Use sub directories to store message files

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

@@ -68,7 +68,7 @@ import javax.mail.internet.InternetAddress;
 // https://developer.android.com/topic/libraries/architecture/room.html
 
 @Database(
-        version = 245,
+        version = 246,
         entities = {
                 EntityIdentity.class,
                 EntityAccount.class,
@@ -2477,6 +2477,12 @@ public abstract class DB extends RoomDatabase {
                         db.execSQL("UPDATE account SET keep_alive_noop = 1" +
                                 " WHERE host = 'outlook.office365.com' AND pop = " + EntityAccount.TYPE_IMAP);
                     }
+                }).addMigrations(new Migration(245, 246) {
+                    @Override
+                    public void migrate(@NonNull SupportSQLiteDatabase db) {
+                        logMigration(startVersion, endVersion);
+                        EntityMessage.convert(context);
+                    }
                 }).addMigrations(new Migration(998, 999) {
                     @Override
                     public void migrate(@NonNull SupportSQLiteDatabase db) {

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

@@ -577,10 +577,28 @@ public class EntityMessage implements Serializable {
     }
 
     static File getFile(Context context, Long id) {
-        File dir = Helper.ensureExists(new File(context.getFilesDir(), "messages"));
+        File root = new File(context.getFilesDir(), "messages");
+        File dir = Helper.ensureExists(new File(root, Long.toString(id / 1000)));
         return new File(dir, id.toString());
     }
 
+    static void convert(Context context) {
+        File root = new File(context.getFilesDir(), "messages");
+        File[] files = root.listFiles();
+        if (files == null)
+            return;
+        for (File file : files)
+            if (file.isFile())
+                try {
+                    long id = Long.parseLong(file.getName());
+                    File target = getFile(context, id);
+                    if (!file.renameTo(target))
+                        throw new IllegalArgumentException("Failed renaming to " + target);
+                } catch (Throwable ex) {
+                    Log.e(ex);
+                }
+    }
+
     File getFile(Context context) {
         return getFile(context, id);
     }

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

@@ -2390,6 +2390,18 @@ public class Helper {
         return size;
     }
 
+    static List<File> listFiles(File dir) {
+        List<File> result = new ArrayList<>();
+        File[] files = dir.listFiles();
+        if (files != null)
+            for (File file : files)
+                if (file.isDirectory())
+                    result.addAll(listFiles(file));
+                else
+                    result.add(file);
+        return result;
+    }
+
     static long getAvailableStorageSpace() {
         StatFs stats = new StatFs(Environment.getDataDirectory().getAbsolutePath());
         return stats.getAvailableBlocksLong() * stats.getBlockSizeLong();

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

@@ -206,7 +206,7 @@ public class WorkerCleanup extends Worker {
             long now = new Date().getTime();
 
             List<File> files = new ArrayList<>();
-            File[] messages = new File(context.getFilesDir(), "messages").listFiles();
+            File[] messages = Helper.listFiles(new File(context.getFilesDir(), "messages")).toArray(new File[0]);
             File[] revision = new File(context.getFilesDir(), "revision").listFiles();
             File[] references = new File(context.getFilesDir(), "references").listFiles();
             File[] encryption = new File(context.getFilesDir(), "encryption").listFiles();