Re: [PATCH 13/27] sparse-index: create extension for compatibility

>
> From: Derrick Stolee <dstolee@microsoft.com>
>
> Previously, we enabled the sparse index format only using
> GIT_TEST_SPARSE_INDEX=1. This is not a feasible direction for users to
> actually select this mode. Further, sparse directory entries are not
> understood by the index formats as advertised.
>
> We _could_ add a new index version that explicitly adds these
> capabilities, but there are nuances to index formats 2, 3, and 4 that
> are still valuable to select as options. For now, create a repo
> extension, "extensions.sparseIndex", that specifies that the tool
> reading this repository must understand sparse directory entries.
>
> This change only encodes the extension and enables it when
> GIT_TEST_SPARSE_INDEX=1. Later, we will add a more user-friendly CLI
> mechanism.

>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  Documentation/config/extensions.txt |  7 ++++++
>  cache.h                             |  1 +
>  repo-settings.c                     |  7 ++++++
>  repository.h                        |  3 ++-
>  setup.c                             |  3 +++
>  sparse-index.c                      | 38 +++++++++++++++++++++++++----
>  6 files changed, 53 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/config/extensions.txt b/Documentation/config/extensions.txt
> index 4e23d73cdca..5c86b364873 100644
> --- a/Documentation/config/extensions.txt
> +++ b/Documentation/config/extensions.txt
> @@ -6,3 +6,10 @@ extensions.objectFormat::
>  Note that this setting should only be set by linkgit:git-init[1] or
>  linkgit:git-clone[1].  Trying to change it after initialization will not
>  work and will produce hard-to-diagnose issues.
> +
> +extensions.sparseIndex::
> +       When combined with `core.sparseCheckout=true` and
> +       `core.sparseCheckoutCone=true`, the index may contain entries
> +       corresponding to directories outside of the sparse-checkout
> +       definition. Versions of Git that do not understand this extension
> +       do not expect directory entries in the index.

> diff --git a/cache.h b/cache.h
> index b05341cc687..dcf089b7006 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1054,6 +1054,7 @@ struct repository_format {
>         int worktree_config;
>         int is_bare;
>         int hash_algo;
> +       int sparse_index;
>         char *work_tree;
>         struct string_list unknown_extensions;
>         struct string_list v1_only_extensions;
> diff --git a/repo-settings.c b/repo-settings.c
> index d63569e4041..9677d50f923 100644
> --- a/repo-settings.c
> +++ b/repo-settings.c
> @@ -85,4 +85,11 @@ void prepare_repo_settings(struct repository *r)
>          * removed.
>          */
>         r->settings.command_requires_full_index = 1;
> +
> +       /*
> +        * Initialize this as off.
> +        */
> +       r->settings.sparse_index = 0;
> +       if (!repo_config_get_bool(r, "extensions.sparseindex", &value) && value)
> +               r->settings.sparse_index = 1;
>  }
> diff --git a/repository.h b/repository.h
> index e06a2301569..a45f7520fd9 100644
> --- a/repository.h
> +++ b/repository.h
> @@ -42,7 +42,8 @@ struct repo_settings {
>
>         int core_multi_pack_index;
>
> -       unsigned command_requires_full_index:1;
> +       unsigned command_requires_full_index:1,
> +                sparse_index:1;
>  };
>
>  struct repository {
> diff --git a/setup.c b/setup.c
> index c04cd25a30d..cd839456461 100644
> --- a/setup.c
> +++ b/setup.c
> @@ -500,6 +500,9 @@ static enum extension_result handle_extension(const char *var,
>                         return error("invalid value for 'extensions.objectformat'");
>                 data->hash_algo = format;
>                 return EXTENSION_OK;
> +       } else if (!strcmp(ext, "sparseindex")) {
> +               data->sparse_index = 1;
> +               return EXTENSION_OK;
>         }
>         return EXTENSION_UNKNOWN;
>  }
> diff --git a/sparse-index.c b/sparse-index.c
> index 5dd0b835b9d..71544095267 100644
> --- a/sparse-index.c
> +++ b/sparse-index.c
> @@ -102,19 +102,47 @@ static int convert_to_sparse_rec(struct index_state *istate,
>         return num_converted - start_converted;
>  }
>
> +static int enable_sparse_index(struct repository *repo)
> +{
> +       const char *config_path = repo_git_path(repo, "config.worktree");
> +
> +       if (upgrade_repository_format(1) < 0) {
> +               warning(_("unable to upgrade repository format to enable sparse-index"));
> +               return -1;
> +       }
> +       git_config_set_in_file_gently(config_path,
> +                                     "extensions.sparseIndex",
> +                                     "true");
> +
> +       prepare_repo_settings(repo);
> +       repo->settings.sparse_index = 1;
> +       return 0;
> +}
> +
>  int convert_to_sparse(struct index_state *istate)
>  {
>         if (istate->split_index || istate->sparse_index ||
>             !core_apply_sparse_checkout || !core_sparse_checkout_cone)
>                 return 0;
>
> +       if (!istate->repo)
> +               istate->repo = the_repository;
> +
> +       /*
> +        * The GIT_TEST_SPARSE_INDEX environment variable triggers the
> +        * extensions.sparseIndex config variable to be on.
> +        */
> +       if (git_env_bool("GIT_TEST_SPARSE_INDEX", 0)) {
> +               int err = enable_sparse_index(istate->repo);
> +               if (err < 0)
> +                       return err;
> +       }
> +
>         /*
> -        * For now, only create a sparse index with the
> -        * GIT_TEST_SPARSE_INDEX environment variable. We will relax
> -        * this once we have a proper way to opt-in (and later still,
> -        * opt-out).
> +        * Only convert to sparse if extensions.sparseIndex is set.
>          */
> -       if (!git_env_bool("GIT_TEST_SPARSE_INDEX", 0))
> +       prepare_repo_settings(istate->repo);
> +       if (!istate->repo->settings.sparse_index)
>                 return 0;
>
>         if (!istate->sparse_checkout_patterns) {
> --
> gitgitgadget