Fix path resolution to dpd for MacOS

[k-cross]

First of all, sorry if this is noise. I was trying to get a better understanding of omicron and had trouble running it through the simulated environment. If there is a different process I should use for PRs, I'm willing to go through that too, but might need to be pointed to a doc I missed.

On macOS, std::env::current_exe() does not resolve symlinks.

The dendrite-stub download creates a symlink:

• out/dendrite-stub/bin -> out/dendrite-stub/root/opt/oxide/dendrite/bin
• dpd's built-in infer_p4_dir() walks up from its path expecting .../opt/oxide/dendrite/bin/dpd, but through the symlink it sees ".../dendrite-stub/bin/dpd and fails.

The real dpd path is resolved and then derive P4_DIR from it to bypass this.

I couldn't run the tests locally due to issues pointed out in docs already about trying this on MacOS. Thanks for your time!

[david-crespo]

If I’m understanding the issue correctly, my workaround for this has been to add the directory to my path.

https://github.com/david-crespo/dotfiles/blob/2c704013a781968bbc1845ede4054ebc7ddee539/.zprofile#L284

It would certainly be nice not to have to do that. In theory, the .envrc also does this but I haven’t been able to get it to work. I’ll take a look at this.

[david-crespo]

Actually this might explain why the .envrc never worked for me — it relies on the symlink path, while my PATH thing does straight to the real thing.

[david-crespo]

I looked into why it's a symlink in the first place. Before #2904 (April 2023), the dendrite stub was unpacked so that out/dendrite-stub/bin/dpd was the real binary. In #2904, dendrite was updated so that dpd infers P4_DIR by walking up from its own executable path. This required preserving the full root/opt/oxide/dendrite/... directory structure from the tarball (previously the download script flattened it).

The symlink out/dendrite-stub/bin → out/dendrite-stub/root/opt/oxide/dendrite/bin was added so that existing PATH entries (including the .envrc) would keep working. So the symlink is backwards compat for a directory layout change from 2023. It has probably never worked on mac without the real dir in the path. Before #9312 I used a script that I shared with people that would build dpd from source, and it included a recommendation to add the bin dir to PATH.

I suspect the cleanest fix would be to remove the symlink altogether and update .envrc and env.sh to point at the real path (out/dendrite-stub/root/opt/oxide/dendrite/bin) and just make everyone update their stuff. I will have to see if other people find this tolerable.