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johnrellis
johnrellis
Brainiac
December 19, 2016

P: SDK: LrTasks.pcall fails when called from an LrExportServiceProvider script or observer function

  • December 19, 2016
  • 5 replies
  • 530 views

The following expression fails when called from an LrExportServiceProvider script:

LrTasks.pcall (LrFileUtils.exists, ".")

It generates the error "attempt to yield across metamethod/C-call boundary". This makes my Debugging Toolkit difficult, if not impossible, to use with an export service.  (The toolkit uses LrTasks.pcall() to catch errors, and it uses LrFileUtils.exists() to implement a more useful require().)

Tested in LR CC 2015.8 / OS X 10.11.5.

To reproduce the bug:

1. Start with a copy of  "ftp_upload.lrdevplugin" from the "Sample Plugins" folder of the Lightroom SDK 6.0.

2. Insert these lines at the top of "FTPUploadServiceProvider.lua":

local LrFileUtils = import 'LrFileUtils'local 
LrTasks = import 'LrTasks'
LrTasks.pcall (LrFileUtils.exists, ".")

3. Try to export a photo with FTP Upload. The post-processing action is never invoked.

4. Go to File > Plug-in Manager and observe that the plugin encountered an error:



5. Comment out the lines inserted in step 2 and reload the plugin in the Plug-in Manager.

6. Export a photo with FTP Upload again and observe that the post-processing action is invoked, prompting for a username and password.

    This topic has been closed for replies.

    5 replies

    johnrellis
    johnrellisAuthor
    Brainiac
    December 27, 2016
    LrTasks.pcall() also fails when called from a property table's observer function, producing the message with "attempt to yield across metamethod/C-call boundary" and sending LR into an infinite loop.

    To reproduce, make a plugin with a single File menu command running this script:
    local LrBinding = import 'LrBinding'
local LrFileUtils = import 'LrFileUtils'
    local LrFunctionContext = import 'LrFunctionContext'
    local LrTasks = import 'LrTasks'

    LrFunctionContext.postAsyncTaskWithContext ("", function (context)
        local prop = LrBinding.makePropertyTable (context)
        prop:addObserver ("x", function ()
            LrTasks.pcall (LrFileUtils.exists, "/")
            end)
        prop.x = 1
        end)
    When the command is executed, File > Plug-in Manager shows the following error:


    The number of diagnostic messages continues to increase indefinitely.

    In practice, it isn't easy to restructure the code to avoid calling LrFileUtils from within LrTasks.pcall(), since there a number of layers of abstraction between the LrTasks.pcall() and the call to LrFileUtils.  Something is obviously wrong with LrTasks.pcall().
    johnrellis
    johnrellisAuthor
    Brainiac
    December 19, 2016
    And I'm pretty sure I tested this with LR 3 (in 2010) and this problem didn't occur.  So it may have been introduced sometime in the last six years.
    johnrellis
    johnrellisAuthor
    Brainiac
    December 19, 2016
    I neglected to add that this also occurs with CC 2015.5 (OS X 10.11.5), the only other Mac version I have installed.  
    F
    Community Manager
    FeedbackCommunityMemberCommunity Manager
    Community Manager
    December 19, 2016
    Smit - I've been working with John on this and I first saw it in 2015.7 but I can't say that's when it started. It's possible it's been around for a while before that too.

    db
    Smit K
    Community Manager
    Smit KCommunity Manager
    Community Manager
    December 19, 2016
    Hi John,

    I have logged a bug for this. Could you confirm if this worked prior to 6.8?

    Thanks,
    Smit Keniya
    Adobe Lightroom Team
    Smit | Lightroom Team
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