Slack integration for Sentry on private domains.

[wolandspb]

If Sentry is behind private domain, it's impossible to set up slack integration. This PR should fix it by adding slack.url-prefix option, allowed you to use external domain to set up slack integration.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[wolandspb]

@cathteng I apologize for the mention, but this PR stuck for several weeks. I'm unsure who to ask for review. Given your approval of the previous Slack-related PR, would you be able to review this one, or perhaps suggest someone who could?

[cathteng]

Hi @wolandspb , the developer docs mention you can set the redirect link on the Slack side https://develop.sentry.dev/integrations/slack/, does this work for you?

On self-hosted you'll need to create your own Slack app

[wolandspb]

the developer docs mention you can set the redirect link on the Slack side https://develop.sentry.dev/integrations/slack/, does this work for you?

Sentry isn't finding the redirect URL on the slack side because it doesn't match the system.url-prefix. I've already tested this.

[cathteng]

I think I'm missing some context. When we test self-hosted locally we use system.url-prefix in the redirect url in Slack and it works, because it needs to hit the domain where your Sentry instance is located. Do you need this because your slack.url-prefix is different from system.url-prefix? Can you explain why it would be different?

[wolandspb]

Sorry, I could have explained this better in the PR description.
Our system.url-prefix is a domain resolvable only by our internal DNS and accessible only from the company network for security. This PR introduces a configurable slack.url-prefix, distinct from system.url-prefix. This allows setting it to an externally resolvable domain accessible from the internet, while restricting access to only <slack.url-prefix>/extensions/slack requests.
I believe there are many users, like ourselves, want to receive alerts in Slack but prefer not to make Sentry publicly accessible.