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-py3-compat - check Python 3 compatibility of Mercurial files
#
# Copyright 2015 Gregory Szorc <gregory.szorc@gmail.com>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
from __future__ import absolute_import, print_function
import ast
import importlib
import os
import sys
import traceback
import warnings
def check_compat_py2(f):
"""Check Python 3 compatibility for a file with Python 2"""
with open(f, 'rb') as fh:
content = fh.read()
root = ast.parse(content)
# Ignore empty files.
if not root.body:
return
futures = set()
haveprint = False
for node in ast.walk(root):
if isinstance(node, ast.ImportFrom):
if node.module == '__future__':
futures |= set(n.name for n in node.names)
elif isinstance(node, ast.Print):
haveprint = True
if 'absolute_import' not in futures:
print('%s not using absolute_import' % f)
if haveprint and 'print_function' not in futures:
print('%s requires print_function' % f)
def check_compat_py3(f):
"""Check Python 3 compatibility of a file with Python 3."""
with open(f, 'rb') as fh:
content = fh.read()
try:
ast.parse(content, filename=f)
except SyntaxError as e:
print('%s: invalid syntax: %s' % (f, e))
return
# Try to import the module.
# For now we only support modules in packages because figuring out module
# paths for things not in a package can be confusing.
if (f.startswith(('hgdemandimport/', 'hgext/', 'mercurial/'))
and not f.endswith('__init__.py')):
assert f.endswith('.py')
name = f.replace('/', '.')[:-3]
try:
importlib.import_module(name)
except Exception as e:
exc_type, exc_value, tb = sys.exc_info()
# We walk the stack and ignore frames from our custom importer,
# import mechanisms, and stdlib modules. This kinda/sorta
# emulates CPython behavior in import.c while also attempting
# to pin blame on a Mercurial file.
for frame in reversed(traceback.extract_tb(tb)):
if frame.name == '_call_with_frames_removed':
continue
if 'importlib' in frame.filename:
continue
if 'mercurial/__init__.py' in frame.filename:
continue
if frame.filename.startswith(sys.prefix):
continue
break
if frame.filename:
filename = os.path.basename(frame.filename)
print('%s: error importing: <%s> %s (error at %s:%d)' % (
f, type(e).__name__, e, filename, frame.lineno))
else:
print('%s: error importing module: <%s> %s (line %d)' % (
f, type(e).__name__, e, frame.lineno))
if __name__ == '__main__':
if sys.version_info[0] == 2:
fn = check_compat_py2
else:
fn = check_compat_py3
for f in sys.argv[1:]:
with warnings.catch_warnings(record=True) as warns:
fn(f)
for w in warns:
print(warnings.formatwarning(w.message, w.category,
w.filename, w.lineno).rstrip())
sys.exit(0)