Fix concurrent access to the same folder

2 files changed, 43 insertions, 5 deletions

diff --git a/internal/imap/connection.go b/internal/imap/connection.go
index 88b1496..358445b 100644
--- a/internal/imap/connection.go
+++ b/internal/imap/connection.go
@@ -99,6 +99,13 @@ func (conn *connection) ensureFolder(folder Folder) error {
 		return nil
 	}
 
+	if conn.mailboxes.locking(folder) {
+		// someone else tried to create the MB -- try again, now that he's done
+		return conn.ensureFolder(folder)
+	} else {
+		defer conn.mailboxes.unlocking(folder)
+	}
+
 	log.Printf("Checking for folder '%s'", folder)
 
 	mbox, found, err := conn.list(folder.str)
diff --git a/internal/imap/mailboxes.go b/internal/imap/mailboxes.go
index d0fdede..f1dc6c6 100644
--- a/internal/imap/mailboxes.go
+++ b/internal/imap/mailboxes.go
@@ -7,8 +7,38 @@ import (
 )
 
 type mailboxes struct {
-	mb map[string]*imap.MailboxInfo
-	mu sync.RWMutex
+	mb          map[string]*imap.MailboxInfo
+	mu          sync.RWMutex
+	changeLocks map[string]chan struct{}
+}
+
+func (mbs *mailboxes) unlocking(elem Folder) {
+	mbs.mu.Lock()
+	defer mbs.mu.Unlock()
+
+	ch, ok := mbs.changeLocks[elem.str]
+	if !ok {
+		panic("Unlocking where nothing is locked")
+	}
+	close(ch)
+	delete(mbs.changeLocks, elem.str)
+}
+
+func (mbs *mailboxes) locking(elem Folder) bool {
+	mbs.mu.Lock()
+	ch, ok := mbs.changeLocks[elem.str]
+	if !ok {
+		ch = make(chan struct{})
+		mbs.changeLocks[elem.str] = ch
+		mbs.mu.Unlock()
+		// we created the lock, we are in charge and done here
+		return false
+	} else {
+		// someone else is working, we wait till he's done
+		mbs.mu.Unlock() // we are not doing anything...
+		<-ch
+		return true
+	}
 }
 
 func (mbs *mailboxes) contains(elem Folder) bool {
@@ -28,7 +58,8 @@ func (mbs *mailboxes) add(elem *imap.MailboxInfo) {
 
 func NewMailboxes() *mailboxes {
 	return &mailboxes{
-		mb: map[string]*imap.MailboxInfo{},
-		mu: sync.RWMutex{},
+		mb:          map[string]*imap.MailboxInfo{},
+		changeLocks: map[string]chan struct{}{},
+		mu:          sync.RWMutex{},
 	}
-}
\ No newline at end of file
+}