bug#41572: 28.0.50; [PATCH] Support plain project marked with file .emacs-project

[Dmitry Gutov <dgutov@yandex.ru>]

On 10.10.2021 19:47, Juri Linkov wrote:
>>> Then maybe the backend could be named 'project-file'
>>> since a special file defines the project root.
>>
>> That's a little more meaningful, though too close to
>> 'project-files'. 'project-markered' or 'project-markerfile' would probably
>> be less ambiguous.
> 
> In 'project-filemarker' I misread "filemarker" as "filmmaker" :-)

Right. :-)

> Another possible name would be "fileroot".

Also sounds more like a method than a project type name.

Maybe project-markered or project-marked, or project-dominated (along 
the lines of 'locate-dominating-file'?). Or something noncommittal like 
project-dirtee.

But all of those sounds like one could put them at any position in 
project-find-functions, which project-fallback explicitly discourages.

>> Suppose somebody puts it before 'vc' to use if for a purpose we did not
>> design it for: make sure that some subproject 'foo' in their monorepo is
>> considered a separate project. 'foo/Makefile' exists, so they add
>> "Makefile" to project-fallback-markers, and it kind of seems to work.
> 
> There are two contradictory needs:
> 
> 1. When a marker list contains both ".dir-locals.el" and "Makefile",
>     it should ignore Makefile files in vc-based project subdirs, e.g.
>     emacs/lisp/Makefile, etc.

Right. That says project-try-fallback going after project-try-vc is a 
good thing.

> 2. OTOH, I often type 'C-x p g' to search all gems of the same
>     ruby version in e.g. ~/.rbenv/versions/2.7.4/lib/ruby/gems
>     But it finds ~/.rbenv/.git and tries to search all ruby versions.
>     I could manually add .dir-locals.el only to a particular version's
>     subdir.

I'm using this setup, FWIW:

   (defun project-try-gem (dir)
     (when (string-match "/\\.rbenv/versions/.*/gems/.*/gems/[^/]+/" dir)
       (cons 'rubygem
             (substring dir 0 (match-end 0)))))

   (cl-defmethod project-root ((project (head rubygem)))
     (cdr project))

   (with-eval-after-load 'project
     (add-hook 'project-find-functions #'project-try-gem))

Which avoids the whole directory-walking routine, probably saving on a 
number of CPU cycles.

> But how to override ~/.rbenv/.git?  Maybe by changing
>     the order of backends in project-find-functions?
>     Then the fallback won't be the last backend anymore.
>     Also the backend priorities will be changed globally
>     for all other projects, and 'C-x p g' in emacs/lisp
>     will find emacs/lisp/Makefile to override emacs/.git.

If we keep the same tool to ensure the priorities (order of functions in 
project-try-vc), that would seem like we should use two different 
backends for these two different purposes.

Say, the first one would be called project-dominating, and the other one 
- still project-fallback.

project-find-functions will be

   '(project-try-dominating project-try-vc project-try-fallback

Alternatively, one backend could combine these, but it would have, like, 
configurable logic with two different sets of predicates -- one 
universal (whether the file is inside a VCS repo or not), and another 
for when the file is strictly outside of VCS repos. But that sounds 
trickier, both to configure and to understand. Also, it seems like with 
this approach the backend *should* ignore the .gitignore entires, which 
is fine for .rbenv/versions/.*/gems, but it's bound not to work for some 
other purposes some users are going to try to use it for.

So, since I don't know of any other use cases for the project-dominating 
code path rather than Ruby gems inside ~/.rbenv, I figured not to try to 
solve this use case yet. But we can catalogue similar use cases (maybe 
other versions switchers, for Ruby and other languages?; not sure if 
installed libraries for other languages also fit this approach) and add 
another backend for them later. Help welcome, of course.