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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 15:11:05 -0700	[thread overview]
Message-ID: <xmqqpn84n3pi.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <xmqqk0yconyw.fsf@gitster.c.googlers.com> (Junio C. Hamano's message of "Wed, 05 Aug 2020 13:08:07 -0700")

Junio C Hamano <gitster@pobox.com> writes:

> 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?

Shot in the dark.  Perhaps clearing of .add_tip and .known_common in
the .next method was done to catch a wrong calling sequence where
mark_complete_and_common_ref() gets called after mark_tips() and/or
find_common() have by forcing the code to segfault?  If so, this
patch removes the safety and we may want to add an equivalent safety
logic.  Perhaps by adding a state field in the negotiator instance
to record that mark_tips() and/or find_common() have been used and
call a BUG() if mark_complete_and_common_ref() gets called after that,
if enforcing such an invariant was the original reason why these
fields were cleared.

  reply	other threads:[~2020-08-05 22:11 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
2020-08-05 22:11       ` Junio C Hamano [this message]
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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