Re: [PATCH] reset cached ident date before creating objects

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> Linus suggested resetting the timestamp after making the commit, to
> clear the way for the next commit. But if we reset any cached value
> _before_ making the commit, this has a few advantages:

I guess our mails crossed ;-).

> It does feel a little backwards to cache by default, and then try to
> catch all the places that want to reset. Another way of thinking about
> it would be to almost _never_ cache, but let a few callsites like (the
> commit object creation) explicitly ask for a stable timestamp between
> the author and committer.

... and the reflog?

I would say that the approach taken by this version is perfectly
sensible, if we don't look at it as a "cache" and instead look at it
as a "snapshot" of the clock for the duration of the operation.
"reset" is like "now we are starting another operation, so grab a
snapshot please".

The changes to both tagging and committing look sensible.  Thanks.

> -- >8 --
> Subject: reset cached ident date before creating objects
>
> When we compute the date to put in author/committer lines of
> commits, or tagger lines of tags, we get the current date
> once and then cache it for the rest of the program.  This is
> a good thing in some cases, like "git commit", because it
> means we do not racily assign different times to the
> author/committer fields of a single commit object.
>
> But as more programs start to make many commits in a single
> process (e.g., the recently builtin "git am"), it means that
> you'll get long strings of commits with identical committer
> timestamps (whereas before, we invoked "git commit" many
> times and got true timestamps).
>
> This patch addresses it by letting callers reset the cached
> time, which means they'll get a fresh time on their next
> call to git_committer_info() or git_author_info().  We do so
> automatically before filling in the ident fields of commit
> and tag objects. That retains the property that committers
> and authors in a single object will match, but means that
> separate objects we create should always get their own
> fresh timestamps.
>
> There's no automated test, because it would be inherently
> racy (it depends on whether the program takes multiple
> seconds to run). But you can see the effect with something
> like:
>
>   # make a fake 100-patch series; use --first-parent
>   # so that we pretend merges are just more patches
>   top=$(git rev-parse HEAD)
>   bottom=$(git rev-list --first-parent -100 HEAD | tail -n 1)
>   git log --format=email --reverse --first-parent \
>           --binary -m -p $bottom..$top >patch
>
>   # now apply it; this presumably takes multiple seconds
>   git checkout --detach $bottom
>   git am <patch
>
>   # now count the number of distinct committer times;
>   # prior to this patch, there would only be one, but
>   # now we'd typically see several.
>   git log --format=%ct $bottom.. | sort -u
>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  builtin/tag.c | 1 +
>  cache.h       | 1 +
>  commit.c      | 1 +
>  ident.c       | 5 +++++
>  4 files changed, 8 insertions(+)
>
> diff --git a/builtin/tag.c b/builtin/tag.c
> index 50e4ae5..3025e7f 100644
> --- a/builtin/tag.c
> +++ b/builtin/tag.c
> @@ -225,6 +225,7 @@ static void create_tag(const unsigned char *object, const char *tag,
>  	if (type <= OBJ_NONE)
>  	    die(_("bad object type."));
>  
> +	reset_ident_date();
>  	header_len = snprintf(header_buf, sizeof(header_buf),
>  			  "object %s\n"
>  			  "type %s\n"
> diff --git a/cache.h b/cache.h
> index b5f76a4..31e65f9 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1269,6 +1269,7 @@ extern const char *ident_default_email(void);
>  extern const char *git_editor(void);
>  extern const char *git_pager(int stdout_is_tty);
>  extern int git_ident_config(const char *, const char *, void *);
> +extern void reset_ident_date(void);
>  
>  struct ident_split {
>  	const char *name_begin;
> diff --git a/commit.c b/commit.c
> index 71a360d..7ddbffe 100644
> --- a/commit.c
> +++ b/commit.c
> @@ -1548,6 +1548,7 @@ int commit_tree_extended(const char *msg, size_t msg_len,
>  	}
>  
>  	/* Person/date information */
> +	reset_ident_date();
>  	if (!author)
>  		author = git_author_info(IDENT_STRICT);
>  	strbuf_addf(&buffer, "author %s\n", author);
> diff --git a/ident.c b/ident.c
> index 139c528..e20a772 100644
> --- a/ident.c
> +++ b/ident.c
> @@ -184,6 +184,11 @@ static const char *ident_default_date(void)
>  	return git_default_date.buf;
>  }
>  
> +void reset_ident_date(void)
> +{
> +	strbuf_reset(&git_default_date);
> +}
> +
>  static int crud(unsigned char c)
>  {
>  	return  c <= 32  ||