feat(server): Support chunked formdata for Crashpad

[jan-auer]

Google Crashpad has a limitation on the length of values for custom attributes. This makes it impossible to send scope information and event payloads using the default Crashpad uploader. In the past, our SDKs had to disable automatic uploading, and implement their own logic, which comes with a set of disadvantages. In the future, Crashpad will support custom attachments on all platforms, but until that is available, we need a workaround on our end.

An upcoming version of Electron will send long crashpad annotations in chunks to work around this limitation. When a long value is detected, the annotation is duplicated with a postfix of __n, where n is the 1-based index of the chunk.

This PR joins these chunks, assuming they come in random order, and then parses JSON to extract the payload. An event id is not allowed in these payloads, as it cannot be parsed efficiently in the endpoint. Instead, Relay generates a random event id and always overrides the one in the payload.

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[jan-auer]

event_id_from_formdata still cannot handle this, and it's not clear to me how to implement that efficiently yet. Without that, we are not able to extract the event ID in time to respond correctly to the client.

relay/relay-server/src/endpoints/common.rs

Line 235 in 11f760d

} else if entry.key() == "sentry[event_id]" {

[jan-auer]

Since this is meant for Crashpad, it would be acceptable to define that the event ID should not be set when using this mechanism. Relay will assign one automatically and return it in the response, which can be picked up to render user feedback dialogs.

@HazAT do you think that would be a fair limitation?

[untitaker]

lgtm though I will say that I feel like we're building too many sdk specific things into relay

[jan-auer]

I feel like we're building too many sdk specific things into relay

@untitaker I see your point, this is very borderline. I will update the PR description with more reasoning on this.