bpo-33173: Return Underlying fileobj's Seekability in GzipFile.seekable

[waltaskew]

Does something along these lines seem reasonable?
I ran into an issue where the seekable() method was returning true despite the underlying fileobj not being seekable, which lead to errors when a consumer of the GzipFile used the results of the seekable() method as evidence it was safe to call seek.

https://bugs.python.org/issue33173

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

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[vadmium]

Why not seek(0, SEEK_CUR)? (typical implementation of tell)

[serhiy-storchaka]

Would be nice to add tests.

[serhiy-storchaka]

IOError is an alias of OSError in Python 3.

[waltaskew]

Sorry, had nearly forgotten about this!
I checked bz2 and zipfile to see what they do. They simply return the result of seekable() on their underlying buffer rather than trying to do the clever stuff I was doing with checking if the buffer implemented seek & tell:
https://github.com/python/cpython/blob/master/Lib/zipfile.py#L1019
https://github.com/python/cpython/blob/master/Lib/zipfile.py#L825
https://github.com/python/cpython/blob/master/Lib/bz2.py#L149

I simplified this pull request to mimic that behaviour and added a test.

Also, I have signed the CLA, just hadn't before make this pull request.

[brettcannon]

@waltaskew it looks like there was a hiccup with your CLA and it isn't recorded as such on bugs.python.org. Could you email contributors@python.org and try to track down what's happened?

[csabella]

@waltaskew, I'm going to close this pending the CLA. Once you get it sorted per @brettcannon's request, feel free to re-open this pull request.