api/v2: Check if cluster peer is nil -> release-0.16

[mxinden]

When users start Alertmanager with --cluster.listen-address=, the
cluster will not be initialized, hence api.peer will be nil.

With this patch, api v2 skips populating the peers array in case
api.peer is nil.

Fixes #1725

[yeya24]

I think these codes above will panic too

name := api.peer.Name()
status := api.peer.Status()

[mxinden]

@yeya24 right, thanks for the hint, sorry for the low quality patch. Added a test and the fix.

[stuartnelson3]

I would reverse this comment. "If alertmanager cluster feature is disabled, then api.peers == nil".

[mxinden]

Looks like we will need to remove cluster in alertmanagerStatus from the required fields making it optional. I will follow up with that.

[mxinden]

@stuartnelson3 @simonpasquier any comments on this (bugix for release-0.16, delaying the breaking change to master in #1728)?

[stuartnelson3]

Looks good to me

[simonpasquier]

👍

[knweiss]

@mxinden I just ran into this issue when I was testing HEAD of the alertmanager master branch. Is there a reason why you didn't merge #1726 into the master branch as well?

[mxinden]

Instead of merging this patch into the master branch, we decided to merge a breaking fix into master via #1728. I have just checked out current master, build Alertmanager and ran it with --cluster.listen-address= which returns the following json on api/v2/status:

  "cluster": {
    "peers": null,
    "status": "disabled"
  },

It does not trigger a nil pointer exception in the Go http handler.

@knweiss if this issue still persists for you, would you mind creating a new Github issue? Thanks for the help!

[knweiss]

@mxinden Sorry, I'm a little bit in a hurry and only have time for a quick reply: It no longer triggers the nil pointer exception but when I click "Status" in the web UI I get the following error:

Unexpected response from api: Problem with the value at json.cluster.peers: null Expecting a LIST

It comes from this line:

ui/app/src/Utils/Api.elm:            "Unexpected response from api: " ++ err_ 

(My api/v2/status is the same as you mentioned above.)

[mxinden]

@knweiss we changed peers to be optional along with the patch above:

type alias ClusterStatus =
    { name : Maybe String
    , status : Status
    , peers : Maybe (List PeerStatus)
}

alertmanager/ui/app/src/Data/ClusterStatus.elm

Line 25 in f40ecee

, peers : Maybe (List PeerStatus)

Can you make sure your browser is not using a cached version of the front-end?

[knweiss]

@mxinden Branch release-0.16 (tag v0.16.1 ) worked fine but master branch (commit f40ecee as of today) did not. I still got the error line and could reproduce the problem by switching between both executables.

However, reloading without Firefox cache did the trick. Now the master branch executable renders the Status page, too! Thanks!

(I could be wrong but I also think I'm seeing the red "disabled" label for the first time in the v0.16.1 version. I'll keep an eye on caching effects from now on.)

[mxinden]

@knweiss I think we should disable caching of our web assets to prevent confusions like this. I prepared #1817. Let me know what you think.