don't notify @mnetioned user who are not members of channel (#1912)

mattermost-plugin/server/notifications.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/mattermost/focalboard/server/services/notify"
@@ -67,3 +68,11 @@ func (da *pluginAPIAdapter) GetTeamMember(teamID string, userID string) (*model.
 func (da *pluginAPIAdapter) GetChannelByID(channelID string) (*model.Channel, error) {
 	return da.client.Channel.Get(channelID)
 }
+
+func (da *pluginAPIAdapter) GetChannelMember(channelID string, userID string) (*model.ChannelMember, error) {
+	return da.client.Channel.GetMember(channelID, userID)
+}
+
+func (da *pluginAPIAdapter) IsErrNotFound(err error) bool {
+	return errors.Is(err, pluginapi.ErrNotFound)
+}

server/services/notify/plugindelivery/plugin_delivery.go

@@ -30,6 +30,13 @@ type PluginAPI interface {
 
 	// GetChannelByID gets a Channel by its ID.
 	GetChannelByID(channelID string) (*model.Channel, error)
+
+	// GetChannelMember gets a channel member by userID.
+	GetChannelMember(channelID string, userID string) (*model.ChannelMember, error)
+
+	// IsErrNotFound returns true if `err` or one of its wrapped children are the `ErrNotFound`
+	// as defined in the plugin API.
+	IsErrNotFound(err error) bool
 }
 
 // PluginDelivery provides ability to send notifications to direct message channels via Mattermost plugin API.
@@ -64,6 +71,16 @@ func (pd *PluginDelivery) Deliver(mentionUsername string, extract string, evt no
 		}
 	}
 
+	// check that user is a member of the channel
+	_, err = pd.api.GetChannelMember(evt.Workspace, member.UserId)
+	if err != nil {
+		if pd.api.IsErrNotFound(err) {
+			// mentioned user is not a member of the channel; fail silently.
+			return nil
+		}
+		return fmt.Errorf("cannot fetch channel member for user %s: %w", member.UserId, err)
+	}
+
 	author, err := pd.api.GetUserByID(evt.UserID)
 	if err != nil {
 		return fmt.Errorf("cannot find user: %w", err)

server/services/notify/plugindelivery/user_test.go

@@ -140,6 +140,14 @@ func (m pluginAPIMock) GetChannelByID(channelID string) (*mm_model.Channel, erro
 	return nil, ErrNotFound{}
 }
 
+func (m pluginAPIMock) GetChannelMember(channelID string, userID string) (*mm_model.ChannelMember, error) {
+	return nil, ErrNotFound{}
+}
+
+func (m pluginAPIMock) IsErrNotFound(err error) bool {
+	return false
+}
+
 type ErrNotFound struct{}
 
 func (e ErrNotFound) Error() string {