PERFORMANCE: Refactor auditrecords.go to loop only once #1570 (#1658)

* stash

* Use rejectif idea

* rename

* wip!

* Convert old systems to new

* fixup!

* fix typo

pkg/rejectif/audit.go

@@ -0,0 +1,46 @@
+package rejectif
+
+import (
+	"github.com/StackExchange/dnscontrol/v3/models"
+)
+
+// Auditor stores a list of checks to be executed during Audit().
+type Auditor struct {
+	checksFor map[string][]checker
+}
+
+type checker = func(*models.RecordConfig) error
+
+// Add registers a function to call on each record of a given type.
+func (aud *Auditor) Add(rtype string, fn checker) {
+	if aud.checksFor == nil {
+		aud.checksFor = map[string][]checker{}
+	}
+	aud.checksFor[rtype] = append(aud.checksFor[rtype], fn)
+
+	// SPF records get any checkers that TXT records do.
+	if rtype == "TXT" {
+		aud.Add("SPF", fn)
+	}
+}
+
+// Audit performs the audit. For each record it calls each function in
+// the list of checks.
+func (aud *Auditor) Audit(records models.Records) (errs []error) {
+	// No checks? Exit early.
+	if aud.checksFor == nil {
+		return nil
+	}
+
+	// For each record, call the checks for that type, gather errors.
+	for _, rc := range records {
+		for _, f := range aud.checksFor[rc.Type] {
+			e := f(rc)
+			if e != nil {
+				errs = append(errs, e)
+			}
+		}
+	}
+
+	return errs
+}