[downloader/dash] Add fragment retry capability

YouTube may often return 404 HTTP error for a fragment causing the
whole download to fail. However if the same fragment is immediately
retried with the same request data this usually succeeds (1-2 attemps
is usually enough) thus allowing to download the whole file successfully.
So, we will retry all fragments that fail with 404 HTTP error for now.

youtube_dl/downloader/dash.py

@@ -4,6 +4,7 @@ import os
 import re
 
 from .fragment import FragmentFD
+from ..compat import compat_urllib_error
 from ..utils import (
     sanitize_open,
     encodeFilename,
@@ -36,20 +37,41 @@ class DashSegmentsFD(FragmentFD):
 
         segments_filenames = []
 
-        def append_url_to_file(target_url, target_filename):
-            success = ctx['dl'].download(target_filename, {'url': combine_url(base_url, target_url)})
-            if not success:
+        fragment_retries = self.params.get('fragment_retries', 0)
+
+        def append_url_to_file(target_url, tmp_filename, segment_name):
+            target_filename = '%s-%s' % (tmp_filename, segment_name)
+            count = 0
+            while count <= fragment_retries:
+                try:
+                    success = ctx['dl'].download(target_filename, {'url': combine_url(base_url, target_url)})
+                    if not success:
+                        return False
+                    down, target_sanitized = sanitize_open(target_filename, 'rb')
+                    ctx['dest_stream'].write(down.read())
+                    down.close()
+                    segments_filenames.append(target_sanitized)
+                    break
+                except (compat_urllib_error.HTTPError, ) as err:
+                    # YouTube may often return 404 HTTP error for a fragment causing the
+                    # whole download to fail. However if the same fragment is immediately
+                    # retried with the same request data this usually succeeds (1-2 attemps
+                    # is usually enough) thus allowing to download the whole file successfully.
+                    # So, we will retry all fragments that fail with 404 HTTP error for now.
+                    if err.code != 404:
+                        raise
+                    # Retry fragment
+                    count += 1
+                    if count <= fragment_retries:
+                        self.report_retry_fragment(segment_name, count, fragment_retries)
+            if count > fragment_retries:
+                self.report_error('giving up after %s fragment retries' % fragment_retries)
                 return False
-            down, target_sanitized = sanitize_open(target_filename, 'rb')
-            ctx['dest_stream'].write(down.read())
-            down.close()
-            segments_filenames.append(target_sanitized)
 
         if initialization_url:
-            append_url_to_file(initialization_url, ctx['tmpfilename'] + '-Init')
+            append_url_to_file(initialization_url, ctx['tmpfilename'], 'Init')
         for i, segment_url in enumerate(segment_urls):
-            segment_filename = '%s-Seg%d' % (ctx['tmpfilename'], i)
-            append_url_to_file(segment_url, segment_filename)
+            append_url_to_file(segment_url, ctx['tmpfilename'], 'Seg%d' % i)
 
         self._finish_frag_download(ctx)