[PATCH v2] maintenance: add prune-remote-refs task

From: Shubham Kanodia <shubham.kanodia10@gmail.com>

Remote-tracking refs can accumulate in local repositories even as branches are deleted on remotes, impacting git performance negatively. Existing alternatives to keep refs pruned have a few issues — 

1. Running `git fetch` with either `--prune` or `fetch.prune=true` set will
prune stale refs, but requires a manual operation and also pulls in new
refs from remote which can be an undesirable side-effect.

2.`git remote prune` cleans up refs without adding to the existing list but requires periodic user intervention.

This adds a new maintenance task 'prune-remote-refs' that runs 'git remote prune' for each configured remote daily. This provides an automated way to clean up stale remote-tracking refs — especially when users may not do a full fetch.

This task is disabled by default.

Signed-off-by: Shubham Kanodia <shubham.kanodia10@gmail.com>
---
    maintenance: add prune-remote-refs task
    
    As discussed previously on:
    https://lore.kernel.org/git/xmqqwmfr112w.fsf@gitster.g/T/#t
    
    Remote-tracking refs can accumulate in local repositories even as
    branches are deleted on remotes, impacting git performance negatively.
    Existing alternatives to keep refs pruned have a few issues — 
    
     1. The fetch.prune config automatically cleans up remote ref on fetch,
        but also pulls in new ref from remote which is an undesirable
        side-effect.
    
    2.git remote prune cleans up refs without adding to the existing list
    but requires periodic user intervention.
    
    This adds a new maintenance task 'prune-remote-refs' that runs 'git
    remote prune' for each configured remote daily. This provides an
    automated way to clean up stale remote-tracking refs — especially when
    users may not do a full fetch.
    
    This task is disabled by default.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1838%2Fpastelsky%2Fsk%2Fadd-remote-prune-maintenance-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1838/pastelsky/sk/add-remote-prune-maintenance-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1838

Range-diff vs v1:

 1:  72e27d3ebe6 ! 1:  4d6c143c970 maintenance: add prune-remote-refs task
     @@ Commit message
          are deleted on remotes, impacting git performance negatively. Existing
          alternatives to keep refs pruned have a few issues — 
      
     -    1. The `fetch.prune` config automatically cleans up remote ref on fetch,
     -    but also pulls in new ref from remote which is an undesirable side-effect.
     +    1. Running `git fetch` with either `--prune` or `fetch.prune=true` set will
     +    prune stale refs, but requires a manual operation and also pulls in new
     +    refs from remote which can be an undesirable side-effect.
      
          2.`git remote prune` cleans up refs without adding to the existing list
          but requires periodic user intervention.
     @@ Documentation/git-maintenance.txt: pack-refs::
      ++
      +--
      +* When using selective fetching (e.g., `git fetch origin +foo:refs/remotes/origin/foo`)
     -+  where `fetch.prune` would not affect refs outside the fetched hierarchy
     ++  where `fetch.prune` would only affect refs that are explicitly fetched.
      +* When third-party tools might perform unexpected full fetches, and you want
     -+  periodic cleanup independently of fetch operations
     ++  periodic cleanup independently of fetch operations.
      +--
      +
       OPTIONS
     @@ builtin/gc.c: static int maintenance_opt_schedule(const struct option *opt, cons
       	return 0;
       }
       
     -+static int collect_remote(struct remote *remote, void *cb_data)
     ++static int prune_remote(struct remote *remote, void *cb_data UNUSED)
      +{
     -+	struct string_list *list = cb_data;
     ++	struct child_process child = CHILD_PROCESS_INIT;
      +
      +	if (!remote->url.nr)
      +		return 0;
      +
     -+	string_list_append(list, remote->name);
     -+	return 0;
     ++	child.git_cmd = 1;
     ++	strvec_pushl(&child.args, "remote", "prune", remote->name, NULL);
     ++
     ++	return !!run_command(&child);
      +}
      +
     -+static int maintenance_task_prune_remote(struct maintenance_run_opts *opts UNUSED,
     ++static int maintenance_task_prune_remote(struct maintenance_run_opts *opts,
      +					 struct gc_config *cfg UNUSED)
      +{
     -+	struct string_list_item *item;
     -+	struct string_list remotes_list = STRING_LIST_INIT_NODUP;
     -+	struct child_process child = CHILD_PROCESS_INIT;
     -+	int result = 0;
     -+
     -+	for_each_remote(collect_remote, &remotes_list);
     -+
     -+	for_each_string_list_item (item, &remotes_list) {
     -+		const char *remote_name = item->string;
     -+		child.git_cmd = 1;
     -+		strvec_pushl(&child.args, "remote", "prune", remote_name, NULL);
     -+
     -+		if (run_command(&child))
     -+			result = error(_("failed to prune '%s'"), remote_name);
     ++	if (for_each_remote(prune_remote, opts)) {
     ++		error(_("failed to prune remotes"));
     ++		return 1;
      +	}
      +
     -+	string_list_clear(&remotes_list, 0);
     -+	return result;
     ++	return 0;
      +}
      +
       /* Remember to update object flag allocation in object.h */

 Documentation/git-maintenance.txt | 20 ++++++++++++++
 builtin/gc.c                      | 32 ++++++++++++++++++++++
 t/t7900-maintenance.sh            | 44 +++++++++++++++++++++++++++++++
 3 files changed, 96 insertions(+)