Juju writes to mongo without an actual change occurring

[Ian Booth]

This issue was first mentioned in bug 1344940.

Juju is constantly writing to Mongo been though there's been no change which would warrant a database update being required. Hence the writes are no-ops but cause I/O starvation and fill txn logs etc.

<snip>
the problem is 1000s of writes without an actual change, ie. the transaction log is full of these

    stateServers:apiHostPorts update {u'$set': {u'apihostports': [[{u'networkscope': u'public', u'addresstype': u'hostname', u'port': 17070, u'value': u'pbdpm.cluster.mallards'}, {u'networkscope': u'local-cloud',\
 u'addresstype': u'ipv4', u'port': 17070, u'value': u'192.168.9.74'}, {u'networkscope': u'local-machine', u'addresstype': u'ipv4', u'port': 17070, u'value': u'127.0.0.1'}, {u'networkscope': u'local-machine', u'ad\
dresstype': u'ipv6', u'port': 17070, u'value': u'::1'}, {u'addresstype': u'ipv6', u'port': 17070, u'value': u'fe80::225:90ff:fe4c:e79e'}]]}}
2014/07/19-15:41:13 applied
    stateServers:apiHostPorts update {u'$set': {u'apihostports': [[{u'networkscope': u'public', u'addresstype': u'hostname', u'port': 17070, u'value': u'pbdpm.cluster.mallards'}, {u'networkscope': u'local-cloud',\
 u'addresstype': u'ipv4', u'port': 17070, u'value': u'192.168.9.74'}, {u'networkscope': u'local-machine', u'addresstype': u'ipv4', u'port': 17070, u'value': u'127.0.0.1'}, {u'networkscope': u'local-machine', u'ad\
dresstype': u'ipv6', u'port': 17070, u'value': u'::1'}, {u'addresstype': u'ipv6', u'port': 17070, u'value': u'fe80::225:90ff:fe4c:e79e'}]]}}

we should not be writing to the database when there is no actual change.

here's a pretty print of the last 100 txns in the transaction collection.

http://paste.ubuntu.com/7820254/

<snip>

[Andrew Wilkins (axwalk)]

The change I made doesn't actually fix the issue, because the readonly assertion that I added to prevent races also spams the oplog. There's only one caller at the moment, which I'm pretty sure I can modify to elide no-op calls.

[Andrew Wilkins (axwalk)]

Another partial fix for master: https://github.com/juju/juju/pull/389/files
There's more work for master, because of some broken host-port sorting code relating to on-going IPv6 work. Cherry picking just these commits should be good enough to fix 1.20 though (I'll confirm and propose shortly).