Serialize image downloading

This prevents slow connections from being saturized and causes cached images to be reused

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

@@ -81,7 +81,7 @@ public class HtmlHelper {
     private static final List<String> tails = Collections.unmodifiableList(Arrays.asList(
             "h1", "h2", "h3", "h4", "h5", "h6", "p", "ol", "ul", "li"));
 
-    private static final ExecutorService executor = Executors.newCachedThreadPool(Helper.backgroundThreadFactory);
+    private static final ExecutorService executor = Executors.newSingleThreadExecutor(Helper.backgroundThreadFactory);
 
     static String removeTracking(Context context, String html) {
         SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
@@ -268,7 +268,7 @@ public class HtmlHelper {
         return (body == null ? "" : body.html());
     }
 
-    static Drawable decodeImage(final String source, long id, boolean show, final TextView view) {
+    static Drawable decodeImage(final String source, final long id, boolean show, final TextView view) {
         SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(view.getContext());
         boolean compact = prefs.getBoolean("compact", false);
         int zoom = prefs.getInt("zoom", compact ? 0 : 1);
@@ -351,36 +351,35 @@ public class HtmlHelper {
                 return d;
             }
 
-        // Get cache file name
-        File dir = new File(view.getContext().getCacheDir(), "images");
-        if (!dir.exists())
-            dir.mkdir();
-        final File file = new File(dir, id + "_" + Math.abs(source.hashCode()) + ".png");
-
-        if (file.exists()) {
-            Log.i("Using cached " + file);
-            Bitmap bm = BitmapFactory.decodeFile(file.getAbsolutePath());
-            if (bm == null) {
-                Drawable d = res.getDrawable(R.drawable.baseline_broken_image_24, theme);
-                d.setBounds(0, 0, px, px);
-                return d;
-            } else {
-                Drawable d = new BitmapDrawable(res, bm);
-                d.setBounds(0, 0, bm.getWidth(), bm.getHeight());
-                return d;
-            }
-        }
-
         final LevelListDrawable lld = new LevelListDrawable();
         Drawable wait = res.getDrawable(R.drawable.baseline_hourglass_empty_24, theme);
         lld.addLevel(0, 0, wait);
         lld.setBounds(0, 0, px, px);
 
+        final Context context = view.getContext().getApplicationContext();
         final Handler handler = new Handler(view.getContext().getMainLooper());
         executor.submit(new Runnable() {
             @Override
             public void run() {
                 try {
+                    // Get cache file name
+                    File dir = new File(context.getCacheDir(), "images");
+                    if (!dir.exists())
+                        dir.mkdir();
+                    File file = new File(dir, id + "_" + Math.abs(source.hashCode()) + ".png");
+
+                    // Get cached image
+                    if (file.exists() && !BuildConfig.DEBUG) {
+                        Log.i("Using cached " + file);
+                        Bitmap bm = BitmapFactory.decodeFile(file.getAbsolutePath());
+                        if (bm != null) {
+                            Drawable d = new BitmapDrawable(res, bm);
+                            d.setBounds(0, 0, bm.getWidth(), bm.getHeight());
+                            post(d, source);
+                            return;
+                        }
+                    }
+
                     BitmapFactory.Options options = new BitmapFactory.Options();
                     Log.i("Probe " + source);
                     try (InputStream probe = new URL(source).openStream()) {