message: Implement service layer, stubs for #81

I've made some effort to wire the manager into the controllers, but
tests are currently failing.

pkg/message/manager.go

@@ -0,0 +1,86 @@
+package message
+
+import (
+	"io"
+
+	"github.com/jhillyerd/enmime"
+	"github.com/jhillyerd/inbucket/pkg/storage"
+)
+
+// Manager is the interface controllers use to interact with messages.
+type Manager interface {
+	GetMetadata(mailbox string) ([]*Metadata, error)
+	GetMessage(mailbox, id string) (*Message, error)
+	PurgeMessages(mailbox string) error
+	RemoveMessage(mailbox, id string) error
+	SourceReader(mailbox, id string) (io.ReadCloser, error)
+}
+
+// StoreManager is a message Manager backed by the storage.Store.
+type StoreManager struct {
+	Store storage.Store
+}
+
+// GetMetadata returns a slice of metadata for the specified mailbox.
+func (s *StoreManager) GetMetadata(mailbox string) ([]*Metadata, error) {
+	messages, err := s.Store.GetMessages(mailbox)
+	if err != nil {
+		return nil, err
+	}
+	metas := make([]*Metadata, len(messages))
+	for i, sm := range messages {
+		metas[i] = makeMetadata(sm)
+	}
+	return metas, nil
+}
+
+// GetMessage returns the specified message.
+func (s *StoreManager) GetMessage(mailbox, id string) (*Message, error) {
+	sm, err := s.Store.GetMessage(mailbox, id)
+	if err != nil {
+		return nil, err
+	}
+	r, err := sm.RawReader()
+	if err != nil {
+		return nil, err
+	}
+	env, err := enmime.ReadEnvelope(r)
+	if err != nil {
+		return nil, err
+	}
+	_ = r.Close()
+	header := makeMetadata(sm)
+	return &Message{Metadata: *header, Envelope: env}, nil
+}
+
+// PurgeMessages removes all messages from the specified mailbox.
+func (s *StoreManager) PurgeMessages(mailbox string) error {
+	return s.Store.PurgeMessages(mailbox)
+}
+
+// RemoveMessage deletes the specified message.
+func (s *StoreManager) RemoveMessage(mailbox, id string) error {
+	return s.Store.RemoveMessage(mailbox, id)
+}
+
+// SourceReader allows the stored message source to be read.
+func (s *StoreManager) SourceReader(mailbox, id string) (io.ReadCloser, error) {
+	sm, err := s.Store.GetMessage(mailbox, id)
+	if err != nil {
+		return nil, err
+	}
+	return sm.RawReader()
+}
+
+// makeMetadata populates Metadata from a StoreMessage.
+func makeMetadata(m storage.StoreMessage) *Metadata {
+	return &Metadata{
+		Mailbox: m.Mailbox(),
+		ID:      m.ID(),
+		From:    m.From(),
+		To:      m.To(),
+		Date:    m.Date(),
+		Subject: m.Subject(),
+		Size:    m.Size(),
+	}
+}