use exported functions when registering for config changes / db updates to
allow hot upgrading
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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
Fix For: 1.1
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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