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Autor: Evilham
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A: devuan developers internal list
Assumpte: Re: [devuan-dev] [amprolla3] patches to fix locking
On dv., set. 20 2019, Ralph Ronnquist wrote:

> .. looks fine to me. (Not that I know python much)


Good to know :-).

> I wonder if this also means that the two program suite copies
> (in
> "amprolla" and "packages.devuan.org") can be coalesced into a
> single
> program suite (eg in "bin") with only two different config's for
> the two
> different instantiations. Or maybe that's not the python way?


The "python way" regarding this would probably be to accept the
configuration file as a CLI parameter, but the current "amprolla
way" is that there is the expectation that "from lib import
config" is something that works. That gets tricky the way things
are done there.
I didn't want to touch the code too much so this patch was simple.
TL;DR: no, it can't be done after this patch, but it can be done.

>
> Currenly it appears the "packages.devuan.org" suite is obtained
> by
> copying the "amprolla" program suite, then applying the patch
> file in
> "patches". It makes a separate complication for installing this
> change
> to both instances.


Those are secretly git repositories, so applying the same patch to
both should be fine and not too annoying.
--
Evilham


> Evilham wrote on 20/9/19 12:34 am:
>> Quick reminder that this is important and is awaiting review.
>> --
>> Evilham
>>
>> On ds., set. 14 2019, Evilham wrote:
>>
>>> Hello,
>>>
>>> due to the way file systems work, merely using the existence
>>> of a
>>> file as a lock is not robust as has been observed.
>>>
>>> Please make the time to review a patch for amprolla3 that
>>> makes
>>> sure only one amprolla-related operation is running at any
>>> given
>>> time and does not have lingering-lock-file-breaks-everything
>>> as an
>>> issue.
>>> man 2 flock explains this in better detail than I could.
>>>
>>> The patch itself:
>>> https://git.devuan.org/devuan-infrastructure/amprolla3/merge_requests/2
>>>
>>>
>>> Then there is a bit of an issue with the orchestrate.sh
>>> script,
>>> and indeed, with locking in general.
>>>
>>> The way I understand things, there are three processes (A,B,C)
>>> A. amprolla_update is run with orchestrate.sh (very often!):
>>> 1. there is always a consistent working set in merged which
>>> usually points to merged-production
>>> 2. before amprolla_update, merged switches to merged-staging
>>> (why
>>> here and not after amprolla_update?)
>>> 3. amprolla_update works against -volatile, during this
>>> process
>>> that directory is not necessarily fully consistent
>>> 4. after amprolla_update, merged-volatile is synchronised to
>>> merged-production
>>> 5. merged switches to merged-production
>>> 6. merged-volatile is synchronised to merged-staging
>>> 7. merged-production is synchronised to pkgmaster
>>>
>>> B. Sometimes amprolla_merge is run
>>> C. Sometimes amprolla_merge_contents + amprolla_merge are run
>>>
>>>
>>> The *intent* of the implemented locks appears to be that only
>>> one
>>> process out of A,B,C is active at a time, but the reality is
>>> that
>>> this is not formally provided, timing just happens to match,
>>> but
>>> shouldn't be relied upon:
>>> Existence of an active lock does prevent A, B or C from
>>> starting.
>>> But in A the lock is only active while point A.3. is executed,
>>> everything else runs without an active lock, so if e.g. A.4 to
>>> A.7
>>> take long enough that A.4 is running again while A.7 is
>>> happening,
>>> bogus data will be synchronised to pkgmaster.
>>> Since A.7 is a network operation and A.5 and A.6 are disk
>>> operations, delays *could* happen and they could lead to this
>>> scenario (maybe it has happened at some point).
>>>
>>> That's why I added the support for the --no-lock-I-am-sure
>>> argument to amprolla_update, and instead obtain the lock as a
>>> step
>>> A.0 that is valid throughout all process A in the
>>> orchestrate.sh
>>> script.
>>>
>>> With that, I think there is now only need for 2 directories:
>>> -volatile and -production, with A being redefined as:
>>>
>>> A. amprolla_update is run with orchestrate.sh (very often!):
>>> 0. obtain amprolla lock and exit if it can't be obtained
>>> 1. there is always a consistent working set in merged which
>>> usually points to merged-production
>>> 2. amprolla_update --no-lock-I-am-sure works against
>>> -volatile,
>>> during this process that directory is not necessarily fully
>>> consistent
>>> 3. merged switches to merged-volatile
>>> 4. merged-volatile is synchronised to merged-production
>>> 5. merged switches to merged-production
>>> 7. merged-production is synchronised to pkgmaster
>>>
>>>
>>> I have adapted orchestrate.sh and therefore merged-staging is
>>> not
>>> used with these patches and could be removed in theory.
>>>
>>> I hope this makes sense.
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