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.
[package]
name = "hg-cpython"
version = "0.1.0"
authors = ["Georges Racinet <gracinet@anybox.fr>"]
[lib]
name='rusthg'
crate-type = ["cdylib"]
[features]
default = ["python27"]
python27 = ["cpython/python27-sys",
"cpython/extension-module-2-7",
"python27-sys",
]
python3 = ["python3-sys", "cpython/python3-sys", "cpython/extension-module"]
[dependencies]
hg-core = { path = "../hg-core" }
libc = '*'
[dependencies.cpython]
version = "*"
default-features = false
[dependencies.python27-sys]
version = "0.2.1"
optional = true
[dependencies.python3-sys]
version = "0.2.1"
optional = true