kill filter process in case of borg exceptions, fixes #6401

in the finally-block, we wait for the filter process to die. but it only dies
voluntarily if all data was processed by the filter and it terminates due to EOF.

otoh, if borg has thrown an early exception, e.g. "archive already exists",
we need to kill the filter process to bring it to an early end. in that
case, we also do not need to check the filter rc, because we know we killed it.

src/borg/helpers/process.py

@@ -329,6 +329,14 @@ def create_filter_process(cmd, stream, stream_close, inbound=True):
     try:
         yield stream
 
+    except Exception:
+        # something went wrong with processing the stream by borg
+        logger.debug('Exception, killing the filter...')
+        proc.kill()
+        borg_succeeded = False
+        raise
+    else:
+        borg_succeeded = True
     finally:
         if stream_close:
             stream.close()
@@ -339,5 +347,6 @@ def create_filter_process(cmd, stream, stream_close, inbound=True):
             logger.debug('filter cmd exited with code %d', rc)
             if filter_stream_close:
                 filter_stream.close()
-            if rc:
+            if borg_succeeded and rc:
+                # if borg did not succeed, we know that we killed the filter process
                 raise Error('filter %s failed, rc=%d' % (cmd, rc))