Re: [PATCH v3 4/4] notes: support an external command to display notes

> git notes is a very very helpful feature to show user-supplied
> information about a commit alongside its message transparently.
> 
> For distributed teams working on large git repos (huge number of
> branches/refs, files, etc.) and using the notes feature to mark
> information on git commits, the problem is often not that two users
> update the same note object at the same time. It is that the local
> notes state used while reading history can be stale.
> 
> In kernel work, the same logical upstream fix can appear as different
> commit objects across many downstream branches, such as the stable
> branches and vendor-specific branches (based on which the released
> kernel is actually built). Different developers may be working on those
> branches in parallel, and a review decision recorded for one backport
> is useful context for the others.
> 
> Today, seeing that decision in ordinary history output requires first
> synchronizing the local notes ref, and then interpreting those notes
> for the branch being inspected. The latter step is workflow-specific
> and can be cheap, but keeping the local notes state fresh enough can be
> expensive in a large kernel repository with a large shared notes
> history (and if we are to extrapolate, a slow git server conn/ops can
> be a factor too).
> 
> This TOCTOU problem exacerbates on scale (rapid updates, more devs,
> larger repos, more git server traffic, etc).
> 
> One solution to this is to move the freshness policy out of git so that
> it is someone else's problem. We can have a realtime fetch or faster
> updation via external helper means. But unfortunately we lose the
> coherence in the display of information, and so the user would end up
> reinventing git log in his quest to have same workflow.

> Let's add support for notes.externalCommand, a protected-configuration
> command that git runs as a long-lived helper when displaying notes. git
> sends commit IDs to the helper and displays any returned text through
> the existing notes formatting path. This keeps presentation in git
> while letting the helper decide how fresh note text is obtained.