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.
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----