SCP Plugin for QGIS 3.44 (on MacOS Tahoe) Spawns Multiple copies of QGIS when Activated #447

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1. NR3Y commented on Jul 16, 2026

   

   NR3Y

   on Jul 16, 2026

   Author

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   More detailed information (after spending a couple of hours running suggestions from Anthropic)

   Title: macOS: activating SCP 8.5.0 on QGIS 3.44 spawns multiple full QGIS instances via multiprocessing spawn misconfiguration — persists even after correcting set_executable() path

   Environment: macOS Tahoe, Apple Silicon, QGIS 3.44, SCP 8.5.0

   Summary:
   Checking the "activate" checkbox for SCP in the QGIS Plugin Manager (no tool run required) immediately spawns 4–5 full QGIS.app instances. The original becomes unresponsive. Terminating a spawned instance causes another to immediately reappear; only force-quitting all instances stops it. Reproduces every time.

   Console output (./QGIS from Contents/MacOS):

   ERROR: Status 2: File /Applications/QGIS.app/Contents/MacOS/-c could not be found
   ERROR: Status 2: File /Applications/QGIS.app/Contents/MacOS/from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=18, pipe_handle=55) could not be found
   ERROR: Status 2: File /Applications/QGIS.app/Contents/MacOS/--multiprocessing-fork could not be found
   ERROR: Status 2: File /Applications/QGIS.app/Contents/MacOS/from multiprocessing.resource_tracker import main;main(17) could not be found
   

   Root cause (confirmed):
   sys.executable inside QGIS's embedded Python resolves to the QGIS launcher itself:

   >>> sys.exec_prefix
   '/Applications/QGIS.app/Contents/Frameworks/'
   >>> sys.executable
   '/Applications/QGIS.app/Contents/MacOS/QGIS'

   When multiprocessing.set_start_method('spawn') runs (unconditionally, at module import) without a working set_executable() redirect, each spawned worker re-invokes sys.executable — the QGIS launcher — with -c "..." / --multiprocessing-fork args. QGIS can't interpret these as files to open and opens a new blank window per attempt instead.

   Existing macOS path detection is outdated for this build:
   The darwin branch in semiautomaticclassificationplugin.py's __init__ tries:

   1. sys.exec_prefix/bin/python3
   2. sys.exec_prefix/../Resources/bin/python3

   Neither exists here. The real interpreter is at /Applications/QGIS.app/Contents/MacOS/python3.12, confirmed via find.

   I patched the darwin branch locally to add a third fallback (path.join(sys.exec_prefix, '../MacOS/python3.12')), confirmed to resolve correctly and point to an existing file. This did not resolve the issue — identical crash even with a correct, confirmed set_executable() path in place. This suggests either:

   • remotior_sensus.Session(...) (called later in __init__ with multiprocess_module=multiprocessing) resets/bypasses the executable configuration set earlier, or
   • the crash originates from a different multiprocessing entry point than the darwin branch.

   Request: Since patching the known darwin branch didn't fix it, could you point to where else multiprocessing's context/executable gets configured or reset? Happy to test further patches.
2. semiautomaticgit commented on Jul 20, 2026

   

   semiautomaticgit

   on Jul 20, 2026

   Owner

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   Hello @NR3Y ,

   about your issue it is likely a QGIS installation issue, possibly related to other Python environments in the system.
   In general to solve these kind of issues, one should first clean the whole system (removing all the Python versions), then reinstall QGIS.
   Unfortunately I don't have experience in how to clean the environment with macOS. However, I suggest to install QGIS 4 because maybe it could configure a different Python environment.
   Also, I recommend installing the new SCP version 9 for QGIS 4.
   I'm moving this to discussions, because the SCP v. 8.5 is outdated.
3. Repository owner locked and limited conversation to collaborators on Jul 20, 2026
4. semiautomaticgit

   converted this issue into a discussion #448 on Jul 20, 2026