[jira] [Closed] (COUCHDB-755) use exported functions when registering for config changes / db updates to allow hot upgrading

["Adam Kocoloski (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/COUCHDB-755?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Adam Kocoloski closed COUCHDB-755.
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    Skill Level: Committers Level (Medium to Hard)  (was: Regular Contributors Level (Easy to Medium))

> use exported functions when registering for config changes / db updates to allow hot upgrading
> ----------------------------------------------------------------------------------------------
>
>                 Key: COUCHDB-755
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-755
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: Database Core
>    Affects Versions: 0.11
>            Reporter: Adam Kocoloski
>            Assignee: Adam Kocoloski
>             Fix For: 2.0.0
>
>
> This is a common idiom in our current codebase:
> couch_config:register(fun("couchdb", "os_process_timeout", NewTimeout) ->
>     couch_os_process:set_timeout(Pid, list_to_integer(NewTimeout))
> end),
> It proves problematic for hot code upgrades.  The issue is that the anonymous fun is held by the couch_config process but belongs to the module in which it is defined (here, couch_external_server.erl).  If an expert user loads a new version of couch_external_server, the anonymous fun will belong to the 'old' version of the module.  If she upgrades the module again, couch_config will be killed.  That's no good.
> The solution is to export the function that handles the configuration change or db update event.  For instance
> -export(config_change/3).
> ...
> couch_config:register(fun ?MODULE:config_change/3),
> ...
> config_change("couchdb", "os_process_timeout", NewTimeout) ->
>     couch_os_process:set_timeout(Pid, list_to_integer(NewTimeout)).
> I have a patch for this that we've been using for some time in Cloudant, just need to clean it up and apply it to trunk.



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