[PATCH v3 0/4] maintenance: use systemd timers on Linux

Hello,

Thanks again for your valuable feedback! I’ve reworked this submission to hopefully address all the raised concerns.

As it’s becoming bigger, I’ve split it in several patches:

* cache.h: rename "xdg_config_home" to "xdg_config_home_git"

In order to honor `$XDG_CONFIG_HOME` to built the path of systemd units, a lot of code from the `xdg_config_home()` function was duplicated. This existing function couldn’t be used as-is because it was hard-coding the `git` sub-directory below `$XDG_CONFIG_HOME`.

This first preparatory patch re-purposes `xdg_config_home()` to make
it more generic by removing the `git` path it used to append to
`$XDG_CONFIG_HOME`.
Thanks to this refactoring, `xdg_config_home()` can be used to build
paths like:
* `$XDG_CONFIG_HOME/git/…` it’s the new `xdg_config_home_git()`
function and
* `$XDG_CONFIG_HOME/systemd/user/…` it’s the new
`xdg_config_home_systemd()` function which will be introduced later,
in the last patch of this series.

* maintenance: introduce ENABLE/DISABLE for code clarity

Some functions in git maintenance are doing one thing and the exact opposite, depending on a parameter that could be `0` or `1`. This could sometimes be confusing. This patch introduces an enum with `ENABLE` and `DISABLE` values to make the code more explicit.

* maintenance: `git maintenance run` learned `--scheduler=<scheduler>`

This patch contains all the code that is related to the addition of the new `--scheduler` parameter of the `git maintenance start` command, independently of the systemd timers.

This part has significantly changed to review the testing mechanisms. The `GIT_TEST_MAINT_SCHEDULER` environment variable can now take a comma separated list of schedulers and their mock in order to be able to test cases where multiple schedulers are available.

It allows to test that `git maintenance start --scheduler=XXX` not only enables the scheduler XXX, but it also disables all the other available ones. We can also test that `git maintenance stop`, stops all the available schedulers.

In order to remove the `cmd` parameter that was passed to a lot of functions, the parsing of `GIT_TEST_MAINT_SCHEDULER` is now factorized in a `get_schedule_cmd()` function that is invoked from the leaves of the function call tree. It means that this function is called many times to parse `GIT_TEST_MAINT_SCHEDULER` again and again. It is inefficient but as it is used for test only, it shouldn’t be a concern.

`int get_schdule_cmd(const char **cmd, int *is_available)`

This function returns `true` if `GIT_TEST_MAINT_SCHEDULER` is defined. `false` otherwise.

`*is_available` is set to `true` if `*cmd` is present in `$GIT_TEST_MAINT_SCHEDULER`, and to `false` otherwise.

If `*cmd` is present in `$GIT_TEST_MAINT_SCHEDULER`, it’s value is updated to its mock.

* maintenance: optionally use systemd timers on Linux

This last patch adds the support for systemd timers.

Lénaïc Huard (4):
  cache.h: rename "xdg_config_home" to "xdg_config_home_git"
  maintenance: introduce ENABLE/DISABLE for code clarity
  maintenance: `git maintenance run` learned `--scheduler=<scheduler>`
  maintenance: optionally use systemd timers on Linux

 Documentation/git-maintenance.txt |  60 ++++
 attr.c                            |   2 +-
 builtin/config.c                  |   2 +-
 builtin/credential-cache.c        |   2 +-
 builtin/credential-store.c        |   2 +-
 builtin/gc.c                      | 549 ++++++++++++++++++++++++++----
 cache.h                           |  12 +-
 config.c                          |   2 +-
 dir.c                             |   2 +-
 path.c                            |  35 +-
 sequencer.c                       |   2 +-
 t/t7900-maintenance.sh            | 110 +++++-
 12 files changed, 685 insertions(+), 95 deletions(-)