Fetching error when pulling multiple images that have the same image base

[geektortoise]

When we add a software user repository and fetch all the targeted repositories, sometimes I get "Error: unable to build: conveyor failed to get: no descriptor found for reference".

It seems that, as our external softwares are often based on the same image base (cytomine/python-software for example), we got a multithread issue with concurrent fetch.

There is example in the related issue
https://github.com/sylabs/singularity/issues/3634 and https://github.com/sylabs/singularity/issues/5020

Short solution : implement a pullingQueue to assure that there is no concurrency pulling.
Later solution : Should be resolved with singularity 3.6