Add initial set of interfaces for modules

module/auth.go

@@ -0,0 +1,7 @@
+package module
+
+// AuthProvider is the interface implemented by modules providing authentication using
+// username:password pairs.
+type AuthProvider interface {
+	CheckPlain(username, password string) bool
+}

module/filter.go

@@ -0,0 +1,68 @@
+package module
+
+import (
+	"bytes"
+	"errors"
+	"net"
+)
+
+// DeliveryContext structure is created for each message and passed with it
+// to each step of pipeline.
+//
+// Module instances must not retain references to context structure after
+// Apply function returns.
+type DeliveryContext struct {
+	// Message sender address.
+	From string
+
+	// Recipient addresses as specified by message sender.
+	// Pipeline steps are allowed to modify this field.
+	To []string
+
+	// If message is received over SMTP and client is authenticated
+	// non-anonymously - this field contains used username.
+	AuthUser string
+
+	// If message is received over SMTP - this field contains
+	// FQDN sent by client in EHLO/HELO command.
+	SrcHostname string
+	// If message is received over SMTP - this field contains
+	// network address of client.
+	SrcIP net.Addr
+
+	// Arbitrary context meta-data that can be modified by any module in
+	// pipeline. It is passed unchanged to next module in chain.
+	//
+	// For example, spam filter may set Ctx[spam] to true to tell storage
+	// backend to mark message as spam.
+	Ctx map[string]interface{}
+
+	// Custom options passed to filter from server configuration.
+	Opts map[string]interface{}
+}
+
+var ErrSilentDrop = errors.New("message is dropped by filter")
+
+// Filter is the interface implemented by modules that can perform arbitrary
+// transformations on incoming/outdoing messages.
+type Filter interface {
+	// Apply is called for each message that should be processed by module
+	// instance.
+	//
+	// If it returns non-nil error - message processing stops and error will be
+	// returned to message source.  However, if Apply returns ErrSilentDrop
+	// error value - message processing stops and message source will be told
+	// that message is processed successfully.
+	Apply(ctx *DeliveryContext, rcpt *bytes.Buffer) error
+}
+
+// DeliveryTarget is a special versionof Filter tht can't mutate message or its
+// delivery context.
+type DeliveryTarget interface {
+	// Deliver is called for each message that should be processed by module
+	// instance.
+	//
+	// If it returns non-nil error - message processing stops and error will be
+	// returned to message source.
+	Deliver(ctx DeliveryContext, rcpt bytes.Buffer) error
+}

module/storage.go

@@ -0,0 +1,30 @@
+package module
+
+import imapbackend "github.com/emersion/go-imap/backend"
+
+// Storage interface is a slightly modified go-imap's Backend interface
+// (authentication is removed).
+type Storage interface {
+	// GetUser returns User associated with user account specified by name.
+	//
+	// If it doesn't exists - it should be created.
+	GetUser(username string) (imapbackend.User, error)
+}
+
+// IMAPBackend is structure that binds together implementations of Storage and
+// AuthProvider interfaces to get full go-imap Backend implementation.
+type IMAPBackend struct {
+	Auth  AuthProvider
+	Store Storage
+}
+
+// Force check to ensure that we really implement go-imap interface.
+var _ imapbackend.Backend = IMAPBackend{}
+
+func (be IMAPBackend) Login(username, password string) (imapbackend.User, error) {
+	if !be.Auth.CheckPlain(username, password) {
+		return nil, imapbackend.ErrInvalidCredentials
+	}
+
+	return be.Store.GetUser(username)
+}