tree 3e3ec9ee44ef29cd4496ff93585a5d9849b43b09
parent 94d1aac44948afee11a785c7dfb8b774543ceec3
author Marcin Czech <maczech@gmail.com> 1589549584 +0200
committer Luca Milanesio <luca.milanesio@gmail.com> 1589577101 +0000

Do not block ref-updates during the configuration reload

For setups with a large number of remote configurations, configuration
reload can be a time consuming operation. During the reload, the list of
all destinations is blocked by a synchronized and thus any ref-update
in Gerrit is blocked and cause a general outage until the replication
plugin configuration is reloaded.

This situation happens because of a ref-update post action
GitReferenceUpdatedListener.onGitReferenceUpdated(Event). This is
a blocking operation and it is calling another blocking method
ReplicationQueue.fire(NameKey,String, String, ReplicationState, boolean)
and finally synchronized method DestinationsCollection.getAll(FilterType)
is called. If DestinationCollection is during the configuration reload
ref-update have to wait on DestinationsCollection.getAll, this is causing
general lock for all operations against Gerrit instance.

With NoteDb, almost every operation in Gerrit causes a ref-update and,
therefore, would be blocked in that chain, causing a unique single sync
in the entire Gerrit JVM that can bring an entire server down.

Solution to that issue is to remove synchronization on
DestinationsCollection.getAll to stop blocking ref-updates. Removing
synchronize keyword is still thread safe because we store destinations
as an immutable list the only way of updating it is to replace it with
a new list. Since it is volatile, the update of destinations is atomic
and thread safe.

Bug: Issue 12592
Change-Id: Idcba1ef20a0ecde31e6a7ef6d827bf08746e5cb2