[GODT-797] EXPUNGE waits for APPEND to prevent data loss when Outlook moves from Spam to Inbox

internal/imap/mailbox.go

@@ -177,6 +177,17 @@ func (im *imapMailbox) Check() error {
 // Expunge permanently removes all messages that have the \Deleted flag set
 // from the currently selected mailbox.
 func (im *imapMailbox) Expunge() error {
+	// Wait for any APPENDS to finish in order to avoid data loss when
+	// Outlook sends commands too quickly STORE \Deleted, APPEND, EXPUNGE,
+	// APPEND FINISHED:
+	//
+	// Based on Outlook APPEND request we will not create new message but
+	// move the original to desired mailbox. If the message is currently
+	// in Trash or Spam and EXPUNGE happens before APPEND processing is
+	// finished the message is deleted from Proton instead of moved to
+	// the desired mailbox.
+	im.user.waitForAppend()
+
 	im.user.backend.setUpdatesBeBlocking(im.user.currentAddressLowercase, im.name, operationDeleteMessage)
 	defer im.user.backend.unsetUpdatesBeBlocking(im.user.currentAddressLowercase, im.name, operationDeleteMessage)
 

internal/imap/mailbox_message.go

@@ -70,6 +70,9 @@ func (im *imapMailbox) CreateMessage(flags []string, date time.Time, body imap.L
 	// Called from go-imap in goroutines - we need to handle panics for each function.
 	defer im.panicHandler.HandlePanic()
 
+	im.user.appendStarted()
+	defer im.user.appendFinished()
+
 	m, _, _, readers, err := message.Parse(body)
 	if err != nil {
 		return err

internal/imap/user.go

@@ -20,6 +20,7 @@ package imap
 import (
 	"errors"
 	"strings"
+	"sync"
 
 	"github.com/ProtonMail/proton-bridge/pkg/pmapi"
 	imapquota "github.com/emersion/go-imap-quota"
@@ -39,6 +40,8 @@ type imapUser struct {
 	storeAddress storeAddressProvider
 
 	currentAddressLowercase string
+
+	appendInProcess sync.WaitGroup
 }
 
 // This method should eventually no longer be necessary. Everything should go via store.
@@ -238,3 +241,15 @@ func (iu *imapUser) CreateMessageLimit() *uint32 {
 	upload := uint32(maxUpload)
 	return &upload
 }
+
+func (iu *imapUser) appendStarted() {
+	iu.appendInProcess.Add(1)
+}
+
+func (iu *imapUser) appendFinished() {
+	iu.appendInProcess.Done()
+}
+
+func (iu *imapUser) waitForAppend() {
+	iu.appendInProcess.Wait()
+}