worker: manually buffer reads from pickle stream My previous fix (D8051, cb52e619c99e, which added Python's built-in buffering to the pickle stream) has the problem that the selector will ignore the buffer. When multiple pickled objects are read from the pipe into the buffer at once, only one object will be loaded. This can repeat until the buffer is full and delays the processing of completed items until the worker exits, at which point the pipe is always considered readable and all remaining items are processed. This changeset reverts D8051, removing the buffer again. Instead, on Python 3 only, we use a wrapper to modify the "read" provided to the Unpickler to behave more like a buffered read. We never read more bytes from the pipe than the Unpickler requests, so the selector behaves as expected. Also add a test case for "pickle data was truncated" issue. https://phab.mercurial-scm.org/D8051#119193 Differential Revision: https://phab.mercurial-scm.org/D8076