options/internal: SIGSEGV in locking code

[Dennisbonke]

While testing git on managarm, the following bug came to my attention.
By doing the following operations in order, the program can be killed with an SIGSEGV
To reproduce:

• Have git installed and a git repo available, in this test case managarm
• Do the required git setup, for managarm, this is touch /dev/null, and setting the global username and email (git will complain about this)
• In the git repo, touch a file, and run git add .
• Run git commit -m <message>, where message is the commit message (irrelevant for triggering this bug) and watch it crash.

Running addr2line on libc.so gives the following line

dennis@DENNIS-MAIN-LAPTOP-SIDUCTION:~/managarm-clone/build$ addr2line --e system-root/usr/lib/libc.so 
0x987d0
/home/dennis/managarm-clone/build/pkg-builds/mlibc/../../../src/mlibc/options/internal/include/mlibc/lock.hpp:24

which translates to

if(__atomic_compare_exchange_n(&_futex, &expected, 1,
							false, __ATOMIC_ACQUIRE, __ATOMIC_ACQUIRE)) {

where the first part of the if statement is line 24 in lock.hpp.