[FragmentFD] Respect --no-continue

* discard partial fragment on `--no-continue`
* continue with correct progress display otherwise

Resolves #21467

youtube_dl/downloader/common.py

@@ -88,17 +88,21 @@ class FileDownloader(object):
             return '---.-%'
         return '%6s' % ('%3.1f%%' % percent)
 
-    @staticmethod
-    def calc_eta(start, now, total, current):
+    @classmethod
+    def calc_eta(cls, start_or_rate, now_or_remaining, *args):
+        if len(args) < 2:
+            rate, remaining = (start_or_rate, now_or_remaining)
+            if None in (rate, remaining):
+                return None
+            return int(float(remaining) / rate)
+        start, now = (start_or_rate, now_or_remaining)
+        total, current = args
         if total is None:
             return None
         if now is None:
             now = time.time()
-        dif = now - start
-        if current == 0 or dif < 0.001:  # One millisecond
-            return None
-        rate = float(current) / dif
-        return int((float(total) - float(current)) / rate)
+        rate = cls.calc_speed(start, now, current)
+        return rate and int((float(total) - float(current)) / rate)
 
     @staticmethod
     def format_eta(eta):
@@ -123,6 +127,12 @@ class FileDownloader(object):
     def format_retries(retries):
         return 'inf' if retries == float('inf') else '%.0f' % retries
 
+    @staticmethod
+    def filesize_or_none(unencoded_filename):
+        fn = encodeFilename(unencoded_filename)
+        if os.path.isfile(fn):
+            return os.path.getsize(fn)
+
     @staticmethod
     def best_block_size(elapsed_time, bytes):
         new_min = max(bytes / 2.0, 1.0)