GitLab Mattermost 503 after gitlab-ctl reconfigure in 9.2.0-rc7

Summary

Mattermost is no longer reachable (503 Service unavailable). cat /var/log/gitlab/mattermost/current is flooded with messages pasted below:

2017-05-21_08:53:25.10173 [2017/05/21 10:53:25 CEST] [INFO] Pinging SQL master database
2017-05-21_08:53:25.10896 [2017/05/21 10:53:25 CEST] [INFO] Pinging SQL replica-0 database
2017-05-21_08:53:25.11499 [2017/05/21 10:53:25 CEST] [INFO] Pinging SQL search-replica-0 database
2017-05-21_08:53:25.11506 [2017/05/21 10:53:25 CEST] [CRIT] Failed to ping DB err:missing "=" after "[]" in connection info string"

Steps to reproduce

• upgrade to latest gitlab
• run gitlab-ctl reconfigure
• check availability of mattermost

What is the current bug behavior?

/var/opt/gitlab/mattermost/config.json shows following settings in sql section

    "SqlSettings": {
        "DriverName": "postgres",
        "DataSource": "user=gitlab_mattermost host=/var/opt/gitlab/postgresql port=5432 dbname=mattermost_production",
        "DataSourceReplicas": ["user=gitlab_mattermost host=/var/opt/gitlab/postgresql port=5432 dbname=mattermost_production"],
        "MaxIdleConns": 10,
        "MaxOpenConns": 10,
        "Trace": false,
        "AtRestEncryptKey": "REDACTED",
        "DataSourceSearchReplicas": "[]"
    },

What is the expected correct behavior?

Mattermost should operate normally