v3completion: dynamic completion loading

In this small patch I want to introduce a way to dynamically load completion scripts for external subcommands.

A few years ago, you would put a completion script (which defines a Bash function _git_foo for a custom git subcommand foo) into /etc/bash_completion.d, or save it somewhere in your $HOME and source it manually in your .bashrc.

Starting with bash-completion v2.0 (or, to be absolutely exact, the preview versions started at v1.90), completion scripts are not sourced at bash startup anymore. Rather, when typing in a command and trigger completion by pressing the TAB key, the new bash-completion's main script looks for a script with the same name as the typed command in the completion directory, sources it, and provides the completions defined in this script.

However, that only works for top level commands. After a completion script has been found, the logic is delegated to this script. This means, that the completion of subcommands must be handled by the corresponding completion script.

As it is now, git is perfectly able to call custom completion functions, iff they are already defined when calling the git completion. With my patch, git uses an already defined loader function of bash-completion (or continues silently if this function is not found), loads an external completion script, and provides its completions.

An example for an external completion script would be /usr/share/bash-completion/completions/git-foo for a git subcommand foo.

Changes since v2:

-   Replaced __load_completion with _completion_loader, which was introduced 
    way before the former and should therefor be available even in older 
    distributions

-   Updated commit message with explanations from Szeder Gábor

Changes since v1 (RFC):

-   Re-arranged if-fi statement to increase readability (suggested by Junio C 
    Hamano)

-   Re-worded commit message to include the exakt version of bashcomp that 
    introduced dynamic loading (suggested by Stefan Beller)

Florian Gamböck (1):
  completion: load completion file for external subcommand

 contrib/completion/git-completion.bash | 10 ++++++++++
 1 file changed, 10 insertions(+)

Adding external subcommands to Git is as easy as to put an executable file git-foo into PATH. Packaging such subcommands for a Linux distribution can be achieved by unpacking the executable into /usr/bin of the user's system. Adding system-wide completion scripts for new subcommands, however, can be a bit tricky.

Since bash-completion started to use dynamical loading of completion scripts since v1.90 (preview of v2.0), it is no longer sufficient to drop a completion script of a subcommand into the standard completions path, /usr/share/bash-completion/completions, since this script will not be loaded if called as a git subcommand.

For example, look at https://bugs.gentoo.org/544722. To give a short summary: The popular git-flow subcommand provides a completion script, which gets installed as /usr/share/bash-completion/completions/git-flow.

If you now type into a Bash shell:

    git flow <TAB>

You will not get any completions, because bash-completion only loads completions for git and git has no idea that git-flow is defined in another file. You have to load this script manually or trigger the dynamic loader with:

    git-flow <TAB> # Please notice the dash instead of whitespace

This will not complete anything either, because it only defines a Bash function, without generating completions. But now the correct completion script has been loaded and the first command can use the completions.

So, the goal is now to teach the git completion script to consider the possibility of external completion scripts for subcommands, but of course without breaking current workflows.

I think the easiest method is to use a function that was defined by bash-completion v1.90, namely _completion_loader. It will take care of loading the correct script if present. Afterwards, the git completion script behaves as usual.

_completion_loader was introduced in commit 20c05b43 of bash-completion (https://github.com/scop/bash-completion.git) back in 2011, so it should be available in even older LTS distributions. This function searches for external completion scripts not only in the default path /usr/share/bash-completion/completions, but also in the user's home directory via $XDG_DATA_HOME and in a user specified directory via $BASH_COMPLETION_USER_DIR.

The only "drawback" (if it even can be called as such) is, that if _completion_loader does not find a completion script, it automatically registers a minimal function for basic path completion. In practice, however, this will not matter, because in this case the given command is a git command in its dashed form, e.g. 'git-diff-index', and those have been deprecated for a long time.

This way we can leverage bash-completion's dynamic loading for git subcommands and make it easier for developers to distribute custom completion scripts.

Signed-off-by: Florian Gamböck <mail@floga.de>
---
 contrib/completion/git-completion.bash | 10 ++++++++++
 1 file changed, 10 insertions(+)

On Sun, Apr 29, 2018 at 6:42 PM, Florian Gamböck <mail@floga.de> wrote:

The patch and the updated commit message both look good to me. Thanks for following through.