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.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo qqq>qqq.txt
rollback dry run without rollback information
$ hg rollback
no rollback information available
[1]
add file
$ hg add
adding qqq.txt
commit first revision
$ hg ci -m 1
set bookmark
$ hg book test
$ echo www>>qqq.txt
commit second revision
$ hg ci -m 2
set bookmark
$ hg book test2
update to -2 (deactivates the active bookmark)
$ hg update -r -2
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(leaving bookmark test2)
$ echo eee>>qqq.txt
commit new head
$ hg ci -m 3
created new head
bookmarks updated?
$ hg book
test 1:25e1ee7a0081
test2 1:25e1ee7a0081
strip to revision 1
$ hg strip 1
saved backup bundle to $TESTTMP/.hg/strip-backup/*-backup.hg (glob)
list bookmarks
$ hg book
test 0:5c9ad3787638
test2 0:5c9ad3787638