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From Alan Carroll <solidwallofc...@verizonmedia.com>
Subject Re: Remap plugin DSO reloading - enabling and disabling
Date Fri, 08 May 2020 14:33:24 GMT
Sudheer, I understand the point you are making, I just consider it
irrelevant. Let me give Leif an example to illustrate why - TxnBox. It
shares data between the global and remap configurations at run time via
static variables. If you enable remap DSO reloading for TxnBox, it will
crash on the first transaction that hits a remap rule. It doesn't matter if
it's actually been reloaded or not. However your organization does plugin
updates, TxnBox will still crash in that situation. Even in your example,
Sudheer, there's no _choice_ about whether a particular plugin can be DSO
reloaded, it's a result of the implementation. As you yourself write, you
can't enable it for those plugins without changing the code. No
configuration cleverness will get around that.

For plugins that do support DSO reloading, the enablement is still per
plugin, not per remap rule. Moreover, if we went with option (3) it would
be simple to have to plugin support a configuration / load time option to
enable or disable DSO reloading. In general, if the plugin can be DSO
reloaded, it's unclear why it shouldn't be except in unusual circumstances
which are depending on the plugin implementation.

For Sudheer, I remain unclear on what exact flexibility you want, given the
constraints created by a specific plugin's implementation. I've re-read
your note and AFAICT it assumes doing DSO reload or not *per plugin*, which
is also my point. I dislike (1) because it makes no sense to me to have
this change between remap rules for a specific plugin. I think it's better
to have the plugin decide if that's possible and, if needed, provide
configuration to disable it if needed. Speaking specifically for TxnBox, I
must forbid you from enabling DSO reloading. Even in your case, it might be
reasonable to have this for plugins that you have not yet updated (which is
actually the case with TxnBox - I'm limited by a requirement for ATS 7
compatibility, so I can't change that feature at the current time).

On Thu, May 7, 2020 at 10:11 PM Leif Hedstrom <zwoop@apache.org> wrote:

>
>
> On May 7, 2020, at 8:12 PM, Alan Carroll <solidwallofcode@verizonmedia.com>
> wrote:
>
> Leif;
>
> If the plugin can be global or remap but not both, I don't see why (2)
> limits anything. The entire issue is irrelevant for such plugins, because
> the situation of reloading the remap DSO but not the global cannot occur,
> In fact, option (3) or (4) would enable detecting this and issuing a
> warning.
>
>
> Ah yes, good point. However, still the same problem, one can very much
> want to use say header_rewrite as both global and remap plugin at the same
> time, and be fine with the fact that it doesn’t reload as a “global”, but
> you want it to reload as a remap. We use that plugin in this way for
> example.
>
> I still feel that option 2) is a bad option, but I’m ok with the others
> (still with a preference towards #1). I think a finer granular control
> mechanism here is a good idea.
>
> I’d also be curious to hear which of the core plugins are having problems
> here, in most cases, there’s a no dependency between the global
> instantiation, and the per remap instantiation. Sudheer and LinkedIn have
> many internal plugins that do experience this problem however, so I’m
> guessing that maybe you have similar custom internal plugins?
>
> Cheers,
>
> — Leif
>
>
> Approach (1) was my first thought, but I think the problem there is
> whether the plugin can work as a global and a reloadable remap is a
> property of the plugin implementation, not any particular remap rule. That
> is, for a specific plugin, there's really no choice about whether to use
> "@plugin" or "@global" - the configuration must get it right or the plugin
> crashes. Every time. Every rule. It is for this reason I disagree with
> Leif's view the user should decide. The user's opinion is irrelevant - the
> plugin works in this mode, or it doesn't. And as our friends at LinkedIn
> discovered, some rather basic C++ decisions (such as using static
> variables) will prevent a plugin from working in this mode. On the other
> hand, if the plugin uses the "User Args" feature then it can work, in which
> case what's the point of disabling the DSO reload? Unless the plugin
> implementor is concerned about code skew between the global and remap
> versions, which again the user is not qualified to decide.
>
> My personal preference is (3), but I suspect after mysterious crashes with
> plugins, we will have been happier with (4).
>
> On Thu, May 7, 2020 at 7:42 PM Sudheer Vinukonda <
> sudheervinukonda@yahoo.com> wrote:
>
>> +1 on the general idea to make the reloadability customizable per plugin.
>>
>> However, I think it'd be more simple, cleaner and intuitive to not tie it
>> to whether or not a plugin is used both as a global and remap plugin.
>>
>> In other words, approach (1) below but, instead of calling it "@global",
>> we could add a param which says "@reloadable=false" (the default value for
>> "@reloadable" can be "true").
>>
>> The same param can then be used, when we eventually add relodability to
>> global plugins as well.
>>
>> Thoughts?
>>
>>
>>
>>
>> On Thursday, May 7, 2020, 05:24:09 PM PDT, Alan Carroll <
>> solidwallofcode@verizonmedia.com> wrote:
>>
>>
>> As part of the ATS 9 upgrade, a feature was added so that remap plugins
>> could have their DSO reloaded. This means not just the configuration, but
>> the implementation itself. While very useful, this has some
>> unfortunate side effects with plugins that are used in both a global and
>> remap context. To alleviate this, a configuration variable as added to
>> disable the feature.
>>
>> Although reasonable, this is a rather heavy handed way to deal with the
>> problem. What would be better is the ability to reload the DSO or not on a
>> per remap plugin basis. I have a few ways this could be done:
>>
>> 1) Add the keyword "@global" to "remap.config". This would behave exactly
>> as "@plugin" except it would prohibit reloading of the DSO for that plugin.
>>
>> 2) Have the remap reload configuration check to see if the plugin is also
>> a global plugin and disable remap DSO reload for that plugin.
>>
>> 3) Add a flag to the global plugin registry information which can be set
>> during TSPluginInit which disables DSO reloading for that plugin, should it
>> occur in "remap.config". This is similar to (2) but requires a  plugin to
>> prohibit DSO reloading. The call woud be TSPluginDSOReloadEnable(flag) and
>> would only be valid when called from TSPluginInit.
>>
>> 4) As (3), except the flag is set by default and must be cleared to
>> enable DSO reloading in "remap.config".
>>
>> I'm willing to see if I can make this work, but I would like to have some
>> feedback on the preferred approach first.
>>
>>
>

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