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Jon Ross-Perkins
3070e5cfc6
When finding an executable, this validates that the returned binary is a symlink back to the same thing as /proc/self/exe, also using that as a fallback for different things. Looking back at #3912, we started using `findProgramByName` in order to avoid path canonicalization done by `GetMainExecutable`. That created issues as in #5096, wherein an `argv[0]` that's not explicit enough (`llvm-symbolizer` instead of the full path, done in [LLVM's Signals.cpp](https://github.com/llvm/llvm-project/blob/4f60f45130c6bd96c79e468fe9927a29af760f56/llvm/lib/Support/Signals.cpp#L198)) leads to incorrect results (finding an `llvm-symbolizer` in `$PATH`). One option to fix this would be to patch LLVM to provide an absolute path for `llvm-symbolizer`. However, I'll suggest that passing a filename in `argv[0]` is not terribly uncommon, and could be a migration limitation if we force it. The failure mode is also opaque; for example: ``` $ /bin/sh -c "exec -a llvm-symbolizer ./bazel-bin/toolchain/carbon" error: expected carbon-busybox symlink at `/usr/lib/llvm-19/bin/llvm-symbolizer` ``` Combined with the `setenv` of `LLVM_SYMBOLIZER_PATH` in `busybox_main.cpp`, this is intended to fix #5096.