bug#70036: 30.0.50; Move file-truename to the C level

[Felician Nemeth <felician.nemeth@gmail.com>]

Theodor Thornhill <theo@thornhill.no> writes:
> Felician Nemeth <felician.nemeth@gmail.com> writes:

>> Theo, can you email me the relevant messages that your server sends
>> to Emacs?  Does the server send lots of similar diagnostics messages
>> frequently?

> I'll try to include such a report a little later today. 

Thanks, that would be helpful.

> [2. text/x-diff; 0001-Don-t-use-file-truepath-in-Eglot-bug-70036.patch]...

I think using find-buffer-visiting instead of get-file-buffer and
file-truename instead of expand-file-name in Eglot is problematic.
Let's say we have these files:

/project/a.c
/project/a.h -> /other/a.h

Eglot will communicate these file names to the LSP server: /project/a.c
and /other/a.h.  Then the server cannot "associate" a.h with a.c.
Additionally, a.h will be outside of the LSP workspace.

This indeed confuses clangd a bit: it only takes into account the
changes of buffer a.h when it is saved.  (Because it assumes
/project/a.h is not opened in the editor.)

------

Regarding the patch itself, cache invalidation is missing from it.  The
user might kill a buffer or save it under a different name with
write-file.  Changing (PATH -> BUFFER) to (PATH -> (BUFFER, FILENAME))
would probably work.  Eglot should save the current buffer-file-name
when it inserts a new item into the hash of managed-buffers.  And when
it retrieves an item, it should verify whether the buffer-file-name is
the same as the saved file-name.

Can file-truepath change while buffer-file-name remains the same?  Yes,
but I think Eglot could ignore those rare cases, or handle it elsewhere.
(For example, it could update the cache entry after a buffer is saved.)