patchreftable: document an alternate cleanup method on Windows

> From: Han-Wen Nienhuys <hanwen@google.com>
>
> The new method uses the update_index counter, which isn't susceptible to clock
> inaccuracies.
>
> Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
> ---
>     reftable: document an alternate cleanup method on Windows
>     
>     The new method uses the update_index counter, which isn't susceptible to
>     clock inaccuracies.
>     
>     Signed-off-by: Han-Wen Nienhuys hanwen@google.com

>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1001%2Fhanwen%2Fwindows-2-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1001/hanwen/windows-2-v1
> Pull-Request: https://github.com/git/git/pull/1001
>
>  Documentation/technical/reftable.txt | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/technical/reftable.txt b/Documentation/technical/reftable.txt
> index 3ef169af27d8..d7c3b645cfb0 100644
> --- a/Documentation/technical/reftable.txt
> +++ b/Documentation/technical/reftable.txt
> @@ -1011,8 +1011,13 @@ reftable stack, reload `tables.list`, and delete any tables no longer mentioned
>  in `tables.list`.
>  
>  Irregular program exit may still leave about unused files. In this case, a
> -cleanup operation can read `tables.list`, note its modification timestamp, and
> -delete any unreferenced `*.ref` files that are older.
> +cleanup operation should proceed as follows:
> +
> +* take a lock `tables.list.lock` to prevent concurrent modifications
> +* refresh the reftable stack, by reading `tables.list`
> +* for each `*.ref` file, remove it if
> +** it is not mentioned in `tables.list`, and
> +** its max update_index is not beyond the max update_index of the stack
>  
>  
>  Alternatives considered
>
> base-commit: 89b43f80a514aee58b662ad606e6352e03eaeee4