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.
#!/usr/bin/env python
from __future__ import absolute_import, print_function
import sys
from mercurial import (
commands,
localrepo,
ui as uimod,
)
print_ = print
def print(*args, **kwargs):
"""print() wrapper that flushes stdout buffers to avoid py3 buffer issues
We could also just write directly to sys.stdout.buffer the way the
ui object will, but this was easier for porting the test.
"""
print_(*args, **kwargs)
sys.stdout.flush()
u = uimod.ui.load()
print('% creating repo')
repo = localrepo.instance(u, b'.', create=True)
f = open('test.py', 'w')
try:
f.write('foo\n')
finally:
f.close
print('% add and commit')
commands.add(u, repo, b'test.py')
commands.commit(u, repo, message=b'*')
commands.status(u, repo, clean=True)
print('% change')
f = open('test.py', 'w')
try:
f.write('bar\n')
finally:
f.close()
# this would return clean instead of changed before the fix
commands.status(u, repo, clean=True, modified=True)