Crash when updating settings while rendering

[CodeFHD]

Observed in BLC but recorded under LuxCore because it sounds like a core bug:

Blender 5.1, on Archlinux

While testing another bug with a latest, local build, I encountered occasional crashes when changing shader parameters while viewport render was active. Specifically, I was changing UV Scale or Rotation on the 2D Mapping-Node.
The crash does not happen every time, which leads me to believe it might be due to a race condition.

The following error is printed in the console:

/usr/include/c++/16.1.1/bits/stl_bvector.h:1158: constexpr std::vector<bool, _Alloc>::reference std::vector<bool, _Alloc>::operator[](size_type) [with _Alloc = std::allocator<bool>; reference = std::vector<bool>::reference; size_type = long unsigned int]: Assertion '__n < this->size()' failed.
(core dumped) ./blender

backtrace from gdb (until first occurrences in LuxCore):

#0  0x00007fffddc9a29c in ?? () from /usr/lib/libc.so.6
#1  0x00007fffddc3e7d0 in raise () from /usr/lib/libc.so.6
#2  0x00007fffddc25681 in abort () from /usr/lib/libc.so.6
#3  0x00007fffeefbf460 in std::__glibcxx_assert_fail(char const*, int, char const*, char const*) ()
   from /home/jojo/Blender/blender-5.1.0-linux-x64/lib/libOpenEXR.so.33
#4  0x00007fff6379a0c3 in std::vector<bool, std::allocator<bool> >::operator[] (this=0x7fff1ae12d00, __n=3)
    at /usr/include/c++/16.1.1/bits/stl_bvector.h:1158
#5  0x00007fff63b18c8a in slg::ImageMapCache::DefineImageMap (this=0x7fff1ae12c90, 
    im=std::shared_ptr<slg::ImageMap> (use count 1, weak count 0) = {...})
    at /home/jojo/swdev/LUXCORERENDER/LuxCore/src/slg/imagemap/imagemapcache.cpp:157
#6  0x00007fff63b18999 in slg::ImageMapCache::DefineImageMap (this=0x7fff1ae12c90, im=...)
    at /home/jojo/swdev/LUXCORERENDER/LuxCore/src/slg/imagemap/imagemapcache.cpp:134

[CodeFHD]

I encountered another crash when linking nodes inputs/outputs while rendering is active:

[Exporter] Update because of: MATERIAL | OBJECT | REQUIRES_SCENE_EDIT | WORLD
Deleting object:TLd4e4d076ccbc5efd_0
Deleting object:TLd4e4d076ccbc5efd_1
/usr/include/c++/16.1.1/array:219: constexpr const std::array<_Tp, _Nm>::value_type& std::array<_Tp, _Nm>::operator[](size_type) const [with _Tp = unsigned char; long unsigned int _Nm = 3; const_reference = const unsigned char&; size_type = long unsigned int]: Assertion '__n < this->size()' failed.

Slightly different, but same assert.

[howetuft]

At first glance, ImageMapCache C++ implementation is absolutely not thread-safe! I don't know what BLC expects, but if concurrent accesses are planned, a code refactoring may be necessary.
However:

• this issue is not a regression, IMHO (existed before)
• and it may not be restricted to ImageMapCache

So I would propose to postpone it.