativeKeyEventData * NSKeyEventRef;
 // These are defined in llappviewermacosx.cpp.
 bool initViewer();
 void handleQuit();
-bool runMainLoop();
+bool pumpMainLoop();
 void initMainLoop();
 void cleanupViewer();
 void handleUrl(const char* url);
diff --git a/indra/mac_crash_logger/llcrashloggermac.cpp b/indra/mac_crash_logger/llcrashloggermac.cpp
index 3149fad6e8..ec3616e26a 100644
--- a/indra/mac_crash_logger/llcrashloggermac.cpp
+++ b/indra/mac_crash_logger/llcrashloggermac.cpp
@@ -64,7 +64,7 @@ void LLCrashLoggerMac::gatherPlatformSpecificFiles()
 {
 }
 
-bool LLCrashLoggerMac::mainLoop()
+bool LLCrashLoggerMac::frame()
 {
 
     if (mCrashBehavior == CRASH_BEHAVIOR_ALWAYS_SEND)
diff --git a/indra/mac_crash_logger/llcrashloggermac.h b/indra/mac_crash_logger/llcrashloggermac.h
index 6d8f63ecac..05ef8c9f53 100644
--- a/indra/mac_crash_logger/llcrashloggermac.h
+++ b/indra/mac_crash_logger/llcrashloggermac.h
@@ -37,7 +37,7 @@ public:
 	LLCrashLoggerMac(void);
 	~LLCrashLoggerMac(void);
 	virtual bool init();
-	virtual bool mainLoop();
+	virtual bool frame();
 	virtual bool cleanup();
 	virtual void gatherPlatformSpecificFiles();
 };
diff --git a/indra/mac_crash_logger/mac_crash_logger.cpp b/indra/mac_crash_logger/mac_crash_logger.cpp
index 95d4e65207..54e41a1954 100644
--- a/indra/mac_crash_logger/mac_crash_logger.cpp
+++ b/indra/mac_crash_logger/mac_crash_logger.cpp
@@ -49,7 +49,7 @@ int main(int argc, char **argv)
     {
 //        return NSApplicationMain(argc, (const char **)argv);
     }
-	app.mainLoop();
+	app.frame();
 	app.cleanup();
 
 	LL_INFOS() << "Crash reporter finished normally." << LL_ENDL;
diff --git a/indra/newview/llappdelegate-objc.mm b/indra/newview/llappdelegate-objc.mm
index 549df80fa1..be8877328d 100644
--- a/indra/newview/llappdelegate-objc.mm
+++ b/indra/newview/llappdelegate-objc.mm
@@ -48,16 +48,19 @@
 - (void) applicationDidFinishLaunching:(NSNotification *)notification
 {
 	frameTimer = nil;
-	
+
 	[self languageUpdated];
-	
+
 	if (initViewer())
 	{
-		frameTimer = [NSTimer scheduledTimerWithTimeInterval:0.0 target:self selector:@selector(mainLoop) userInfo:nil repeats:YES];
+		// Set up recurring calls to oneFrame (repeating timer with timeout 0)
+		// until applicationShouldTerminate.
+		frameTimer = [NSTimer scheduledTimerWithTimeInterval:0.0 target:self
+							  selector:@selector(oneFrame) userInfo:nil repeats:YES];
 	} else {
 		handleQuit();
 	}
-	
+
 	[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(languageUpdated) name:@"NSTextInputContextKeyboardSelectionDidChangeNotification" object:nil];
 
  //   [[NSAppleEventManager sharedAppleEventManager] setEventHandler:self andSelector:@selector(handleGetURLEvent:withReplyEvent:) forEventClass:kInternetEventClass andEventID:kAEGetURL];
@@ -96,22 +99,29 @@
 
 - (NSApplicationDelegateReply) applicationShouldTerminate:(NSApplication *)sender
 {
-	if (!runMainLoop())
+	// run one frame to assess state
+	if (!pumpMainLoop())
 	{
+		// pumpMainLoop() returns true when done, false if it wants to be
+		// called again. Since it returned false, do not yet cancel
+		// frameTimer.
 		handleQuit();
 		return NSTerminateCancel;
 	} else {
+		// pumpMainLoop() returned true: it's done. Okay, done with frameTimer.
 		[frameTimer release];
 		cleanupViewer();
 		return NSTerminateNow;
 	}
 }
 
-- (void) mainLoop
+- (void) oneFrame
 {
-	bool appExiting = runMainLoop();
+	bool appExiting = pumpMainLoop();
 	if (appExiting)
 	{
+		// Once pumpMainLoop() reports that we're done, cancel frameTimer:
+		// stop the repetitive calls.
 		[frameTimer release];
 		[[NSApplication sharedApplication] terminate:self];
 	}
diff --git a/indra/newview/llappviewer.cpp b/indra/newview/llappviewer.cpp
index b6d02ea2f8..604e45f314 100644
--- a/indra/newview/llappviewer.cpp
+++ b/indra/newview/llappviewer.cpp
@@ -769,9 +769,6 @@ bool LLAppViewer::init()
 	//
 	// Start of the application
 	//
-#ifdef LL_DARWIN
-	mMainLoopInitialized = false;
-#endif
 
 	// initialize LLWearableType translation bridge.
 	// Memory will be cleaned up in ::cleanupClass()
@@ -1220,6 +1217,23 @@ bool LLAppViewer::init()
         boost::bind(&LLControlGroup::getU32, boost::ref(gSavedSettings), _1),
         boost::bind(&LLControlGroup::declareU32, boost::ref(gSavedSettings), _1, _2, _3, LLControlVariable::PERSIST_ALWAYS));
 
+	/*----------------------------------------------------------------------*/
+	// nat 2016-06-29 moved the following here from the former mainLoop().
+	mMainloopTimeout = new LLWatchdogTimeout();
+
+	// Create IO Pump to use for HTTP Requests.
+	gServicePump = new LLPumpIO(gAPRPoolp);
+
+	// Note: this is where gLocalSpeakerMgr and gActiveSpeakerMgr used to be instantiated.
+
+	LLVoiceChannel::initClass();
+	LLVoiceClient::getInstance()->init(gServicePump);
+	LLVoiceChannel::setCurrentVoiceChannelChangedCallback(boost::bind(&LLFloaterIMContainer::onCurrentChannelChanged, _1), true);
+
+	joystick = LLViewerJoystick::getInstance();
+	joystick->setNeedsReset(true);
+	/*----------------------------------------------------------------------*/
+
 	return true;
 }
 
@@ -1294,297 +1308,255 @@ static LLTrace::BlockTimerStatHandle FTM_AGENT_UPDATE("Update");
 // externally visible timers
 LLTrace::BlockTimerStatHandle FTM_FRAME("Frame");
 
-bool LLAppViewer::mainLoop()
+bool LLAppViewer::frame()
 {
-#ifdef LL_DARWIN
-	if (!mMainLoopInitialized)
-#endif
-	{
-        LL_INFOS() << "Entering main_loop" << LL_ENDL;
-		mMainloopTimeout = new LLWatchdogTimeout();
-		
-		//-------------------------------------------
-		// Run main loop until time to quit
-		//-------------------------------------------
-		
-		// Create IO Pump to use for HTTP Requests.
-		gServicePump = new LLPumpIO(gAPRPoolp);
-		
-		// Note: this is where gLocalSpeakerMgr and gActiveSpeakerMgr used to be instantiated.
-		
-		LLVoiceChannel::initClass();
-		LLVoiceClient::getInstance()->init(gServicePump);
-		LLVoiceChannel::setCurrentVoiceChannelChangedCallback(boost::bind(&LLFloaterIMContainer::onCurrentChannelChanged, _1), true);
-		
-		joystick = LLViewerJoystick::getInstance();
-		joystick->setNeedsReset(true);
-		
-#ifdef LL_DARWIN
-		// Ensure that this section of code never gets called again on OS X.
-		mMainLoopInitialized = true;
-#endif
-	}
-	// As we do not (yet) send data on the mainloop LLEventPump that varies
-	// with each frame, no need to instantiate a new LLSD event object each
-	// time. Obviously, if that changes, just instantiate the LLSD at the
-	// point of posting.
-	
 	LLEventPump& mainloop(LLEventPumps::instance().obtain("mainloop"));
-	
-    LLSD newFrame;
-	
+	LLSD newFrame;
+
 	LLTimer frameTimer,idleTimer;
 	LLTimer debugTime;
-	
+
 	//LLPrivateMemoryPoolTester::getInstance()->run(false) ;
 	//LLPrivateMemoryPoolTester::getInstance()->run(true) ;
 	//LLPrivateMemoryPoolTester::destroy() ;
 
-	// Handle messages
-#ifdef LL_DARWIN
-	if (!LLApp::isExiting())
-#else
-	while (!LLApp::isExiting())
-#endif
+	LL_RECORD_BLOCK_TIME(FTM_FRAME);
+	LLTrace::BlockTimer::processTimes();
+	LLTrace::get_frame_recording().nextPeriod();
+	LLTrace::BlockTimer::logStats();
+
+	LLTrace::get_thread_recorder()->pullFromChildren();
+
+	//clear call stack records
+	LL_CLEAR_CALLSTACKS();
+
+	//check memory availability information
+	checkMemory() ;
+
+	try
 	{
-		LL_RECORD_BLOCK_TIME(FTM_FRAME);
-		LLTrace::BlockTimer::processTimes();
-		LLTrace::get_frame_recording().nextPeriod();
-		LLTrace::BlockTimer::logStats();
+		pingMainloopTimeout("Main:MiscNativeWindowEvents");
+
+		if (gViewerWindow)
+		{
+			LL_RECORD_BLOCK_TIME(FTM_MESSAGES);
+			gViewerWindow->getWindow()->processMiscNativeEvents();
+		}
 
-		LLTrace::get_thread_recorder()->pullFromChildren();
+		pingMainloopTimeout("Main:GatherInput");
 
-		//clear call stack records
-		LL_CLEAR_CALLSTACKS();
+		if (gViewerWindow)
+		{
+			LL_RECORD_BLOCK_TIME(FTM_MESSAGES);
+			if (!restoreErrorTrap())
+			{
+				LL_WARNS() << " Someone took over my signal/exception handler (post messagehandling)!" << LL_ENDL;
+			}
 
-		//check memory availability information
-		checkMemory() ;
+			gViewerWindow->getWindow()->gatherInput();
+		}
+
+#if 1 && !LL_RELEASE_FOR_DOWNLOAD
+		// once per second debug info
+		if (debugTime.getElapsedTimeF32() > 1.f)
+		{
+			debugTime.reset();
+		}
 		
-		try
+#endif
+		//memory leaking simulation
+		LLFloaterMemLeak* mem_leak_instance =
+			LLFloaterReg::findTypedInstance<LLFloaterMemLeak>("mem_leaking");
+		if(mem_leak_instance)
 		{
-			pingMainloopTimeout("Main:MiscNativeWindowEvents");
+			mem_leak_instance->idle() ;				
+		}							
+
+		// canonical per-frame event
+		mainloop.post(newFrame);
 
-			if (gViewerWindow)
+		if (!LLApp::isExiting())
+		{
+			pingMainloopTimeout("Main:JoystickKeyboard");
+
+			// Scan keyboard for movement keys.  Command keys and typing
+			// are handled by windows callbacks.  Don't do this until we're
+			// done initializing.  JC
+			if ((gHeadlessClient || gViewerWindow->getWindow()->getVisible())
+				&& gViewerWindow->getActive()
+				&& !gViewerWindow->getWindow()->getMinimized()
+				&& LLStartUp::getStartupState() == STATE_STARTED
+				&& (gHeadlessClient || !gViewerWindow->getShowProgress())
+				&& !gFocusMgr.focusLocked())
 			{
-				LL_RECORD_BLOCK_TIME(FTM_MESSAGES);
-				gViewerWindow->getWindow()->processMiscNativeEvents();
+				joystick->scanJoystick();
+				gKeyboard->scanKeyboard();
 			}
-		
-			pingMainloopTimeout("Main:GatherInput");
-			
-			if (gViewerWindow)
+
+			// Update state based on messages, user input, object idle.
 			{
-				LL_RECORD_BLOCK_TIME(FTM_MESSAGES);
-				if (!restoreErrorTrap())
-				{
-					LL_WARNS() << " Someone took over my signal/exception handler (post messagehandling)!" << LL_ENDL;
-				}
+				pauseMainloopTimeout(); // *TODO: Remove. Messages shouldn't be stalling for 20+ seconds!
+				
+				LL_RECORD_BLOCK_TIME(FTM_IDLE);
+				idle();
 
-				gViewerWindow->getWindow()->gatherInput();
+				resumeMainloopTimeout();
 			}
 
-#if 1 && !LL_RELEASE_FOR_DOWNLOAD
-			// once per second debug info
-			if (debugTime.getElapsedTimeF32() > 1.f)
+			if (gDoDisconnect && (LLStartUp::getStartupState() == STATE_STARTED))
 			{
-				debugTime.reset();
+				pauseMainloopTimeout();
+				saveFinalSnapshot();
+				disconnectViewer();
+				resumeMainloopTimeout();
 			}
-			
-#endif
-			//memory leaking simulation
-			LLFloaterMemLeak* mem_leak_instance =
-				LLFloaterReg::findTypedInstance<LLFloaterMemLeak>("mem_leaking");
-			if(mem_leak_instance)
-			{
-				mem_leak_instance->idle() ;				
-			}							
-
-            // canonical per-frame event
-            mainloop.post(newFrame);
 
-			if (!LLApp::isExiting())
+			// Render scene.
+			// *TODO: Should we run display() even during gHeadlessClient?  DK 2011-02-18
+			if (!LLApp::isExiting() && !gHeadlessClient)
 			{
-				pingMainloopTimeout("Main:JoystickKeyboard");
-				
-				// Scan keyboard for movement keys.  Command keys and typing
-				// are handled by windows callbacks.  Don't do this until we're
-				// done initializing.  JC
-				if ((gHeadlessClient || gViewerWindow->getWindow()->getVisible())
-					&& gViewerWindow->getActive()
-					&& !gViewerWindow->getWindow()->getMinimized()
-					&& LLStartUp::getStartupState() == STATE_STARTED
-					&& (gHeadlessClient || !gViewerWindow->getShowProgress())
-					&& !gFocusMgr.focusLocked())
-				{
-					joystick->scanJoystick();
-					gKeyboard->scanKeyboard();
-				}
+				pingMainloopTimeout("Main:Display");
+				gGLActive = TRUE;
+				display();
+				pingMainloopTimeout("Main:Snapshot");
+				LLFloaterSnapshot::update(); // take snapshots
+				gGLActive = FALSE;
+			}
+		}
 
-				// Update state based on messages, user input, object idle.
-				{
-					pauseMainloopTimeout(); // *TODO: Remove. Messages shouldn't be stalling for 20+ seconds!
-					
-					LL_RECORD_BLOCK_TIME(FTM_IDLE);
-					idle();
+		pingMainloopTimeout("Main:Sleep");
 
-					resumeMainloopTimeout();
-				}
- 
-				if (gDoDisconnect && (LLStartUp::getStartupState() == STATE_STARTED))
-				{
-					pauseMainloopTimeout();
-					saveFinalSnapshot();
-					disconnectViewer();
-					resumeMainloopTimeout();
-				}
+		pauseMainloopTimeout();
 
-				// Render scene.
-				// *TODO: Should we run display() even during gHeadlessClient?  DK 2011-02-18
-				if (!LLApp::isExiting() && !gHeadlessClient)
+		// Sleep and run background threads
+		{
+			LL_RECORD_BLOCK_TIME(FTM_SLEEP);
+			
+			// yield some time to the os based on command line option
+			if(mYieldTime >= 0)
+			{
+				LL_RECORD_BLOCK_TIME(FTM_YIELD);
+				ms_sleep(mYieldTime);
+			}
+
+			// yield cooperatively when not running as foreground window
+			if (   (gViewerWindow && !gViewerWindow->getWindow()->getVisible())
+					|| !gFocusMgr.getAppHasFocus())
+			{
+				// Sleep if we're not rendering, or the window is minimized.
+				S32 milliseconds_to_sleep = llclamp(gSavedSettings.getS32("BackgroundYieldTime"), 0, 1000);
+				// don't sleep when BackgroundYieldTime set to 0, since this will still yield to other threads
+				// of equal priority on Windows
+				if (milliseconds_to_sleep > 0)
 				{
-					pingMainloopTimeout("Main:Display");
-					gGLActive = TRUE;
-					display();
-					pingMainloopTimeout("Main:Snapshot");
-					LLFloaterSnapshot::update(); // take snapshots
-					gGLActive = FALSE;
+					ms_sleep(milliseconds_to_sleep);
+					// also pause worker threads during this wait period
+					LLAppViewer::getTextureCache()->pause();
+					LLAppViewer::getImageDecodeThread()->pause();
 				}
 			}
-
-			pingMainloopTimeout("Main:Sleep");
 			
-			pauseMainloopTimeout();
+			if (mRandomizeFramerate)
+			{
+				ms_sleep(rand() % 200);
+			}
 
-			// Sleep and run background threads
+			if (mPeriodicSlowFrame
+				&& (gFrameCount % 10 == 0))
 			{
-				LL_RECORD_BLOCK_TIME(FTM_SLEEP);
-				
-				// yield some time to the os based on command line option
-				if(mYieldTime >= 0)
-				{
-					LL_RECORD_BLOCK_TIME(FTM_YIELD);
-					ms_sleep(mYieldTime);
-				}
+				LL_INFOS() << "Periodic slow frame - sleeping 500 ms" << LL_ENDL;
+				ms_sleep(500);
+			}
+
+			const F64Milliseconds max_idle_time = llmin(.005f*10.f*(F32Milliseconds)gFrameTimeSeconds, F32Milliseconds(5)); // 5 ms a second
+			idleTimer.reset();
+			S32 total_work_pending = 0;
+			S32 total_io_pending = 0;	
+			while(1)
+			{
+				S32 work_pending = 0;
+				S32 io_pending = 0;
+				F32 max_time = llmin(gFrameIntervalSeconds.value() *10.f, 1.f);
+
+				work_pending += updateTextureThreads(max_time);
 
-				// yield cooperatively when not running as foreground window
-				if (   (gViewerWindow && !gViewerWindow->getWindow()->getVisible())
-						|| !gFocusMgr.getAppHasFocus())
 				{
-					// Sleep if we're not rendering, or the window is minimized.
-					S32 milliseconds_to_sleep = llclamp(gSavedSettings.getS32("BackgroundYieldTime"), 0, 1000);
-					// don't sleep when BackgroundYieldTime set to 0, since this will still yield to other threads
-					// of equal priority on Windows
-					if (milliseconds_to_sleep > 0)
-					{
-						ms_sleep(milliseconds_to_sleep);
-						// also pause worker threads during this wait period
-						LLAppViewer::getTextureCache()->pause();
-						LLAppViewer::getImageDecodeThread()->pause();
-					}
+					LL_RECORD_BLOCK_TIME(FTM_VFS);
+ 					io_pending += LLVFSThread::updateClass(1);
 				}
-				
-				if (mRandomizeFramerate)
 				{
-					ms_sleep(rand() % 200);
+					LL_RECORD_BLOCK_TIME(FTM_LFS);
+ 					io_pending += LLLFSThread::updateClass(1);
 				}
 
-				if (mPeriodicSlowFrame
-					&& (gFrameCount % 10 == 0))
+				if (io_pending > 1000)
 				{
-					LL_INFOS() << "Periodic slow frame - sleeping 500 ms" << LL_ENDL;
-					ms_sleep(500);
+					ms_sleep(llmin(io_pending/100,100)); // give the vfs some time to catch up
 				}
 
-				const F64Milliseconds max_idle_time = llmin(.005f*10.f*(F32Milliseconds)gFrameTimeSeconds, F32Milliseconds(5)); // 5 ms a second
-				idleTimer.reset();
-				S32 total_work_pending = 0;
-				S32 total_io_pending = 0;	
-				while(1)
-				{
-					S32 work_pending = 0;
-					S32 io_pending = 0;
-					F32 max_time = llmin(gFrameIntervalSeconds.value() *10.f, 1.f);
-
-					work_pending += updateTextureThreads(max_time);
-
-					{
-						LL_RECORD_BLOCK_TIME(FTM_VFS);
-	 					io_pending += LLVFSThread::updateClass(1);
-					}
-					{
-						LL_RECORD_BLOCK_TIME(FTM_LFS);
-	 					io_pending += LLLFSThread::updateClass(1);
-					}
-
-					if (io_pending > 1000)
-					{
-						ms_sleep(llmin(io_pending/100,100)); // give the vfs some time to catch up
-					}
-
-					total_work_pending += work_pending ;
-					total_io_pending += io_pending ;
-					
-					if (!work_pending || idleTimer.getElapsedTimeF64() >= max_idle_time)
-					{
-						break;
-					}
-				}
-				gMeshRepo.update() ;
+				total_work_pending += work_pending ;
+				total_io_pending += io_pending ;
 				
-				if(!total_work_pending) //pause texture fetching threads if nothing to process.
+				if (!work_pending || idleTimer.getElapsedTimeF64() >= max_idle_time)
 				{
-					LLAppViewer::getTextureCache()->pause();
-					LLAppViewer::getImageDecodeThread()->pause();
-					LLAppViewer::getTextureFetch()->pause(); 
-				}
-				if(!total_io_pending) //pause file threads if nothing to process.
-				{
-					LLVFSThread::sLocal->pause(); 
-					LLLFSThread::sLocal->pause(); 
-				}									
-
-				//texture fetching debugger
-				if(LLTextureFetchDebugger::isEnabled())
-				{
-					LLFloaterTextureFetchDebugger* tex_fetch_debugger_instance =
-						LLFloaterReg::findTypedInstance<LLFloaterTextureFetchDebugger>("tex_fetch_debugger");
-					if(tex_fetch_debugger_instance)
-					{
-						tex_fetch_debugger_instance->idle() ;				
-					}
+					break;
 				}
+			}
+			gMeshRepo.update() ;
+			
+			if(!total_work_pending) //pause texture fetching threads if nothing to process.
+			{
+				LLAppViewer::getTextureCache()->pause();
+				LLAppViewer::getImageDecodeThread()->pause();
+				LLAppViewer::getTextureFetch()->pause(); 
+			}
+			if(!total_io_pending) //pause file threads if nothing to process.
+			{
+				LLVFSThread::sLocal->pause(); 
+				LLLFSThread::sLocal->pause(); 
+			}									
 
-				if ((LLStartUp::getStartupState() >= STATE_CLEANUP) &&
-					(frameTimer.getElapsedTimeF64() > FRAME_STALL_THRESHOLD))
+			//texture fetching debugger
+			if(LLTextureFetchDebugger::isEnabled())
+			{
+				LLFloaterTextureFetchDebugger* tex_fetch_debugger_instance =
+					LLFloaterReg::findTypedInstance<LLFloaterTextureFetchDebugger>("tex_fetch_debugger");
+				if(tex_fetch_debugger_instance)
 				{
-					gFrameStalls++;
+					tex_fetch_debugger_instance->idle() ;				
 				}
-				frameTimer.reset();
-
-				resumeMainloopTimeout();
-	
-				pingMainloopTimeout("Main:End");
-			}	
-		}
-		catch(std::bad_alloc)
-		{			
-			LLMemory::logMemoryInfo(TRUE) ;
+			}
 
-			//stop memory leaking simulation
-			LLFloaterMemLeak* mem_leak_instance =
-				LLFloaterReg::findTypedInstance<LLFloaterMemLeak>("mem_leaking");
-			if(mem_leak_instance)
+			if ((LLStartUp::getStartupState() >= STATE_CLEANUP) &&
+				(frameTimer.getElapsedTimeF64() > FRAME_STALL_THRESHOLD))
 			{
-				mem_leak_instance->stop() ;				
-				LL_WARNS() << "Bad memory allocation in LLAppViewer::mainLoop()!" << LL_ENDL ;
+				gFrameStalls++;
 			}
-			else
-			{
-				//output possible call stacks to log file.
-				LLError::LLCallStacks::print() ;
+			frameTimer.reset();
 
-				LL_ERRS() << "Bad memory allocation in LLAppViewer::mainLoop()!" << LL_ENDL ;
-			}
+			resumeMainloopTimeout();
+
+			pingMainloopTimeout("Main:End");
+		}	
+	}
+	catch(std::bad_alloc)
+	{			
+		LLMemory::logMemoryInfo(TRUE) ;
+
+		//stop memory leaking simulation
+		LLFloaterMemLeak* mem_leak_instance =
+			LLFloaterReg::findTypedInstance<LLFloaterMemLeak>("mem_leaking");
+		if(mem_leak_instance)
+		{
+			mem_leak_instance->stop() ;				
+			LL_WARNS() << "Bad memory allocation in LLAppViewer::frame()!" << LL_ENDL ;
+		}
+		else
+		{
+			//output possible call stacks to log file.
+			LLError::LLCallStacks::print() ;
+
+			LL_ERRS() << "Bad memory allocation in LLAppViewer::frame()!" << LL_ENDL ;
 		}
 	}
 
@@ -1618,7 +1590,7 @@ bool LLAppViewer::mainLoop()
 		LL_INFOS() << "Exiting main_loop" << LL_ENDL;
 	}
 
-	return LLApp::isExiting();
+	return ! LLApp::isRunning();
 }
 
 S32 LLAppViewer::updateTextureThreads(F32 max_time)
diff --git a/indra/newview/llappviewer.h b/indra/newview/llappviewer.h
index b5e674bd7b..f7c1bb58b4 100644
--- a/indra/newview/llappviewer.h
+++ b/indra/newview/llappviewer.h
@@ -79,7 +79,7 @@ public:
 	//
 	virtual bool init();			// Override to do application initialization
 	virtual bool cleanup();			// Override to do application cleanup
-	virtual bool mainLoop(); // Override for the application main loop.  Needs to at least gracefully notice the QUITTING state and exit.
+	virtual bool frame(); // Override for application body logic
 
 	// Application control
 	void flushVFSIO(); // waits for vfs transfers to complete
@@ -283,7 +283,6 @@ private:
 	std::string mSerialNumber;
 	bool mPurgeCache;
     bool mPurgeOnExit;
-	bool mMainLoopInitialized;
 	LLViewerJoystick* joystick;
 
 	bool mSavedFinalSnapshot;
diff --git a/indra/newview/llappviewerlinux.cpp b/indra/newview/llappviewerlinux.cpp
index f5742b29cf..6f32aab851 100644
--- a/indra/newview/llappviewerlinux.cpp
+++ b/indra/newview/llappviewerlinux.cpp
@@ -95,10 +95,8 @@ int main( int argc, char **argv )
 	}
 
 		// Run the application main loop
-	if(!LLApp::isQuitting()) 
-	{
-		viewer_app_ptr->mainLoop();
-	}
+	while (! viewer_app_ptr->frame()) 
+	{}
 
 	if (!LLApp::isError())
 	{
diff --git a/indra/newview/llappviewermacosx.cpp b/indra/newview/llappviewermacosx.cpp
index ca219fda59..4fe1e31668 100644
--- a/indra/newview/llappviewermacosx.cpp
+++ b/indra/newview/llappviewermacosx.cpp
@@ -117,12 +117,17 @@ void handleQuit()
 	LLAppViewer::instance()->userQuit();
 }
 
-bool runMainLoop()
+// This function is called pumpMainLoop() rather than runMainLoop() because
+// it passes control to the viewer's main-loop logic for a single frame. Like
+// LLAppViewer::frame(), it returns 'true' when it's done. Until then, it
+// expects to be called again by the timer in LLAppDelegate
+// (llappdelegate-objc.mm).
+bool pumpMainLoop()
 {
 	bool ret = LLApp::isQuitting();
 	if (!ret && gViewerAppPtr != NULL)
 	{
-		ret = gViewerAppPtr->mainLoop();
+		ret = gViewerAppPtr->frame();
 	} else {
 		ret = true;
 	}
diff --git a/indra/newview/llappviewerwin32.cpp b/indra/newview/llappviewerwin32.cpp
index 4786f83bfd..a7f248ab5a 100644
--- a/indra/newview/llappviewerwin32.cpp
+++ b/indra/newview/llappviewerwin32.cpp
@@ -317,10 +317,8 @@ int APIENTRY WINMAIN(HINSTANCE hInstance,
 	}
 	
 	// Run the application main loop
-	if(!LLApp::isQuitting()) 
-	{
-		viewer_app_ptr->mainLoop();
-	}
+	while (! viewer_app_ptr->frame()) 
+	{}
 
 	if (!LLApp::isError())
 	{
diff --git a/indra/test/llapp_tut.cpp b/indra/test/llapp_tut.cpp
index aa5c0672e6..98e714a497 100644
--- a/indra/test/llapp_tut.cpp
+++ b/indra/test/llapp_tut.cpp
@@ -41,7 +41,7 @@ namespace tut
 		public:
 			virtual bool init() { return true; }
 			virtual bool cleanup() { return true; }
-			virtual bool mainLoop() { return true; }
+			virtual bool frame() { return true; }
 		};
 		LLTestApp* mApp;
 		application()
diff --git a/indra/win_crash_logger/llcrashloggerwindows.cpp b/indra/win_crash_logger/llcrashloggerwindows.cpp
index 23c29e6b18..167acf6ac7 100644
--- a/indra/win_crash_logger/llcrashloggerwindows.cpp
+++ b/indra/win_crash_logger/llcrashloggerwindows.cpp
@@ -454,7 +454,7 @@ void LLCrashLoggerWindows::gatherPlatformSpecificFiles()
 	//mDebugLog["DisplayDeviceInfo"] = gDXHardware.getDisplayInfo();  //Not initialized.
 }
 
-bool LLCrashLoggerWindows::mainLoop()
+bool LLCrashLoggerWindows::frame()
 {	
 	LL_INFOS() << "CrashSubmitBehavior is " << mCrashBehavior << LL_ENDL;
 
@@ -503,14 +503,14 @@ bool LLCrashLoggerWindows::mainLoop()
 			TranslateMessage(&msg);
 			DispatchMessage(&msg);
 		}
-		return msg.wParam;
+		return true; // msg.wParam;
 	}
 	else
 	{
 		LL_WARNS() << "Unknown crash behavior " << mCrashBehavior << LL_ENDL;
-		return 1;
+		return true; // 1;
 	}
-	return 0;
+	return true; // 0;
 }
 
 void LLCrashLoggerWindows::updateApplication(const std::string& message)
diff --git a/indra/win_crash_logger/llcrashloggerwindows.h b/indra/win_crash_logger/llcrashloggerwindows.h
index 1812e2737e..f89b8708dc 100644
--- a/indra/win_crash_logger/llcrashloggerwindows.h
+++ b/indra/win_crash_logger/llcrashloggerwindows.h
@@ -46,7 +46,7 @@ public:
 	static LLCrashLoggerWindows* sInstance; 
 
 	virtual bool init();
-	virtual bool mainLoop();
+	virtual bool frame();
 	virtual void updateApplication(const std::string& message = LLStringUtil::null);
 	virtual bool cleanup();
 	virtual void gatherPlatformSpecificFiles();
diff --git a/indra/win_crash_logger/win_crash_logger.cpp b/indra/win_crash_logger/win_crash_logger.cpp
index 366edd894b..7466dbb766 100644
--- a/indra/win_crash_logger/win_crash_logger.cpp
+++ b/indra/win_crash_logger/win_crash_logger.cpp
@@ -52,7 +52,7 @@ int APIENTRY WinMain(HINSTANCE hInstance,
 	}
 
 	app.processingLoop();
-	app.mainLoop();
+	app.frame();
 	app.cleanup();
 	LL_INFOS() << "Crash reporter finished normally." << LL_ENDL;
 	return 0;