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

On Mon, Aug 01, 2016 at 02:00:47PM -0400, Jeff King wrote:

Here's that patch.

I still just put in a bare "reset_ident_date()" in git-am. This _could_ be wrapped in: begin_logical_commit() or something, but it would just be a wrapper around resetting the ident_date. So I'm inclined not to worry about such semantic obfuscation until we actually have a second thing to reset. :)

I also didn't go looking for other spots which might want similar treatment. Junio mentioned that the sequencer code still uses an external commit process, so cherry-pick/revert are OK. git-fast-import creates a stream of commits, but already deals with this issue in a separate way. And I couldn't think of other programs that want to make a string of commits.

-- >8 --
Subject: [PATCH] am: reset cached ident date for each patch

When we compute the date to go 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(). The first caller to do so is "git am", which resets the time for each patch it applies.

It would be nice if we could just do this automatically before filling in the ident fields of commit and tag objects. Unfortunately, it's hard to know where a particular logical operation begins and ends.

For instance, if commit_tree_extended() were to call reset_ident_date() before getting the committer/author ident, that doesn't quite work; sometimes the author info is passed in to us as a parameter, and it may or may not have come from a previous call to ident_default_date(). So in those cases, we lose the property that the committer and the author timestamp always match.

You could similarly put a date-reset at the end of commit_tree_extended(). That actually works in the current code base, but it's fragile. It makes the assumption that after commit_tree_extended() finishes, the caller has no other operations that would logically want to fall into the same timestamp.

So instead we provide the tool to easily do the reset, and let the high-level callers use it to annotate their own logical operations.

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
  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>
Helped-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/am.c | 2 ++
 cache.h      | 1 +
 ident.c      | 5 +++++
 3 files changed, 8 insertions(+)