Sage silently resets `SAGE_NUM_THREADS` environment variable

[maxale]

Steps To Reproduce

Run Sage from shell via

SAGE_NUM_THREADS=8 /usr/local/sage/sage

and execute:

sage: import os
sage: os.environ["SAGE_NUM_THREADS"]
'1'

Expected Behavior

We should see value '8' that we set up.

Actual Behavior

Our value was overwritten with '1'.

Additional Information

The bug was introduced somewhere in between version 10.7.beta8 (which works as expected) and current 10.8.beta4.

Environment

• OS: Ubuntu 24.04.3 LTS
• Sage Version: 10.8.beta4

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[antonio-rojas]

The bug is in https://github.com/sagemath/sage/blob/10.8.beta5/src/bin/sage-env#L583:

        # First, figure out the right values for SAGE_NUM_THREADS (default
        # number of threads) and SAGE_NUM_THREADS_PARALLEL (default number of
        # threads when parallel execution is asked explicitly).
        sage_num_threads_array=$(sage-num-threads.py 2>/dev/null || echo 1 2 1)

The path of sage-num-threads.py is not specified, so the script isn't found and the fallback 1 2 1 is always used.

However, this code hasn't been touched in over 5 years. Are you sure that this worked for you with the exact same setup?

[maxale]

@antonio-rojas Thank you for the analysis. I can confirm that the reported issue I observed only recently, so the reason may be that the control flow was somehow changed and the "old code" kicked in just now. Checking why this code did not break things in the release version 10.6 could shed light on what's going on behind the hood.

[antonio-rojas]

@antonio-rojas Thank you for the analysis. I can confirm that the reported issue I observed only recently, so the reason may be that the control flow was somehow changed and the "old code" kicked in just now. Checking why this code did not break things in the release version 10.6 could shed light on what's going on behind the hood.

Since the code in question did not change, the only possible explanation I can think of is that the dir where sage-num-threads.py lives was previously in PATH at the point where sage-env is imported, and it no longer is. Sounds like #39030 could be related, but I don't use sage-the-distro myself so I can't confirm.

[maxale]

I'm not sure what is sage-the-distro. I build Sage from sources from this github. The latest version, which I seen without this issue, was 10.7.beta8.