Lua Script GIMP doesn't waits for GIMP locks when it's previously opened

Description of the Issue

The contrib/gimp.lua script fails to open images in GIMP when an instance of GIMP is already running. The image is exported by Darktable, but GIMP displays a "No such file or directory" error, and the image is never re-imported into Darktable.

The Root Cause

Race Condition The script's logic relies on the GIMP process being blocking (waiting until the application closes) to proceed to the file cleanup/re-import phase.

Scenario A (GIMP Closed): The script works. The process blocks, user edits, closes GIMP, and the script continues.

Scenario B (GIMP Open): Modern GIMP behavior (especially GIMP 3.0 AppImages or modern 2.10 distros using DBus) defaults to "Single Instance" mode. The command sends a signal to the running instance and returns control immediately (non-blocking) to the shell.

Because of this non-blocking return, the Lua script immediately proceeds to df.file_move, moving/renaming the source file before the GIMP instance (which queues the open request as an idle job) has a chance to read it.

Verification

GIMP 3.0 (AppImage): Confirmed failure due to immediate process return.

GIMP 2.10 (Ubuntu Studio Repo): Confirmed failure on a standard, fresh install, ruling out custom environment issues.

CLI Reproduction: gimp image.jpg && mv image.jpg destination.jpg reproduces the failure in the terminal (GIMP fails to read because mv happens instantly).

The Solution According to GIMP Developer Jehan (discussed in GNOME/gimp #14827), the correct way to maintain the script's intended sequential workflow (Edit -> Wait -> Re-import) is to force a new instance.

Adding the --new-instance flag forces GIMP to spawn a new process that blocks the terminal until that specific window is closed, preventing the race condition.

Proposed Change In contrib/gimp.lua, modify the command construction:
Lua

-- Current implementation (around line 99) gimpStartCommand = gimp_executable .. " " .. img_list

-- Proposed fix
gimpStartCommand = gimp_executable .. " --new-instance " .. img_list

Result of the Fix I verified this locally. With --new-instance, the script correctly waits for the editing session to close, the file remains available for GIMP to read, and the re-import to Darktable succeeds upon closing the window.

Open contrib/gimp.lua and inspect the GIMP command construction around line 99. Reproduce the workflow with GIMP already running, then apply the proposed command change and verify that the editing session waits, the source file remains available, and re-import into darktable succeeds after closing GIMP.