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.
'''
Examples of useful python hooks for Mercurial.
'''
from __future__ import absolute_import
from mercurial import (
patch,
util,
)
def diffstat(ui, repo, **kwargs):
'''Example usage:
[hooks]
commit.diffstat = python:/path/to/this/file.py:diffstat
changegroup.diffstat = python:/path/to/this/file.py:diffstat
'''
if kwargs.get('parent2'):
return
node = kwargs['node']
first = repo[node].p1().node()
if 'url' in kwargs:
last = repo.changelog.tip()
else:
last = node
diff = patch.diff(repo, first, last)
ui.write(patch.diffstat(util.iterlines(diff)))