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.
from __future__ import absolute_import, print_function
import os
from mercurial import (
pycompat,
ui as uimod,
)
if pycompat.ispy3:
xrange = range
hgrc = os.environ['HGRCPATH']
f = open(hgrc)
basehgrc = f.read()
f.close()
print(' hgrc settings command line options final result ')
print(' quiet verbo debug quiet verbo debug quiet verbo debug')
for i in xrange(64):
hgrc_quiet = bool(i & 1<<0)
hgrc_verbose = bool(i & 1<<1)
hgrc_debug = bool(i & 1<<2)
cmd_quiet = bool(i & 1<<3)
cmd_verbose = bool(i & 1<<4)
cmd_debug = bool(i & 1<<5)
f = open(hgrc, 'w')
f.write(basehgrc)
f.write('\n[ui]\n')
if hgrc_quiet:
f.write('quiet = True\n')
if hgrc_verbose:
f.write('verbose = True\n')
if hgrc_debug:
f.write('debug = True\n')
f.close()
u = uimod.ui.load()
if cmd_quiet or cmd_debug or cmd_verbose:
u.setconfig(b'ui', b'quiet', pycompat.bytestr(bool(cmd_quiet)))
u.setconfig(b'ui', b'verbose', pycompat.bytestr(bool(cmd_verbose)))
u.setconfig(b'ui', b'debug', pycompat.bytestr(bool(cmd_debug)))
check = ''
if u.debugflag:
if not u.verbose or u.quiet:
check = ' *'
elif u.verbose and u.quiet:
check = ' +'
print(('%2d %5s %5s %5s %5s %5s %5s -> %5s %5s %5s%s'
% (i, hgrc_quiet, hgrc_verbose, hgrc_debug,
cmd_quiet, cmd_verbose, cmd_debug,
u.quiet, u.verbose, u.debugflag, check)))