Subject: Re: [PATCH v3 2/3] patch-id: document new behaviour

From: Junio C Hamano <[email-masked]>
Date: Mon, 31 Mar 2014 12:08:36 -0700

"Michael S. Tsirkin" <[email-masked]> writes:

> Clarify that patch ID is now a sum of hashes, not a hash.
> Document --stable and --unstable flags.
>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>
> changes from v2:
> 	explicitly list the kinds of changes against which patch ID is stable
>
>  Documentation/git-patch-id.txt | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/git-patch-id.txt b/Documentation/git-patch-id.txt
> index 312c3b1..30923e0 100644
> --- a/Documentation/git-patch-id.txt
> +++ b/Documentation/git-patch-id.txt
> @@ -8,14 +8,14 @@ git-patch-id - Compute unique ID for a patch
>  SYNOPSIS
>  --------
>  [verse]
> -'git patch-id' < <patch>
> +'git patch-id' [--stable | --unstable] < <patch>

Thanks.  It seems taht we are fairly inconsistent when writing
alternatives on the SYNOPSIS line.  A small minority seems to spell
the above as "[--stable|--unstable]", which may want to be fixed
(outside the context of this series, of course).

>  
>  DESCRIPTION
>  -----------
> -A "patch ID" is nothing but a SHA-1 of the diff associated with a patch, with
> -whitespace and line numbers ignored.  As such, it's "reasonably stable", but at
> -the same time also reasonably unique, i.e., two patches that have the same "patch
> -ID" are almost guaranteed to be the same thing.
> +A "patch ID" is nothing but a sum of SHA-1 of the diff hunks associated with a
> +patch, with whitespace and line numbers ignored.  As such, it's "reasonably
> +stable", but at the same time also reasonably unique, i.e., two patches that
> +have the same "patch ID" are almost guaranteed to be the same thing.

Perhaps "nothing but" can go by now?

>  
>  IOW, you can use this thing to look for likely duplicate commits.
>  
> @@ -27,6 +27,19 @@ This can be used to make a mapping from patch ID to commit ID.
>  
>  OPTIONS
>  -------
> +
> +--stable::
> +	Use a symmetrical sum of hashes as the patch ID.
> +	With this option, reordering file diffs that make up a patch or
> +	splitting a diff up to multiple diffs that touch the same path
> +	does not affect the ID.
> +	This is the default.
> +
> +--unstable::
> +	Use a non-symmetrical sum of hashes, such that reordering
> +	or splitting the patch does affect the ID.
> +	This was the default value for git 1.9 and older.

I am not sure if swapping the default in this series is a wise
decision.  We typically introduce a new shiny toy to play with in a
release and then later when the shiny toy proves to be useful, start
to think about changing the default, but not before.

>  <patch>::
>  	The diff to create the ID of.