expose fewer internals in packages, for easier software reuse

- prometheus is now behind an interface, they aren't dependencies for the
  reusable components anymore.
- some dependencies have been inverted: instead of packages importing a main
  package to get configuration, the main package now sets configuration in
  these packages. that means fewer internals are pulled in.
- some functions now have new parameters for values that were retrieved from
  package "mox-".

mox-/dkimsign.go

@@ -0,0 +1,65 @@
+package mox
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/mjl-/mox/config"
+	"github.com/mjl-/mox/dkim"
+	"github.com/mjl-/mox/dns"
+	"github.com/mjl-/mox/mlog"
+	"github.com/mjl-/mox/smtp"
+)
+
+// DKIMSelectors returns the selectors to use for signing.
+func DKIMSelectors(dkimConf config.DKIM) []dkim.Selector {
+	var l []dkim.Selector
+	for _, sign := range dkimConf.Sign {
+		sel := dkimConf.Selectors[sign]
+		s := dkim.Selector{
+			Hash:          sel.HashEffective,
+			HeaderRelaxed: sel.Canonicalization.HeaderRelaxed,
+			BodyRelaxed:   sel.Canonicalization.BodyRelaxed,
+			Headers:       sel.HeadersEffective,
+			SealHeaders:   !sel.DontSealHeaders,
+			Expiration:    time.Duration(sel.ExpirationSeconds) * time.Second,
+			PrivateKey:    sel.Key,
+			Domain:        sel.Domain,
+		}
+		l = append(l, s)
+	}
+	return l
+}
+
+// DKIMSign looks up the domain for "from", and uses its DKIM configuration to
+// generate DKIM-Signature headers, for inclusion in a message. The
+// DKIM-Signatur headers, are returned. If no domain was found an empty string and
+// nil error is returned.
+func DKIMSign(ctx context.Context, log mlog.Log, from smtp.Path, smtputf8 bool, data []byte) (string, error) {
+	// Add DKIM signature for domain, even if higher up than the full mail hostname.
+	// This helps with an assumed (because default) relaxed DKIM policy. If the DMARC
+	// policy happens to be strict, the signature won't help, but won't hurt either.
+	fd := from.IPDomain.Domain
+	var zerodom dns.Domain
+	for fd != zerodom {
+		confDom, ok := Conf.Domain(fd)
+		if !ok {
+			var nfd dns.Domain
+			_, nfd.ASCII, _ = strings.Cut(fd.ASCII, ".")
+			_, nfd.Unicode, _ = strings.Cut(fd.Unicode, ".")
+			fd = nfd
+			continue
+		}
+
+		selectors := DKIMSelectors(confDom.DKIM)
+		dkimHeaders, err := dkim.Sign(ctx, log.Logger, from.Localpart, fd, selectors, smtputf8, bytes.NewReader(data))
+		if err != nil {
+			return "", fmt.Errorf("dkim sign for domain %s: %v", fd, err)
+		}
+		return dkimHeaders, nil
+	}
+	return "", nil
+}