Batched POP3 purge

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

@@ -2969,17 +2969,28 @@ class Core {
 
     private static void onPurgeFolder(Context context, EntityFolder folder) {
         // POP3
+        int count = 0;
+        int purged = 0;
+        do {
+            if (count > 0) {
+                try {
+                    Thread.sleep(YIELD_DURATION);
+                } catch (InterruptedException ignored) {
+                }
+            }
+
             DB db = DB.getInstance(context);
             try {
                 db.beginTransaction();
-
+                count = db.message().deleteHiddenMessages(folder.id, 100);
-            int purged = db.message().deleteHiddenMessages(folder.id);
-            Log.i(folder.name + " purge count=" + purged);
-
                 db.setTransactionSuccessful();
             } finally {
                 db.endTransaction();
             }
+
+            purged += count;
+            Log.i(folder.name + " purge count=" + count + "/" + purged);
+        } while (count > 0);
     }
 
     private static void onRule(Context context, JSONArray jargs, EntityMessage message) throws JSONException, MessagingException {

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

@@ -947,10 +947,12 @@ public interface DaoMessage {
             " AND NOT uid IS NULL")
     int deleteBrowsedMessages(long folder, long before);
 
-    @Query("DELETE FROM message" +
+    @Query("DELETE FROM message WHERE id IN (" +
+            " SELECT id FROM message" +
             " WHERE folder = :folder" +
-            " AND ui_hide")
+            " AND ui_hide" +
-    int deleteHiddenMessages(long folder);
+            " LIMIT :limit)")
+    int deleteHiddenMessages(long folder, int limit);
 
     @Query("DELETE FROM message" +
             " WHERE folder = :folder" +