Cygwin: OpenBLAS doesn't read environment variables properly

[embray]

In Sage we currently, by default, disable multithreading in OpenBLAS by setting OPENBLAS_NUM_THREADS=1 in sage-env.

While this is not ideal, it does reduce the likelihood of multithreading issues coming up in contexts where it wasn't explicitly requested.

While investigating #27434, I found that there is a subtle and apparently rare bug in OpenBLAS's multithreading on Windows, which was causing the docbuild to hang, particularly while building the plot3d docs (which happen to exercise this area of the code more than anything else in Sage).

It occurred to me, however, that the docbuild should not be exhibiting the bug since we set OPENBLAS_NUM_THREADS=1. It turns out this is due to a bug in OpenBLAS on Cygwin, fixed by the attached patch.

I'm setting this to "critical" since it would be very good to have this fixed for the buildbot. I think this bug has been affecting Cygwin for a long time, but I could never nail down why. Nevertheless since we have lived with this bug thus far I won't consider it a blocker.

Upstream PR: OpenMathLib/OpenBLAS#2060

Upstream: Fixed upstream, but not in a stable release.

Component: porting: Cygwin

Author: Erik Bray

Branch/Commit: abe0d27

Reviewer: Volker Braun

Issue created by migration from https://trac.sagemath.org/ticket/27493

[embray]

New commits:

abe0d27

Trac #27493: Patch OpenBLAS to read environment variables properly on

[embray]

Commit: abe0d27

[embray]

Branch: u/embray/ticket-27493

[embray]

Upstream: Fixed upstream, but not in a stable release.

[embray]

comment:3

Moving all blocker/critical issues from 8.7 to 8.8.

[vbraun]

Reviewer: Volker Braun

[vbraun]

Changed branch from u/embray/ticket-27493 to abe0d27