The followLinksToStore() function could hang indefinitely when encountering
symlink cycles outside the Nix store, causing 100% CPU usage and blocking
any operations that use this function.
This affects multiple commands including nix-store --query, --delete,
--verify, nix-env, and nix-copy-closure when given paths with symlink cycles.
The fix adds a maximum limit of 1024 symlink follows (matching the limit
used by canonPath) and throws an error when exceeded, preventing the
infinite loop while preserving the original semantics of stopping at
the first path inside the store.
Replace non-thread-safe ptsname() calls with a new getPtsName() helper
function that:
- Uses thread-safe ptsname_r() on Linux/BSD platforms
- Uses mutex-protected ptsname() on macOS (which lacks ptsname_r())
This turns out to be a big problem for performance of Bison
generated code, that for whatever reason cannot be made internal
to the shared library. This causes GCC to make a bunch of function
calls go through PLT. Ideally these hot functions (like move/copy ctor) could become
inline in upstream Bison. That will make sure that GCC can do interprocedular
optimizations without -fno-semantic-interposition [^]. Considering that
LLVM already does inlining and whatnot is a good motivation for this change.
I don't know of any case where Nix relies on LD_PRELOAD tricks for the shared
libraries in production use-cases.
[^]: https://maskray.me/blog/2021-05-09-fno-semantic-interposition
Since the parser is now LALR we can easily switch
over to the less ugly sketelon than the default C one.
This would allow us to switch from %union to %define api.value.type variant
in the future to avoid the need for triviall POD types.
1. Saves 24-32 bytes per string (size of std::string)
2. Saves additional bytes by not over-allocating strings (in total we
save ~1% memory)
3. Sets us up to perform a similar transformation on the other Expr
subclasses
4. Makes ExprString trivially moveable (before the string data might
move, causing the Value's pointer to become invalid). This is important
so we can put ExprStrings in an std::vector and refer to them by index
We have introduced a string copy in ParserState::stripIndentation().
This could be removed by pre-allocating the right sized string in the
arena, but this adds complexity and doesn't seem to improve performance,
so for now we've left the copy in.
