Re: [PATCH v2] fetch-pack: write fetched refs to .promisor

> The specification of promisor packfiles (in partial-clone.txt) states
> that the .promisor files that accompany packfiles do not matter (just
> like .keep files), so whenever a packfile is fetched from the promisor
> remote, Git has been writing empty .promisor files. But these files
> could contain more useful information.
> 
> So instead of writing empty files, write the refs fetched to these
> files. This makes it easier to debug issues with partial clones, as we
> can identify what refs (and their associated hashes) were fetched at the
> time the packfile was downloaded, and if necessary, compare those hashes
> against what the promisor remote reports now.
> 
> This is implemented by teaching fetch-pack to write its own non-empty
> .promisor file whenever it knows the name of the pack's lockfile. This
> covers the case wherein the user runs "git fetch" with an internal
> protocol or HTTP protocol v2 (fetch_refs_via_pack() in transport.c sets
> lock_pack) and with HTTP protocol v0/v1 (fetch_git() in remote-curl.c
> passes "--lock-pack" to "fetch-pack").
> 
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
> Changes from v1:
>  - commit message explains scope of change ("it knows the name of the
>    pack's lockfile)
>  - explained what .promisor contains in comment in builtin/repack.c
>  - moved .promisor writing until after we know that index-pack succeeded
> ---