Race condition when downloading the same archive twice at the same time

[kit-ty-kate]

On rare occasions, the cache will be written in at the same time and the following error appears:

#=== ERROR while fetching sources for utop.2.12.0 =============================#
OpamSolution.Fetch_fail("/usr/bin/opam: \"symlink\" failed on /home/opam/.opam/download-cache/sha256/ad/ad19c859a783bec573cd91e810c54d0e6b70f339d0a4fed55ec672ae408aa1ea: File exists")

Steps to reproduce:

• Clear the cache
• Create two switches
• Exec opam install --switch <switch-1> <pkg> & ; opam install --switch <switch-2> <pkg> & ; wait
• Reapeat until one breaks

Version: dbe3066b720e57d8e288e73294999c0b594b2ab0

[rjbou]

from dev meeting: we need to handle the exception. If file exists, checks that it is the good one.