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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#require unix-permissions no-root

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  $ hg init a

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  $ cd a

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  $ echo foo > b

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  $ hg add b

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  $ hg ci -m "b"

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  $ chmod -w .hg/store

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  $ cd ..

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  $ hg clone a b

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  requesting all changes

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  adding changesets

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  adding manifests

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  adding file changes

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  added 1 changesets with 1 changes to 1 files

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  new changesets 97310831fa1a

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  updating to branch default

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  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

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  $ chmod +w a/.hg/store # let test clean up

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  $ cd b

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  $ hg verify

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  checking changesets

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  checking manifests

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  crosschecking files in changesets and manifests

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  checking files

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  checked 1 changesets with 1 changes to 1 files

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  $ cd ..