This is because a dynamic_cast<nix::RootArgs *> of a (n-e-j) MyArgs
returns nullptr even though MyArgs has virtual nix::RootArgs as a
parent.
class MyArgs : virtual public nix::MixEvalArgs,
virtual public nix::MixCommonArgs,
virtual nix::RootArgs { ... };
So this should work right?? But it does not. We found out that it's
caused by -fvisibility=hidden in n-e-j, but honestly this code was bad
anyway.
The trivial solution is to simply stop relying on RTTI working properly
here, which is probably better OO architecture anyway. However, I am not
100% confident *this* is sound, since we have this horrible hierarchy:
Args (defines getRoot)
/ | \
RootArgs MixCommonArgs MixEvalArgs
(overrides)
I am not confident that this is guaranteed to resolve from Args always
in the case of this override.
Assertion failed: (res), function getRoot, file src/libutil/args.cc, line 67.
6MyArgsProcess 60503 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = hit program assert
frame #4: 0x0000000100b1a41c liblixutil.dylib`nix::Args::processArgs(std::__1::list<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>> const&, bool) [inlined] nix::Args::getRoot(this=0x00000001000d0688) at args.cc:67:5 [opt]
64 std::cout << typeid(*p).name();
65
66 auto * res = dynamic_cast<RootArgs *>(p);
-> 67 assert(res);
68 return *res;
69 }
70
Target 0: (nix-eval-jobs) stopped.
(lldb) p this
(MyArgs *) 0x00000001000d0688
(lldb) p *this
(nix::Args) {
longFlags = size=180 { ... }
shortFlags = size=4 { ... }
expectedArgs = size=1 { ... }
processedArgs = size=0 {}
hiddenCategories = size=1 {
[0] = "Options to override configuration settings"
}
parent = nullptr
}
We also found that if we did this:
class [[gnu::visibility("default")]] RootArgs : virtual public Args
it would work properly (???!). This is of course, very strange, because
objdump -Ct output on liblixexpr.dylib is identical both with and
without it.
Possibly related: https://www.qt.io/blog/quality-assurance/one-way-dynamic_cast-across-library-boundaries-can-fail-and-how-to-fix-it
Fixes: lix-project/nix-eval-jobs#2
Change-Id: I6b9ed968ed56420a9c4d2dffd18999d78c2761bd
jade
ac78c1dcd5