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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# showstack.py - extension to dump a Python stack trace on signal

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#

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# binds to both SIGQUIT (Ctrl-\) and SIGINFO (Ctrl-T on BSDs)

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r"""dump stack trace when receiving SIGQUIT (Ctrl-\) or SIGINFO (Ctrl-T on BSDs)

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"""

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from __future__ import absolute_import, print_function

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import signal

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import sys

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import traceback

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def sigshow(*args):

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    sys.stderr.write("\n")

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    traceback.print_stack(args[1], limit=10, file=sys.stderr)

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    sys.stderr.write("----\n")

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def sigexit(*args):

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    sigshow(*args)

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    print('alarm!')

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    sys.exit(1)

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def extsetup(ui):

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    signal.signal(signal.SIGQUIT, sigshow)

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    signal.signal(signal.SIGALRM, sigexit)

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    try:

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        signal.signal(signal.SIGINFO, sigshow)

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    except AttributeError:

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        pass