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From: Junio C Hamano <gitster@pobox.com>
To: Jonathan Tan <jonathantanmy@google.com>
Cc: git@vger.kernel.org, peff@peff.net
Subject: Re: [RFC PATCH 2/7] fetch-pack: allow NULL negotiator->known_common
Date: Wed, 05 Aug 2020 13:08:07 -0700	[thread overview]
Message-ID: <xmqqk0yconyw.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <401227c2220b6b45d80e21b52e29b6821ca139f9.1596590295.git.jonathantanmy@google.com> (Jonathan Tan's message of "Tue, 4 Aug 2020 18:20:45 -0700")

Jonathan Tan <jonathantanmy@google.com> writes:

> In a subsequent patch, a null fetch negotiator will be introduced. This
> negotiator, among other things, will not need any information about
> common objects and will have a NULL known_common. Teach fetch-pack to
> allow this.

Hmph, both the default and the skipping negotiator seem to put NULL
in known_common and add_tip when its next() method is called.  Also
they clear known_common to NULL after add_tip is called even once.

So, how have we survived so far without this patch to "allow this
(i.e.  known_common method to be NULL)"?  Is there something that
makes sure a negotiator never gets called from this function after
its .next or .add_tip method is called?

Puzzled.  Or is this merely an optimization?  If so, it's not like
the change "allows this", but it starts to take advantage of it in
some way.

	... goes and looks at mark_complete_and_common_ref()

The function seems to have an unconditional call to ->known_common(),
so anybody passing a negotiator whose known_common is NULL would
already be segfaulting, so this does not appear to be an optimization
but necessary to keep the code from crashing.  I cannot quite tell
if it is avoiding unnecessary work, or sweeping crashes under the
rug, though.  

Is the untold assumption that mark_complete_and_common_ref() will
never be called after either mark_tips() or find_common() have been
called?

Thanks.

> [NEEDSWORK]
> Optimizing out the ref iteration also affects the execution
> of everything_local(), which relies on COMPLETE being set. (Having said
> that, the typical use case - lazy fetching - would be fine with
> everything_local() always returning that not everything is local.)
>
> This optimization is needed so that in the future, fetch_pack() can be
> used to lazily fetch in a partial clone (without the no_dependents
> flag). This means that fetch_pack() needs a way to execute without
> relying on any targets of refs being present, and thus it cannot use the
> ref iterator (because it checks and lazy-fetches any missing targets).
> (Git currently does not have this problem because we use the
> no_dependents flag, but lazy-fetching will in a subsequent patch be
> changed to use the user-facing fetch command, which does not use this
> flag.)
> [/NEEDSWORK]
>
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
>  fetch-pack.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fetch-pack.c b/fetch-pack.c
> index 6c786f5970..5f5474dbed 100644
> --- a/fetch-pack.c
> +++ b/fetch-pack.c
> @@ -677,6 +677,9 @@ static void mark_complete_and_common_ref(struct fetch_negotiator *negotiator,
>  	int old_save_commit_buffer = save_commit_buffer;
>  	timestamp_t cutoff = 0;
>  
> +	if (!negotiator->known_common)
> +		return;
> +
>  	save_commit_buffer = 0;
>  
>  	trace2_region_enter("fetch-pack", "parse_remote_refs_and_find_cutoff", NULL);

  reply	other threads:[~2020-08-05 20:08 UTC|newest]

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-24 22:38 [RFC PATCH] Modify fetch-pack to no longer die on error? Jonathan Tan
2020-07-24 23:07 ` Junio C Hamano
2020-07-24 23:11 ` Junio C Hamano
2020-07-25 21:41   ` Jeff King
2020-07-25 23:01     ` Junio C Hamano
2020-07-27 17:11       ` Jeff King
2020-07-28 19:23       ` Jonathan Tan
2020-07-28 20:08         ` Jeff King
2020-07-29 18:53           ` Jonathan Tan
2020-07-29 19:29             ` Jeff King
2020-07-29 19:02           ` Junio C Hamano
2020-07-29 22:55             ` Jonathan Tan
2020-08-05  1:20 ` [RFC PATCH 0/7] Lazy fetch with subprocess Jonathan Tan
2020-08-05  1:20   ` [RFC PATCH 1/7] fetch-pack: allow NULL negotiator->add_tip Jonathan Tan
2020-08-05 19:53     ` Junio C Hamano
2020-08-07 20:53       ` Jonathan Tan
2020-08-05  1:20   ` [RFC PATCH 2/7] fetch-pack: allow NULL negotiator->known_common Jonathan Tan
2020-08-05 20:08     ` Junio C Hamano [this message]
2020-08-05 22:11       ` Junio C Hamano
2020-08-07 20:59         ` Jonathan Tan
2020-08-05  1:20   ` [RFC PATCH 3/7] negotiator/null: add null fetch negotiator Jonathan Tan
2020-08-05  1:20   ` [RFC PATCH 4/7] fetch: --stdin Jonathan Tan
2020-08-05 20:33     ` Junio C Hamano
2020-08-07 21:10       ` Jonathan Tan
2020-08-07 21:58         ` Junio C Hamano
2020-08-07 21:10       ` Jonathan Tan
2020-08-05  1:20   ` [RFC PATCH 5/7] fetch: submodule config Jonathan Tan
2020-08-05  1:20   ` [RFC PATCH 6/7] fetch: only populate existing_refs if needed Jonathan Tan
2020-08-05  1:20   ` [RFC PATCH 7/7] promisor-remote: use subprocess to fetch Jonathan Tan
2020-08-11 22:52 ` [PATCH v2 0/7] Lazy fetch with subprocess Jonathan Tan
2020-08-11 22:52   ` [PATCH v2 1/7] negotiator/null: add null fetch negotiator Jonathan Tan
2020-08-12 12:55     ` Derrick Stolee
2020-08-12 16:44       ` Junio C Hamano
2020-08-12 17:29         ` Jonathan Tan
2020-08-11 22:52   ` [PATCH v2 2/7] fetch: allow refspecs specified through stdin Jonathan Tan
2020-08-11 22:52   ` [PATCH v2 3/7] fetch: avoid reading submodule config until needed Jonathan Tan
2020-08-12 17:34     ` Junio C Hamano
2020-08-11 22:52   ` [PATCH v2 4/7] fetch: only populate existing_refs if needed Jonathan Tan
2020-08-12 18:06     ` Junio C Hamano
2020-08-11 22:52   ` [PATCH v2 5/7] fetch-pack: do not lazy-fetch during ref iteration Jonathan Tan
2020-08-12 18:25     ` Junio C Hamano
2020-08-11 22:52   ` [PATCH v2 6/7] promisor-remote: lazy-fetch objects in subprocess Jonathan Tan
2020-08-12 18:28     ` Junio C Hamano
2020-08-11 22:52   ` [PATCH v2 7/7] fetch-pack: remove no_dependents code Jonathan Tan
2020-08-12 12:51   ` [PATCH v2 0/7] Lazy fetch with subprocess Derrick Stolee
2020-08-18  4:01 ` [PATCH v3 " Jonathan Tan
2020-08-18  4:01   ` [PATCH v3 1/7] negotiator/noop: add noop fetch negotiator Jonathan Tan
2020-08-18  4:01   ` [PATCH v3 2/7] fetch: allow refspecs specified through stdin Jonathan Tan
2020-08-18  4:01   ` [PATCH v3 3/7] fetch: avoid reading submodule config until needed Jonathan Tan
2020-08-18  4:01   ` [PATCH v3 4/7] fetch: only populate existing_refs if needed Jonathan Tan
2020-08-18  4:01   ` [PATCH v3 5/7] fetch-pack: do not lazy-fetch during ref iteration Jonathan Tan
2020-08-18  4:01   ` [PATCH v3 6/7] promisor-remote: lazy-fetch objects in subprocess Jonathan Tan
2020-08-18  4:01   ` [PATCH v3 7/7] fetch-pack: remove no_dependents code Jonathan Tan
2020-08-18 19:56   ` [PATCH v3 0/7] Lazy fetch with subprocess Junio C Hamano
2020-08-18 22:32     ` Junio C Hamano
2020-08-18 23:36       ` [PATCH] fixup! promisor-remote: lazy-fetch objects in subprocess Jonathan Tan
2020-08-18 23:57         ` Junio C Hamano

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