Mercurial > hg
diff mercurial/posix.py @ 42562:97ada9b8d51b stable 5.0.2
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.
author | Augie Fackler <augie@google.com> |
---|---|
date | Mon, 08 Jul 2019 13:12:20 -0400 |
parents | faa04f45b5fe |
children | 2372284d9457 |
line wrap: on
line diff
--- a/mercurial/posix.py Wed Jul 03 10:06:39 2019 +0800 +++ b/mercurial/posix.py Mon Jul 08 13:12:20 2019 -0400 @@ -30,7 +30,6 @@ osutil = policy.importmod(r'osutil') -posixfile = open normpath = os.path.normpath samestat = os.path.samestat try: @@ -52,6 +51,19 @@ umask = os.umask(0) os.umask(umask) +if not pycompat.ispy3: + def posixfile(name, mode=r'r', buffering=-1): + fp = open(name, mode=mode, buffering=buffering) + # The position when opening in append mode is implementation defined, so + # make it consistent by always seeking to the end. + if r'a' in mode: + fp.seek(0, os.SEEK_END) + return fp +else: + # The underlying file object seeks as required in Python 3: + # https://github.com/python/cpython/blob/v3.7.3/Modules/_io/fileio.c#L474 + posixfile = open + def split(p): '''Same as posixpath.split, but faster