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Replace #4505 with a different set of workarounds (#4527)

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This restores the symlinks for the installation, but teaches the busybox
info search to look for a relative path to the busybox binary itself
before walking through symlinks. This let's it find the tree structure
when directly invoking `prefix_root/bin/carbon` or similar, either
inside of a Bazel rule or from the command line, and mirrors how we
expect the installed tree to look. This works even when Bazel resolves
the symlink target fully, and potentially to something nonsensical like
a CAS file.

In order to make a convenient Bazel target that can be used with `bazel
run //toolchain`, this adds an override to explicitly set the desired
argv[0] to use when selecting a mode for the busybox and a busybox
binary. Currently, the workaround uses an environment variable because
that required the least amount of plumbing, and seems a useful override
mechanism generally, but I'm open to other approaches.

This should allow a few things to work a bit more nicely:
- It should handle sibling symlinks like `clang++` to `clang` or
  `ld.lld` to `lld`, where that symlink in turn points at the busybox.
  We want to use *initial* `argv[0]` value to select the mode there.
- It avoids bouncing through Python (or other subprocesses) when
  invoking the `carbon` binary in Bazel rules, which will be nice for
  building the example code and benchmarking.

It does come at a cost of removing one feature: the initial symlink
can't be some unrelated alias like `my_carbon_symlink` -- we expect the
*first* argv[0] name to have the meaningful filename for selecting
a busybox mode.

It also trades the complexity of the Python script for some complexity
in the busybox search in order to look for a relative `carbon-busybox`
binary. On the whole, I think that tradeoff is worthwhile, but it isn't
free.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
This commit is contained in:
Chandler Carruth
2024-12-20 00:33:29 +00:00
committed by GitHub
co-authored by Jon Ross-Perkins
parent 85ea848879
commit 13502b7c89
16 changed files with 292 additions and 216 deletions
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toolchain/docs/adding_features.md
+4 -2
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@@ -467,8 +467,10 @@ example, with `toolchain/parse/testdata/basics/empty.carbon`:
- Executes an individual test.
- `bazel run //toolchain -- compile --phase=parse --dump-parse-tree toolchain/parse/testdata/basics/empty.carbon`
- Explicitly runs `carbon` with the provided arguments.
- `bazel-bin/toolchain/install/run_carbon compile --phase=parse --dump-parse-tree toolchain/parse/testdata/basics/empty.carbon`
- Similar to the previous command, but without using `bazel`.
- `bazel-bin/toolchain/carbon compile --phase=parse --dump-parse-tree toolchain/parse/testdata/basics/empty.carbon`
- Similar to the previous command, but without using `bazel run`. This can
be useful with a debugger or other tool that needs to directly run the
binary.
- `bazel run //toolchain -- -v compile --phase=check toolchain/check/testdata/basics/run.carbon`
- Runs using `-v` for verbose log output, and running through the `check`
phase.