Merge branch 'docs-slack-troubleshooting' into 'master'

Add troubleshooting section to Slack Notification Service

See merge request gitlab-org/gitlab-ce!24339

doc/user/project/integrations/slack.md

@@ -23,4 +23,54 @@ The Slack Notifications Service allows your GitLab project to send events (e.g.
 
 Your Slack team will now start receiving GitLab event notifications as configured.
 
-![Slack configuration](img/slack_configuration.png)
+![Slack configuration](img/slack_configuration.png)
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+
+## Troubleshooting
+
+If you're having trouble with the Slack integration not working, then start by
+searching through the [Sidekiq logs](/doc/administration/logs.md#sidekiqlog)
+for errors relating to your Slack service.
+
+### Something went wrong on our end
+
+This is a generic error shown in the GitLab UI and doesn't mean much by itself.
+You'll need to look in [the logs](/doc/administration/logs.md#productionlog) to find
+an error message and keep troubleshooting from there.
+
+### `certificate verify failed`
+
+You may see an entry similar to the following in your Sidekiq log:
+
+```text
+2019-01-10_13:22:08.42572 2019-01-10T13:22:08.425Z 6877 TID-abcdefg ProjectServiceWorker JID-3bade5fb3dd47a85db6d78c5 ERROR: {:class=>"ProjectServiceWorker", :service_class=>"SlackService", :message=>"SSL_connect returned=1 errno=0 state=error: certificate verify failed"}
+```
+
+This is probably a problem either with GitLab communicating with Slack, or GitLab
+communicating with itself. The former is less likely since Slack's security certificates
+should _hopefully_ always be trusted. We can establish which we're dealing with by using
+the below rails console script.
+
+```sh
+# start a rails console:
+sudo gitlab-rails console production
+
+# or for source installs:
+bundle exec rails console production
+```
+
+```ruby
+# run this in the Rails console
+# replace <SLACK URL> with your actual Slack URL
+result = Net::HTTP.get(URI('https://<SLACK URL>'));0
+
+# replace <GITLAB URL> with your actual GitLab URL
+result = Net::HTTP.get(URI('https://<GITLAB URL>'));0
+```
+
+If it's an issue with GitLab not trusting HTTPS connections to itself, then you may simply
+need to [add your certificate to GitLab's trusted certificates](https://docs.gitlab.com/omnibus/settings/ssl.html#install-custom-public-certificates).
+
+If it's an issue with GitLab not trusting connections to Slack, then the GitLab
+OpenSSL trust store probably got messed up somehow. Typically this is from overriding
+the trust store with `gitlab_rails['env'] = {"SSL_CERT_FILE" => "/path/to/file.pem"}`
+or by accidentally modifying the default CA bundle `/opt/gitlab/embedded/ssl/certs/cacert.pem`.
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