feat: better handling of multipart messages

pkg/message/parser/visitor.go

@@ -0,0 +1,55 @@
+package parser
+
+import "regexp"
+
+type Visitor struct {
+	rules    []*rule
+	fallback Rule
+}
+
+func NewVisitor(fallback Rule) *Visitor {
+	return &Visitor{
+		fallback: fallback,
+	}
+}
+
+type Visit func(*Part) (interface{}, error)
+
+type Rule func(*Part, Visit) (interface{}, error)
+
+type rule struct {
+	re string
+	fn Rule
+}
+
+func (v *Visitor) RegisterRule(contentTypeRegex string, fn Rule) *Visitor {
+	v.rules = append(v.rules, &rule{
+		re: contentTypeRegex,
+		fn: fn,
+	})
+
+	return v
+}
+
+func (v *Visitor) Visit(p *Part) (interface{}, error) {
+	t, _, err := p.Header.ContentType()
+	if err != nil {
+		return nil, err
+	}
+
+	if rule := v.getRuleForContentType(t); rule != nil {
+		return rule.fn(p, v.Visit)
+	}
+
+	return v.fallback(p, v.Visit)
+}
+
+func (v *Visitor) getRuleForContentType(contentType string) *rule {
+	for _, rule := range v.rules {
+		if regexp.MustCompile(rule.re).MatchString(contentType) {
+			return rule
+		}
+	}
+
+	return nil
+}