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.
#require no-windows
Dummy extension simulating unsafe long running command
$ cat > sleepext.py <<EOF
> import itertools
> import time
>
> from mercurial.i18n import _
> from mercurial import registrar
>
> cmdtable = {}
> command = registrar.command(cmdtable)
>
> @command(b'sleep', [], _(b'TIME'), norepo=True)
> def sleep(ui, sleeptime=b"1", **opts):
> with ui.uninterruptible():
> for _i in itertools.repeat(None, int(sleeptime)):
> time.sleep(1)
> ui.warn(b"end of unsafe operation\n")
> ui.warn(b"%s second(s) passed\n" % sleeptime)
> EOF
Kludge to emulate timeout(1) which is not generally available.
$ cat > timeout.py <<EOF
> from __future__ import print_function
> import argparse
> import signal
> import subprocess
> import sys
> import time
>
> ap = argparse.ArgumentParser()
> ap.add_argument('-s', nargs=1, default='SIGTERM')
> ap.add_argument('duration', nargs=1, type=int)
> ap.add_argument('argv', nargs='*')
> opts = ap.parse_args()
> try:
> sig = int(opts.s[0])
> except ValueError:
> sname = opts.s[0]
> if not sname.startswith('SIG'):
> sname = 'SIG' + sname
> sig = getattr(signal, sname)
> proc = subprocess.Popen(opts.argv)
> time.sleep(opts.duration[0])
> proc.poll()
> if proc.returncode is None:
> proc.send_signal(sig)
> proc.wait()
> sys.exit(124)
> EOF
Set up repository
$ hg init repo
$ cd repo
$ cat >> $HGRCPATH << EOF
> [extensions]
> sleepext = ../sleepext.py
> EOF
Test ctrl-c
$ python $TESTTMP/timeout.py -s INT 1 hg sleep 2
interrupted!
[124]
$ cat >> $HGRCPATH << EOF
> [experimental]
> nointerrupt = yes
> EOF
$ python $TESTTMP/timeout.py -s INT 1 hg sleep 2
interrupted!
[124]
$ cat >> $HGRCPATH << EOF
> [experimental]
> nointerrupt-interactiveonly = False
> EOF
$ python $TESTTMP/timeout.py -s INT 1 hg sleep 2
shutting down cleanly
press ^C again to terminate immediately (dangerous)
end of unsafe operation
interrupted!
[124]