Closure mangles ERRNO code names, but they are looked up dynamically by nodefs

[cyyynthia]

I ran into weird issues of external C libraries interacting with a filesystem (nodefs) which would behave significantly differently depending on if I was building with --closure 1 or not.

After some troubleshooting, and some time looking at mangled code, I found out that the error handling was attempting to convert the full ERRNO code (as a string) against ERRNO_CODES dynamically, which does work when the code is not mangled, but breaks apart as soon as Closure kicks in and mangles the code names.

This results in operations like stat not behaving as expected (for instance, not returning an error on non-existing files), resulting in undefined behavior as the application behaves according to incorrect information about the state of the filesystem.

Reproduction case

I made a small repository with a minimal reproduction case, with build/run instructions on the readme: https://github.com/cyyynthia/emscripten-fs-bug

Environment information

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 2.0.24 (416685fb964c14cde4be3e8a45ad26d75bac3e33)
clang version 13.0.0 (/startdir/llvm-project 91f147792e815d401ae408989992f3c1530cc18a)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /opt/emscripten-llvm/bin