impossible for milter_headers to add a custom header

[dlatchx]

rspamd has a module to insert custom headers : https://rspamd.com/doc/modules/milter_headers.html

But this filter only support a few hardcoded headers.

I think the problem is here :

filter-rspamd/filter-rspamd.go

Lines 510 to 525 in 9e74486

	/**
	* Prefix auth headers to incoming mail in proper order.
	*/
	if len(authHeaders) > 0 {
	hdrs := []string{
	"ARC-Seal",
	"ARC-Message-Signature",
	"ARC-Authentication-Results",
	"Authentication-Results"}
	for _, h := range hdrs {
	if authHeaders[h] != "" {
	writeHeader(s, token, h, authHeaders[h])
	}
	}
	}

Only the headers in hdrs will be inserted by the filter.

Here is an example of the headers that rspamd can return:

curl -s -X POST http://localhost:11333/checkv2 | jq .milter.add_headers
{
  "X-Spamd-Bar": {
    "value": "+++++++++++++++",
    "order": -1
  },
  "X-Spamd-Result": {
    "value": "default: True [15.00 / 15.00];\r\n\tCOMPLETELY_EMPTY(15.00)[]",
    "order": -1
  },
  "X-Rspamd-Action": {
    "value": "reject",
    "order": -1
  },
  "X-Spam-Status": {
    "value": "Yes, score=15.00",
    "order": -1
  },
  "Authentication-Results": {
    "value": "localhost;\r\n\tnone",
    "order": 1
  },
  "X-Rspamd-Server": {
    "value": "localhost",
    "order": -1
  },
  "X-Spam-Level": {
    "value": "***************",
    "order": -1
  }
}

I can rewrite this, but I'd like to know what is the rationale for this code:

How does the order matter ?
Does this matter for other headers (such as those given above) ?

[dlatchx]

Does the order matter -> yes, it's required by the RFC
https://github.com/rspamd/rspamd/blob/2e2aab12026b0cbd401a364a888d6ecdd5f8d5db/src/plugins/lua/arc.lua#L597-L605

It looks like rspamd's infrastructure for inserting headers in a specified order is very brittle, and it will be hard to fix properly.
So it looks like it we'll have to kip this manual ordering on our side for now.

[junaru]

@dlatchx huge thanks!

Just wasted better part of the weekend trying to figure out why rspamd doesn't add my headers...

Confirming the pull requests works as expected.