Message10496

Originally found while investigating unexpected Jython behaviour in urllib3 issues #743[0] and #747[1].

On CPython and PyPy, when using a zlib.decompressobj, each call to decompress passes Z_SYNC_FLUSH to zlib. This means that, in practice, the call to flush() is a no-op on CPython and PyPy when decompressing zlib compressed streams. This has gone so far that the CPython devs have considered deprecating decompressobj.flush[2].

However, this behaviour is not matched by Jython. Jython uses the standard JDK java.util.zip package to wrap zlib. Specifically, Jython calls the inflate() method with arity 3 as shown here[3], or with arity 1 as shown here[4]. The problem is that these specific overrides of deflate() set the flush mode to Z_NO_FLUSH, as shown in the documentation for JDK 8 here[5] and here[6]. This causes the zlib library to behave very differently for CPython and PyPy.

Sadly, I don't know zlib well enough to know if this matters: it may be that the net effect is totally identical across Jython and CPython and PyPy, but without passing the same flush parameter in all three cases it's hard to be sure.

[0]: https://github.com/shazow/urllib3/issues/743
[1]: https://github.com/shazow/urllib3/pull/747
[2]: https://bugs.python.org/issue23200
[3]: https://bitbucket.org/jython/jython/src/6855fa289d48cf741a3daed2a0bb9c9c87cdf07f/Lib/zlib.py?at=default&fileviewer=file-view-default#zlib.py-200
[4]: https://bitbucket.org/jython/jython/src/6855fa289d48cf741a3daed2a0bb9c9c87cdf07f/Lib/zlib.py?at=default&fileviewer=file-view-default#zlib.py-202
[5]: https://docs.oracle.com/javase/8/docs/api/java/util/zip/Deflater.html#deflate-byte:A-int-int-
[6]: https://docs.oracle.com/javase/8/docs/api/java/util/zip/Deflater.html#deflate-byte:A-