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
# Filter output by pyflakes to control which warnings we check
from __future__ import absolute_import, print_function
import re
import sys
lines = []
for line in sys.stdin:
# We blacklist tests that are too noisy for us
pats = [
r"undefined name 'WindowsError'",
r"redefinition of unused '[^']+' from line",
# for cffi, allow re-exports from pure.*
r"cffi/[^:]*:.*\bimport \*' used",
r"cffi/[^:]*:.*\*' imported but unused",
]
keep = True
for pat in pats:
if re.search(pat, line):
keep = False
break # pattern matches
if keep:
fn = line.split(':', 1)[0]
f = open(fn)
data = f.read()
f.close()
if 'no-' 'check-code' in data:
continue
lines.append(line)
for line in lines:
sys.stdout.write(line)
print()