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Posted to notifications@couchdb.apache.org by "ASF GitHub Bot (JIRA)" <ji...@apache.org> on 2016/08/16 23:40:20 UTC
[jira] [Commented] (COUCHDB-3102) The xxx_config_listener doesn't
restart after a failure
[ https://issues.apache.org/jira/browse/COUCHDB-3102?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15423600#comment-15423600 ]
ASF GitHub Bot commented on COUCHDB-3102:
-----------------------------------------
GitHub user iilyak opened a pull request:
https://github.com/apache/couchdb-config/pull/11
Move responsibility to re-add handler to config
We move responsibility to re-add handler from config_listener behaviour
to config_listener_mon module.
gen_event behaviour is a tricky one. Due to its prehistoric times it
uses links instead of monitors (which didn't exist at that time). This
makes dealing with gen_event manager restart tricky.
We use {swap_handler, Reason, OldState, NewHandler, NewState} reply from
handle_event callback to be able to catch any exception from
Mod:handle_config_change. Effectively preventing gen_event manager from
killing config_listener_mon process. This trick allows us:
- detect all kinds of failures and re-add handler
- detect crash of gen_event manager to restart config_listener_mon
As a result of this change the config_listener behaviour modules need to
be updated to remove following code:
handle_config_terminate(_Server, _Reason, State) ->
spawn(fun() ->
timer:sleep(5000),
config:listen_for_changes(?MODULE, State)
end).
The handler would be re-added automatically. You can disable this
behaviour by returning `remove_handler` atom from handle_config_terminate/3
COUCHDB-3102
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/cloudant/couchdb-config 3102-restart-monitor
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/couchdb-config/pull/11.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #11
----
commit 7402d78539bca11d99e4cd9f29f25263f4107fe6
Author: ILYA Khlopotov <ii...@ca.ibm.com>
Date: 2016-08-15T23:55:09Z
Consider only needed handlers in n_handlers
When testing. Make sure we don't count other config_event handlers
present in the system.
commit e936d6a470d5bcec880c474c00f9bf935c42d404
Author: ILYA Khlopotov <ii...@ca.ibm.com>
Date: 2016-08-16T22:56:34Z
Fix compilation warning
commit 99f38142d5446206f4878e895648304ff8303642
Author: ILYA Khlopotov <ii...@ca.ibm.com>
Date: 2016-08-16T20:52:12Z
Move responsibility to re-add handler to config
We move responsibility to re-add handler from config_listener behaviour
to config_listener_mon module.
gen_event behaviour is a tricky one. Due to its prehistoric times it
uses links instead of monitors (which didn't exist at that time). This
makes dealing with gen_event manager restart tricky.
We use {swap_handler, Reason, OldState, NewHandler, NewState} reply from
handle_event callback to be able to catch any exception from
Mod:handle_config_change. Effectively preventing gen_event manager from
killing config_listener_mon process. This trick allows us:
- detect all kinds of failures and re-add handler
- detect crash of gen_event manager to restart config_listener_mon
As a result of this change the config_listener behaviour modules need to
be updated to remove following code:
handle_config_terminate(_Server, _Reason, State) ->
spawn(fun() ->
timer:sleep(5000),
config:listen_for_changes(?MODULE, State)
end).
The handler would be re-added automatically. You can disable this
behaviour by returning `remove_handler` atom from handle_config_terminate/3
COUCHDB-3102
----
> The xxx_config_listener doesn't restart after a failure
> -------------------------------------------------------
>
> Key: COUCHDB-3102
> URL: https://issues.apache.org/jira/browse/COUCHDB-3102
> Project: CouchDB
> Issue Type: Bug
> Reporter: ILYA
>
> The [config_listener_mon|https://github.com/apache/couchdb-config/blob/master/src/config_listener_mon.erl#L44] monitors the caller.
> However in lots of places we use [following pattern|https://github.com/apache/couchdb-chttpd/blob/master/src/chttpd_config_listener.erl#L45:L49]
> {code}
> handle_config_terminate(_Server, _Reason, State) ->
> spawn(fun() ->
> timer:sleep(5000),
> config:listen_for_changes(?MODULE, State)
> end).
> {code}
> In this case the spawned fun exits right after the monitor has been established.
> Which means config_listener_mon would terminate. Effectively removing event handler from config_event process (gen_event event manager).
> When this happen any further configuration changes get unnoticed by processes.
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