select.select cannot be used for file descriptors > 1024

[chrisburr]

Copying from https://lblogbook.cern.ch/Operations/34170 as this is a bug in DIRAC.

The WorkloadManagement/Optimizers instances in LHCb started failing with:

2021-06-26 06:49:05 UTC WorkloadManagement/Optimizers ERROR: Exception while selecting persistent connections
Traceback (most recent call last):
 File "/opt/dirac/pro/DIRAC/Core/DISET/private/MessageBroker.py", line 151, in __listenAutoReceiveConnections
   inList, _outList, _exList = select.select([pos[1] for pos in sIdList], [], [], 1)
ValueError: filedescriptor out of range in select()

Looking into the cpython sources[1,2] it turns out this is a glibc limitation that file descriptors larger than 1024 can't be monitored with the select() system call and poll/epoll must be used instead.[3]

[1] https://github.com/python/cpython/blob/v2.7.18/Modules/selectmodule.c#L113-L119
[2] https://github.com/python/cpython/blob/v2.7.18/Include/fileobject.h#L89
[3] https://access.redhat.com/solutions/488623

[chaen]

Related issue that I deliberately ignored :D #4819

[chrisburr]

Maybe the correct solution to this issue would be to replace all uses of select.select with selectors.DefaultSelector. It's Python 3 only but there is a backport for Python 2.

[chaen]

It probably is the best option. Do you want to give it a go as hotfix in LHCb, since we are impacted by this on what seems to be a regular basis lately ?

[fstagni]

The newly created v7r2p19 uses DIRACOS v1r23 which includes selectors2