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
#
# check-perf-code - (historical) portability checker for contrib/perf.py
from __future__ import absolute_import
import os
import sys
# write static check patterns here
perfpypats = [
[
(r'(branchmap|repoview|repoviewutil)\.subsettable',
"use getbranchmapsubsettable() for early Mercurial"),
(r'\.(vfs|svfs|opener|sopener)',
"use getvfs()/getsvfs() for early Mercurial"),
(r'ui\.configint',
"use getint() instead of ui.configint() for early Mercurial"),
],
# warnings
[
]
]
def modulewhitelist(names):
replacement = [('.py', ''), ('.c', ''), # trim suffix
('mercurial%s' % ('/'), ''), # trim "mercurial/" path
]
ignored = {'__init__'}
modules = {}
# convert from file name to module name, and count # of appearances
for name in names:
name = name.strip()
for old, new in replacement:
name = name.replace(old, new)
if name not in ignored:
modules[name] = modules.get(name, 0) + 1
# list up module names, which appear multiple times
whitelist = []
for name, count in modules.items():
if count > 1:
whitelist.append(name)
return whitelist
if __name__ == "__main__":
# in this case, it is assumed that result of "hg files" at
# multiple revisions is given via stdin
whitelist = modulewhitelist(sys.stdin)
assert whitelist, "module whitelist is empty"
# build up module whitelist check from file names given at runtime
perfpypats[0].append(
# this matching pattern assumes importing modules from
# "mercurial" package in the current style below, for simplicity
#
# from mercurial import (
# foo,
# bar,
# baz
# )
((r'from mercurial import [(][a-z0-9, \n#]*\n(?! *%s,|^[ #]*\n|[)])'
% ',| *'.join(whitelist)),
"import newer module separately in try clause for early Mercurial"
))
# import contrib/check-code.py as checkcode
assert 'RUNTESTDIR' in os.environ, "use check-perf-code.py in *.t script"
contribpath = os.path.join(os.environ['RUNTESTDIR'], '..', 'contrib')
sys.path.insert(0, contribpath)
checkcode = __import__('check-code')
# register perf.py specific entry with "checks" in check-code.py
checkcode.checks.append(('perf.py', r'contrib/perf.py$', '',
checkcode.pyfilters, perfpypats))
sys.exit(checkcode.main())