Export plugin on Macintosh not honoring LrTasks.startAsyncTask ?

Question

Hi -- I need some help again!
I'm writing an export plugin for my own application. 
On Windows, everything works as expected -- I can start the export and then go back to doing other tasks in Lightroom while my application does its own thing.
But on Macintosh, Lightroom hangs with the rotating "busy" icon until my application quits.
I am baffled by this behavior because I think I've wrapped everything I'm doing in calls to LrTasks.startAsyncTask.
Apparently I'm missing something about how asynchronous tasks work in Lightroom, or at least in Lightroom on Macintosh.
Do you have any suggestions about how I can get the Mac version to keep Lightroom responsive while my exported-to application does its thing?
Thanks!
--Rik
For further information...
Mac OS X 10.9, Lightroom 4.2
Here is the current structure of my ExportToMA.lua file
local myLogger        = LrLogger("myLogger")
myLogger:enable("logfile")
ExportToMA = {}
ExportToMA.outputToLog = function(param)
  myLogger:trace(param)
end
LrTasks.startAsyncTask( function()
    myLogger:trace("Entering outer LrTasks.startAsyncTask function")
    LrTasks.startAsyncTask( function()
        myLogger:trace("Entering inner LrTasks.startAsyncTask function")
        local activeCatalog = LrApplication.activeCatalog()
        local frameSet     = activeCatalog.targetPhotos
        local exportSession = LrExportSession( {
            exportSettings = {
                LR_exportServiceProvider       = "net.mydomain.MAloader",
                LR_exportServiceProviderTitle  = "My Application",
                LR_format                      = "TIFF",
                LR_tiff_compressionMethod      = "compressionMethod_None",
                LR_export_bitDepth             = 16,
                LR_export_colorSpace           = "sRGB",
                LR_minimizeEmbeddedMetadata    = false
            photosToExport = frameSet,
        myLogger:trace("Before doExportOnCurrentTask()")
        exportSession:doExportOnCurrentTask()       
        myLogger:trace("After doExportOnCurrentTask()")
        myLogger:trace("Exiting inner LrTasks.startAsyncTask function")
    end )
    myLogger:trace("Exiting outer LrTasks.startAsyncTask function")
end )
When run on Windows, the log file contains a sequence of messages like this:
12/30/2013 10:36:48 TRACE    Entering outer LrTasks.startAsyncTask function
12/30/2013 10:36:48 TRACE    Exiting outer LrTasks.startAsyncTask function
12/30/2013 10:36:48 TRACE    Entering inner LrTasks.startAsyncTask function
12/30/2013 10:36:48 TRACE    Before doExportOnCurrentTask()
12/30/2013 10:36:49 TRACE    After doExportOnCurrentTask()
12/30/2013 10:36:49 TRACE    Exiting inner LrTasks.startAsyncTask function
   <meanwhile my application continues to run while Lightroom stays responsive>
But when run on Macintosh, this is what happens:
2013-12-30 19:01:50 +0000, TRACE    Entering outer LrTasks.startAsyncTask function
2013-12-30 19:01:50 +0000, TRACE    Exiting outer LrTasks.startAsyncTask function
2013-12-30 19:01:50 +0000, TRACE    Entering inner LrTasks.startAsyncTask function
2013-12-30 19:01:50 +0000, TRACE    Before doExportOnCurrentTask()
  <indefinitely long pause while my application runs>
2013-12-30 19:02:44 +0000, TRACE    After doExportOnCurrentTask()
2013-12-30 19:02:44 +0000, TRACE    Exiting inner LrTasks.startAsyncTask function
Ideas?

Answer 1

> I'm confused by your application: if you already have a renditions loop, then why are you starting another export
I apologize for the confusion. 
In each case I've exercised, there is only one export and one renditions loop. 
The code I've been showing you is what gets executed to service a menu entry in File > Plug-In Extras.  I'm focusing attention on that case because it seems more transparent.  Notably, I get to specify which task the export happens on. 
The renditions loop in my processRenderedPhotos function is in another script.  The renditions loop gets called in two places: 1) from File > Plug-In Extras via the explicit doExportOnCurrentTask (now doExportOnNewTask), and 2) from Lightroom's internal processing of File > Export... and File > Export with Preset.
> did you try doExportOnNewTask?
I tried it just now.  It did not change the behavior.
I also added some more diagnostics.  Here is what happens on the Mac:
2013-12-31 01:09:31 +0000, TRACE    Entering outer LrTasks.startAsyncTask function
2013-12-31 01:09:31 +0000, TRACE    Exiting outer LrTasks.startAsyncTask function
2013-12-31 01:09:31 +0000, TRACE    Entering inner LrTasks.startAsyncTask function
2013-12-31 01:09:31 +0000, TRACE    Before doExportOnNewTask()
2013-12-31 01:09:31 +0000, TRACE    After doExportOnNewTask()
2013-12-31 01:09:31 +0000, TRACE    Exiting inner LrTasks.startAsyncTask function
2013-12-31 01:09:31 +0000, TRACE    MyApplicationUploadTask.processRenderedPhotos prefs.maPath: /Users/myaccount/Desktop/MyApplication.app/Contents/MacOS/JavaApplicationStub
2013-12-31 01:09:31 +0000, TRACE    tempPath: /var/folders/4n/2lxf6v593x5_c3lmx_jghgnw0000gn/T/temp_lre_ma_1230170931
2013-12-31 01:09:31 +0000, TRACE    nPhotos = 2
2013-12-31 01:09:32 +0000, TRACE    source path/Users/myaccount/Desktop/jpeg-8d/testorig.jpg
2013-12-31 01:09:32 +0000, TRACE    destination path: /var/folders/4n/2lxf6v593x5_c3lmx_jghgnw0000gn/T/6F05FC8E-126C-47E0-AD51-C545CE0DA66D/tes torig.tif
2013-12-31 01:09:32 +0000, TRACE    source path/Users/myaccount/Desktop/jpeg-8d/testprog.jpg
2013-12-31 01:09:32 +0000, TRACE    destination path: /var/folders/4n/2lxf6v593x5_c3lmx_jghgnw0000gn/T/6F05FC8E-126C-47E0-AD51-C545CE0DA66D/tes tprog.tif
2013-12-31 01:09:32 +0000, TRACE    just before LrShell.openFilesInApp
   <long pause here, while I let my application run for a while>
2013-12-31 01:10:00 +0000, TRACE    just after LrShell.openFilesInApp
The LrShell.openFilesInApp call happens after my renditions loop, at the very bottom of my processRenderedPhotos function, which is now separated by 3 levels of asynchronous tasks from whatever task initially serviced the menu entry.
Nonetheless, on Macintosh only, as soon as LrShell.openFilesInApp is called, Lightroom comes to a grinding halt heralded by a spinning beach ball. 
It's as if, deep under the covers, LrShell.openFilesInApp (on Macintosh only!) is telling the rest of Lightroom to wait until the spawned application terminates. 
I know, it sounds crazy to me too, and I've been programming for close to 45 years.
At the moment, the only way I see to work around this problem is to provide a separate "application" that is actually a spawn-and-terminate, so that LrShell.openFilesInApp will see a rapid termination of the thing that it launches.  That seems like a lot of trouble for what I've been hoping is a simple misunderstanding on my part.
The one thing I can see different between my app and anybody else's is that mine is written in Java, so the thing that gets launched by LrShell.openFilesInApp is the JavaApplicationStub.
Any other ideas?
--Rik
PS.
> Do you have this problem with hard drive export too?
I doubt it, but I don't know for sure.  On my test Mac, at the moment, there are not enough files to keep hard drive export running long enough to notice.