get_media_requests: support and encourage callback=NO_CALLBACK

scrapy/pipelines/files.py

@@ -22,6 +22,7 @@ from twisted.internet import defer, threads
 
 from scrapy.exceptions import IgnoreRequest, NotConfigured
 from scrapy.http import Request
+from scrapy.http.request import NO_CALLBACK
 from scrapy.pipelines.media import MediaPipeline
 from scrapy.settings import Settings
 from scrapy.utils.boto import is_botocore_available
@@ -517,7 +518,7 @@ class FilesPipeline(MediaPipeline):
     # Overridable Interface
     def get_media_requests(self, item, info):
         urls = ItemAdapter(item).get(self.files_urls_field, [])
-        return [Request(u) for u in urls]
+        return [Request(u, callback=NO_CALLBACK) for u in urls]
 
     def file_downloaded(self, response, request, info, *, item=None):
         path = self.file_path(request, response=response, info=info, item=item)

scrapy/pipelines/images.py

@@ -13,6 +13,7 @@ from itemadapter import ItemAdapter
 
 from scrapy.exceptions import DropItem, NotConfigured, ScrapyDeprecationWarning
 from scrapy.http import Request
+from scrapy.http.request import NO_CALLBACK
 from scrapy.pipelines.files import FileException, FilesPipeline
 
 # TODO: from scrapy.pipelines.media import MediaPipeline
@@ -214,7 +215,7 @@ class ImagesPipeline(FilesPipeline):
 
     def get_media_requests(self, item, info):
         urls = ItemAdapter(item).get(self.images_urls_field, [])
-        return [Request(u) for u in urls]
+        return [Request(u, callback=NO_CALLBACK) for u in urls]
 
     def item_completed(self, results, item, info):
         with suppress(KeyError):

scrapy/pipelines/media.py

@@ -18,6 +18,10 @@ from scrapy.utils.log import failure_to_exc_info
 logger = logging.getLogger(__name__)
 
 
+def _DUMMY_CALLBACK(response):
+    return response
+
+
 class MediaPipeline:
 
     LOG_FAILED_RESULTS = True
@@ -91,7 +95,10 @@ class MediaPipeline:
 
     def _process_request(self, request, info, item):
         fp = self._fingerprinter.fingerprint(request)
-        cb = request.callback or (lambda _: _)
+        if not request.callback or request.callback is NO_CALLBACK:
+            cb = _DUMMY_CALLBACK
+        else:
+            cb = request.callback
         eb = request.errback
         request.callback = NO_CALLBACK
         request.errback = None