api_views: use loop to prevent NFS errors on GET (#41379)

File can disapear more frequently on NFS as open() is not atomic (NFS filehandle
refers to an inode but they don't lock it like an open file descriptor).

petale/api_views.py

@@ -21,8 +21,11 @@ try:
     from functools import reduce
 except ImportError:
     pass
-
 import requests
+try:
+    from time import process_time
+except ImportError:
+    from time import clock as process_time
 
 from django.utils.six.moves.urllib import parse as urlparse
 
@@ -180,30 +183,32 @@ class PetalAPIView(APIView):
 
     @logit
     def get(self, request, partner_name, cut_uuid, petal_name):
-        logger = logging.getLogger(__name__)
         if_none_match = request.META.get('HTTP_IF_NONE_MATCH')
         if_none_match = if_none_match and [x.strip() for x in if_none_match.split(',')]
 
-        petal = self.get_petal(partner_name, cut_uuid, petal_name)
-        if if_none_match:
-            if if_none_match == ['*'] or petal.etag in if_none_match:
-                return Response(
-                    status=status.HTTP_304_NOT_MODIFIED,
-                    headers={
-                        'ETag': petal.etag
-                    }
-                )
-        # verify file exists before creating a StreamingHttpResponse
-        # as StreamingHttpResponse generate its content after the Django global try/catch
-        try:
-            petal.data.open()
-        except IOError as e:
-            logger.error('file not found "%s": %s', petal.data.path, e)
-            return HttpResponse('missing file', status=500)
-        response = StreamingHttpResponse(
-            petal.data.chunks(),
-            content_type=petal.content_type,
-        )
+        start = process_time()
+        while process_time() - start < 10:
+            petal = self.get_petal(partner_name, cut_uuid, petal_name)
+            if if_none_match:
+                if if_none_match == ['*'] or petal.etag in if_none_match:
+                    return Response(
+                        status=status.HTTP_304_NOT_MODIFIED,
+                        headers={
+                            'ETag': petal.etag
+                        }
+                    )
+            # verify file exists before creating a StreamingHttpResponse
+            # as StreamingHttpResponse generate its content after the Django global try/catch
+            try:
+                petal.data.open(mode='rb')
+                response = HttpResponse(petal.data.read(), content_type=petal.content_type)
+            except IOError:
+                continue
+            finally:
+                petal.data.close()
+            break
+        else:
+            return HttpResponse(status=500, reason='Too many updates')
         response['ETag'] = petal.etag
         return response