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.

     1

  $ hg init

     2

  $ touch unknown

     3

     4

  $ touch a

     5

  $ hg add a

     6

  $ hg ci -m "1"

     7

     8

  $ touch b

     9

  $ hg add b

    10

  $ hg ci -m "2"

    11

    12

Should show unknown

    13

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  $ hg status

    15

  ? unknown

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  $ hg revert -r 0 --all

    17

  removing b

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Should show unknown and b removed

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  $ hg status

    22

  R b

    23

  ? unknown

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Should show a and unknown

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  $ ls

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  a

    29

  unknown