posix: always seek to EOF when opening a file in append mode Python 3 already does this, so skip it there. Consider the program: #include <stdio.h> int main() { FILE *f = fopen("narf", "w"); fprintf(f, "narf\n"); fclose(f); f = fopen("narf", "a"); printf("%ld\n", ftell(f)); fprintf(f, "troz\n"); printf("%ld\n", ftell(f)); return 0; } on macOS, FreeBSD, and Linux with glibc, this program prints 5 10 but on musl libc (Alpine Linux and probably others) this prints 0 10 By my reading of https://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html this is technically correct, specifically: > Opening a file with append mode (a as the first character in the > mode argument) shall cause all subsequent writes to the file to be > forced to the then current end-of-file, regardless of intervening > calls to fseek(). in other words, the file position doesn't really matter in append-mode files, and we can't depend on it being at all meaningful unless we perform a seek() before tell() after open(..., 'a'). Experimentally after a .write() we can do a .tell() and it'll always be reasonable, but I'm unclear from reading the specification if that's a smart thing to rely on. This matches what we do on Windows and what Python 3 does for free, so let's just be consistent. Thanks to Yuya for the idea.

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from __future__ import absolute_import

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from __future__ import print_function

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import unittest

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from mercurial import (

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    mdiff,

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)

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class splitnewlinesTests(unittest.TestCase):

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    def test_splitnewlines(self):

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        cases = {b'a\nb\nc\n': [b'a\n', b'b\n', b'c\n'],

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                 b'a\nb\nc': [b'a\n', b'b\n', b'c'],

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                 b'a\nb\nc\n\n': [b'a\n', b'b\n', b'c\n', b'\n'],

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                 b'': [],

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                 b'abcabc': [b'abcabc'],

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                 }

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        for inp, want in cases.items():

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            self.assertEqual(mdiff.splitnewlines(inp), want)

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if __name__ == '__main__':

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    import silenttestrunner

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    silenttestrunner.main(__name__)