We accidentally started using the non-sbrk code path for the main arena in a glibc 2.42 development version of glibc. This led to several bug reports:

• libc-alpha bug report This turned out to be an LLVM bug: [MachO] Improve bounds check #141083
• Bug 32996 - malloc regression on i686, also: Bug 2368545 - malloc regression on i686 blocks the rebuild of python-pyside6

We tried to trigger this code in testing using an obstruction that forces malloc away from the sbrk code paths, but we have not been able to trigger these issues.

We need to either remove this code, or increase its test coverage.