Bug 1931872

Description of problem:

When two 'dnf' instances are running in parallel, the cache is unprotected leading the second instance waiting to fail.

shell #1
# dnf install vim-enhanced

shell #2
# dnf install vim-enhanced


shell #1
- downloads the dependencies, installs and verifies the deps.

shell #2
- notices the downloads are there, waits shell #1 to complete, then fails.


Version-Release number of selected component (if applicable):
dnf-4.2.23-4.el8.noarch

How reproducible:
Always

Steps to Reproduce:
1. Just install the same package using two dnf in parallel

Actual results:
[SKIPPED] pkgconf-1.4.2-1.el8.x86_64.rpm: Already downloaded                                              
[SKIPPED] pkgconf-m4-1.4.2-1.el8.noarch.rpm: Already downloaded                                           
[SKIPPED] pkgconf-pkg-config-1.4.2-1.el8.x86_64.rpm: Already downloaded                                   
[SKIPPED] xml-common-0.6.3-50.el8.noarch.rpm: Already downloaded                                          
Running transaction check                                                                                 
Waiting for process with pid 27640 to finish.                    
Transaction check succeeded.                                                                              
Running transaction test                                  
Error: Cannot open file /var/cache/dnf/beaker-AppStream-822659b7f413e82c/packages/pixman-0.38.4-1.el8.x86_6
4.rpm: [Errno 2] No such file or directory: '/var/cache/dnf/beaker-AppStream-822659b7f413e82c/packages/pixm
an-0.38.4-1.el8.x86_64.rpm'                                                                               
TypeError: an integer is required (got type NoneType)                                                     
FATAL ERROR: python callback ??? failed, aborting! 

Expected results:
To proper synchronize the threads.

Additional info:

The example above is a simplified case to make an easy reproducer.

The real use-case is that there is a daemon installed by Beaker that install packages (rstrnt-package) and that happens while the CI is accessing the server to deploy all the dependencies. That problem always happen when the dependencies coincides.

I can imagine another use case that the user could install something using CLI while the package updater is upgrading as well.