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.
# Helper module to use the Hypothesis tool in tests
#
# Copyright 2015 David R. MacIver
#
# For details see http://hypothesis.readthedocs.org
from __future__ import absolute_import, print_function
import os
import sys
import traceback
try:
# hypothesis 2.x
from hypothesis.configuration import set_hypothesis_home_dir
from hypothesis import settings
except ImportError:
# hypothesis 1.x
from hypothesis.settings import set_hypothesis_home_dir
from hypothesis import Settings as settings
import hypothesis.strategies as st
from hypothesis import given
# hypothesis store data regarding generate example and code
set_hypothesis_home_dir(os.path.join(
os.getenv('TESTTMP'), ".hypothesis"
))
def check(*args, **kwargs):
"""decorator to make a function a hypothesis test
Decorated function are run immediately (to be used doctest style)"""
def accept(f):
# Workaround for https://github.com/DRMacIver/hypothesis/issues/206
# Fixed in version 1.13 (released 2015 october 29th)
f.__module__ = '__anon__'
try:
with settings(max_examples=2000):
given(*args, **kwargs)(f)()
except Exception:
traceback.print_exc(file=sys.stdout)
sys.exit(1)
return accept
def roundtrips(data, decode, encode):
"""helper to tests function that must do proper encode/decode roundtripping
"""
@given(data)
def testroundtrips(value):
encoded = encode(value)
decoded = decode(encoded)
if decoded != value:
raise ValueError(
"Round trip failed: %s(%r) -> %s(%r) -> %r" % (
encode.__name__, value, decode.__name__, encoded,
decoded
))
try:
testroundtrips()
except Exception:
# heredoc swallow traceback, we work around it
traceback.print_exc(file=sys.stdout)
raise
print("Round trip OK")
# strategy for generating bytestring that might be an issue for Mercurial
bytestrings = (
st.builds(lambda s, e: s.encode(e), st.text(), st.sampled_from([
'utf-8', 'utf-16',
]))) | st.binary()