Synchronize message is a critical section

app/src/main/java/eu/faircode/email/Core.java

@@ -77,6 +77,8 @@ import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Properties;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.TimeUnit;
 import java.util.regex.Pattern;
@@ -111,6 +113,7 @@ import static androidx.core.app.NotificationCompat.DEFAULT_SOUND;
 
 class Core {
     private static int lastUnseen = -1;
+    private static ConcurrentMap<Long, Long> lockFolders = new ConcurrentHashMap<>();
 
     private static final int MAX_NOTIFICATION_COUNT = 10; // per group
     private static final int SYNC_CHUNCK_SIZE = 200;
@@ -1369,6 +1372,8 @@ class Core {
             IMAPFolder ifolder, IMAPMessage imessage,
             boolean browsed, boolean download,
             List<EntityRule> rules, State state) throws MessagingException, IOException {
+        lockFolders.putIfAbsent(folder.id, folder.id);
+        synchronized (lockFolders.get(folder.id)) {
             long uid = ifolder.getUID(imessage);
 
             if (imessage.isExpunged()) {
@@ -1528,13 +1533,6 @@ class Core {
                 try {
                     db.beginTransaction();
 
-                // Check if message was added in the meantime
-                EntityMessage existing = db.message().getMessageByUid(message.folder, message.uid);
-                if (existing != null) {
-                    Log.i("Message was already added");
-                    return existing;
-                }
-
                     message.id = db.message().insertMessage(message);
                     Log.i(folder.name + " added id=" + message.id + " uid=" + message.uid);
 
@@ -1696,6 +1694,7 @@ class Core {
 
             return message;
         }
+    }
 
     private static EntityIdentity matchIdentity(Context context, EntityFolder folder, EntityMessage message) {
         DB db = DB.getInstance(context);