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[LinuxVersion] Use "Construct on first use" idiom and add mutex

Closed Florian Schmaus requested to merge flow/emper:linux-version-construct-on-first-use into master

Use the "Construct on first use" idom for UTS Release (aka Linux version) initialization. The previous implementation was fragle and could lead to segfaults like

__s2=0x7fffffffe480 "5.16.10-arch1-1", __s1=<optimized out>)
at /usr/include/c++/11.2.0/bits/char_traits.h:409
__n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
at /usr/include/c++/11.2.0/bits/basic_string.h:359
__n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
at /usr/include/c++/11.2.0/bits/basic_string.h:354
__str=..., this=0x7ffff7fbd2a0 <emper::lib::LinuxVersion::globalVersion[abi:cxx11]>)
at /usr/include/c++/11.2.0/bits/basic_string.h:739
at ../emper/lib/LinuxVersion.cpp:46
at /usr/include/c++/11.2.0/ext/new_allocator.h:79

if globalVersion's constructor was not yet called. This is the so called "static initialization order fiasco" in C++.

While the mutex may not be strictly requires, as we do not call LinuxVersion's non-argument constructor nor getUtsRelease() concurrently, it can not hurt to have one.