Stop throwing if saving a request fails

Saving a cached request may fail for any number of reasons:
* Running out of memory
* Running out of disk space
* Other IO-related errors

Since we have already made the request and received the data we might
as well use it instead of dropping it on the floor just because we can't
save it to satisfy future requests.

cachecontrol/filewrapper.py

@@ -3,6 +3,10 @@
 # SPDX-License-Identifier: Apache-2.0
 
 from io import BytesIO
+import logging
+
+
+logger = logging.getLogger(__name__)
 
 
 class CallbackFileWrapper(object):
@@ -51,9 +55,26 @@ def __is_fp_closed(self):
         # TODO: Add some logging here...
         return False
 
+    def __call_callback(self):
+        if self.__callback is not None and self.__buf is not None:
+            try:
+                self.__callback(self.__buf.getvalue())
+            except Exception as e:
+                self.__callback = None
+                self.__buf = None
+                logger.exception('Failed to save a cached request', exc_info=True)
+
+    def __append(self, data):
+        if self.__callback is not None and self.__buf is not None:
+            try:
+                self.__buf.write(data)
+            except Exception as e:
+                self.__callback = None
+                self.__buf = None
+                logger.exception('Failed to save a cached request', exc_info=True)
+
     def _close(self):
-        if self.__callback:
-            self.__callback(self.__buf.getvalue())
+        self.__call_callback()
 
         # We assign this to None here, because otherwise we can get into
         # really tricky problems where the CPython interpreter dead locks
@@ -64,7 +85,7 @@ def _close(self):
 
     def read(self, amt=None):
         data = self.__fp.read(amt)
-        self.__buf.write(data)
+        self.__append(data)
         if self.__is_fp_closed():
             self._close()
 
@@ -77,7 +98,7 @@ def _safe_read(self, amt):
             # of the chunk.
             return data
 
-        self.__buf.write(data)
+        self.__append(data)
         if self.__is_fp_closed():
             self._close()