Improving Tramp-on-Windows path support (necessary for xref)

[jimporter]

Note: I'm filing this as a discussion since I have a patch for this, but I'm awaiting copyright assignment paperwork (I requested it 2 weeks ago, though I'm not sure how long the process normally takes) (update: got the paperwork and just need to send it back) and don't want to hold things up on that if you'd prefer to fix this more quickly. Just let me know and I can either file a proper issue or submit a PR.

---

When editing a file over Tramp from a Windows client (using clangd), calling xref-find-definitions fails to find definitions in external files (e.g. system headers). It tries to open /sshx:server:path/to/header.h (note the lack of a / before path). This is because eglot--uri-to-path strips the leading / while converting the URI. As I understand it, this is to help convert file:///c:/path/to/file.txt to c:/path/to/file.txt.

That's fine for Windows paths, but not POSIX paths; unfortunately, eglot--uri-to-path and its inverse only check if the client is Windows, so if you're using Tramp, it strips the leading / erroneously. This actually manages to work for paths within the project, since eglot sets rootUri to file:////path/to/file (note that there are 4 slashes). You can see this in the event log:

[client-request] (id:1) Thu Apr 22 19:12:57 2021:
(:jsonrpc "2.0" :id 1 :method "initialize" :params
          (:processId nil :rootPath "e:/home/jim/test/" :rootUri "file:////home/jim/test" :initializationOptions #s(hash-table size 65 test eql rehash-size 1.5 rehash-threshold 0.8125 data

I fixed this locally by only adding/removing the extra / on local paths on Windows clients.

Also note that rootPath has a drive letter, even though it's actually a remote path on a Linux server. That's a separate issue caused by calling expand-file-name after file-local-name in eglot--connect; I think it should be the other way around. I'm not sure if this breaks anything though. At least, I didn't notice any issues...

Like I said, I have a patch for all this, but I didn't want to make things more complicated for you by forcing you to wait until my copyright assignment paperwork is done. If you want me to file a PR though, just let me know.

[joaotavora]

Hello @jimporter , you have a pull quest for this, right? Can you link it here?

[jimporter]

I do. It's #679. I copied all the relevant info over to that PR (well, I think so; maybe I missed something).