gfile.Copy does not overwrite dest file properly on posix filesystems

[tsawada]

Describe the current behavior
gfile.Copy(overwrite=True) does not truncate the destination file before overwriting. That means if the src file is shorter than the dest file, the resulting dest file contains the mix of the two.

Describe the expected behavior
gfile.Copy(overwrite=True) results in having the exact same content of src file in the dest file.

Code to reproduce the issue

$ echo 'aaa' > a.txt
$ echo 'bbbbbb' > b.txt
$ python3 -c "from tensorflow import gfile; gfile.Copy('a.txt', 'b.txt', overwrite=True)"
$ cat b.txt
aaa
bb

Tested with pip3 install tensorflow==1.13.1, python 3.5.2
b.txt should have aaa as the content, not aaa\nbb.

Ref. https://www.tensorflow.org/api_docs/python/tf/gfile/Copy

[tsawada]

CC @Joeper214 who found this issue

[muddham]

@tsawada From the above referenced issue, can see a PR was raised, Can you please check if this is resolved after this change got merged.

[tsawada]

I built latest master 8a8a109e and my o_trunc branch within tensorflow/tensorflow:devel docker container, and confirmed that the problem reproduces on the master, and not on o_trunc.

[mihaimaruseac]

Oh, the PR fixing this hasn't been yet merged (due to my commit causing a merge conflict, sorry about that)

[tensorflow-bot]

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