Added POP3 quick check

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

@@ -2299,12 +2299,33 @@ class Core {
                     ? imessages.length
                     : Math.min(imessages.length, account.max_messages));
 
+            boolean sync = true;
+            if (imessages.length > 0 && folder.last_sync_count != null &&
+                    imessages.length == folder.last_sync_count) {
+                // Check if last message known as new messages indicator
+                MessageHelper helper = new MessageHelper((MimeMessage) imessages[imessages.length - 1], context);
+                String msgid = helper.getMessageID();
+                if (msgid != null) {
+                    int count = db.message().countMessageByMsgId(folder.id, msgid);
+                    if (count == 1) {
+                        Log.i(account.name + " POP having last msgid=" + msgid);
+                        sync = false;
+                    }
+                }
+            }
+
             EntityLog.log(context, account.name + " POP" +
                     " device=" + ids.size() +
                     " server=" + imessages.length +
                     " max=" + max + "/" + account.max_messages +
+                    " last=" + folder.last_sync_count +
+                    " sync=" + sync +
                     " uidl=" + hasUidl);
 
+            folder.last_sync_count = imessages.length;
+            db.folder().setFolderLastSyncCount(folder.id, folder.last_sync_count);
+
+            if (sync) {
                 // Index IDs
                 Map<String, String> uidlMsgId = new HashMap<>();
                 for (TupleUidl id : ids) {
@@ -2550,6 +2571,7 @@ class Core {
                         ((POP3Message) imessage).invalidate(true);
                     }
                 }
+            }
 
             db.folder().setFolderLastSync(folder.id, new Date().getTime());
             EntityLog.log(context, account.name + " POP done");

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

@@ -65,7 +65,7 @@ import static eu.faircode.email.ServiceAuthenticator.AUTH_TYPE_PASSWORD;
 // https://developer.android.com/topic/libraries/architecture/room.html
 
 @Database(
-        version = 202,
+        version = 203,
         entities = {
                 EntityIdentity.class,
                 EntityAccount.class,
@@ -2052,6 +2052,12 @@ public abstract class DB extends RoomDatabase {
                         Log.i("DB migration from version " + startVersion + " to " + endVersion);
                         db.execSQL("UPDATE identity SET use_ip = 0 WHERE host = 'smtp.office365.com'");
                     }
+                }).addMigrations(new Migration(202, 203) {
+                    @Override
+                    public void migrate(@NonNull SupportSQLiteDatabase db) {
+                        Log.i("DB migration from version " + startVersion + " to " + endVersion);
+                        db.execSQL("ALTER TABLE `folder` ADD COLUMN `last_sync_count` INTEGER");
+                    }
                 }).addMigrations(new Migration(998, 999) {
                     @Override
                     public void migrate(@NonNull SupportSQLiteDatabase db) {

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

@@ -353,6 +353,9 @@ public interface DaoFolder {
     @Query("UPDATE folder SET last_sync = :last_sync WHERE id = :id AND NOT (last_sync IS :last_sync)")
     int setFolderLastSync(long id, long last_sync);
 
+    @Query("UPDATE folder SET last_sync_count = :last_sync_count WHERE id = :id AND NOT (last_sync_count IS :last_sync_count)")
+    int setFolderLastSyncCount(long id, Integer last_sync_count);
+
     @Query("UPDATE folder SET read_only = :read_only WHERE id = :id AND NOT (read_only IS :read_only)")
     int setFolderReadOnly(long id, boolean read_only);
 

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

@@ -136,6 +136,7 @@ public class EntityFolder extends EntityOrder implements Serializable {
     public Boolean inferiors = true;
     public String error;
     public Long last_sync;
+    public Integer last_sync_count; // POP3
 
     static final String INBOX = "Inbox";
     static final String OUTBOX = "Outbox";