Emacsclient

Why is the Emacsclient used in the first place?

Magit uses the Emacsclient when editing commit messages. More precisely Magit calls git commit without any other arguments, effectively telling Git to “create a commit message template, and then open that in the editor specified by the environment variable $GIT_EDITOR”. The $GIT_EDITOR can be any reasonable editor, e.g. emacs but Magit uses the with-editor library to make sure an emacsclient executable is used. In other words Magit makes sure “Git knows how to call home”.

The alternative would be to prepare a commit message and then provide that message to Git upfront, as in ~git commit -m “the message”~. This has the considerable drawback that Git cannot prepare the commit message, for the user to edit. The user then always has to write the message from scratch. This would be especially annoying when amending to an existing commit, or when merging.

Why is it problematic to use the Emacsclient?

Magit (With-Editor actually) has to determine an Emacsclient executable which is compatible with the running Emacs instance, and that is problematic for three reasons.

1. On some platforms installing Emacs does not cause its executables to be placed on $PATH. We work around this by also looking in some other directories likely to contain the executable.
2. We cannot just use the first executable found, instead we have to determine the versions of the found executables and then pick the one whose version matches that of the running Emacs instance as closely as possible.

   In the past we did not do so properly; now that we do, there should be no more problems.

3. The Emacsclient cannot be used over Tramp. Well theoretically it could, but that would be too complicated to setup and also insecure.

   To address this With-Editor implements a “sleeping editor” which is a poor man’s emacsclient. While the emacsclient communicates with emacs using a socket, the “sleeping editor” does so on its standard output, where a special process filter is listening.

   This works over Tramp without any further setup. And it can also be used locally, which is useful if we should ever fail to find a suitable Emacsclient executable.

Still doesn’t work for me!

See Configuring With-Editor and Debugging With-Editor.