[core] fix "read" not awaited warning in ContentDecodePolicy for async responses

sdk/core/azure-core/CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Bugs Fixed
 
 - respect text encoding specified in argument (thanks to @ryohji for the contribution)  #20796
+- Fix "coroutine x.read() was never awaited" warning from `ContentDecodePolicy`  #21318
 - fix type check for `data` input to `azure.core.rest` for python 2.7 users  #21341
 
 ### Other Changes

sdk/core/azure-core/azure/core/pipeline/policies/_universal.py

@@ -28,6 +28,7 @@
 """
 from __future__ import absolute_import  # we have a "requests" module that conflicts with "requests" on Py2.7
 import json
+import inspect
 import logging
 import os
 import platform
@@ -618,10 +619,17 @@ def deserialize_from_http_generics(
 
         # Rely on transport implementation to give me "text()" decoded correctly
         if hasattr(response, "read"):
-            # since users can call deserialize_from_http_generics by themselves
-            # we want to make sure our new responses are read before we try to
-            # deserialize
-            response.read()
+            try:
+                # since users can call deserialize_from_http_generics by themselves
+                # we want to make sure our new responses are read before we try to
+                # deserialize. Only read sync responses since we're in a sync function
+                if not inspect.iscoroutinefunction(response.read):
+                    response.read()
+            except AttributeError:
+                # raises an AttributeError in 2.7 bc inspect.iscoroutinefunction was added in 3.5
+                # Entering here means it's 2.7 and that the response has a read method, so we read
+                # bc it will be sync.
+                response.read()
         return cls.deserialize_from_text(response.text(encoding), mime_type, response=response)
 
     def on_request(self, request):