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.
$ cat >> $HGRCPATH <<EOF
> [extensions]
> drawdag=$TESTDIR/drawdag.py
> bruterebase=$TESTDIR/bruterebase.py
> [experimental]
> evolution.createmarkers=True
> evolution.allowunstable=True
> EOF
$ init() {
> N=`expr ${N:-0} + 1`
> cd $TESTTMP && hg init repo$N && cd repo$N
> hg debugdrawdag
> }
Source looks like "N"
$ init <<'EOS'
> C D
> |\|
> A B Z
> EOS
$ hg debugbruterebase 'all()-Z' Z
A: A':Z
B: B':Z
AB: A':Z B':Z
C: ABORT: cannot rebase 3:a35c07e8a2a4 without moving at least one of its parents
AC: A':Z C':A'B
BC: B':Z C':B'A
ABC: A':Z B':Z C':A'B'
D: D':Z
AD: A':Z D':Z
BD: B':Z D':B'
ABD: A':Z B':Z D':B'
CD: ABORT: cannot rebase 3:a35c07e8a2a4 without moving at least one of its parents
ACD: A':Z C':A'B D':Z
BCD: B':Z C':B'A D':B'
ABCD: A':Z B':Z C':A'B' D':B'
Moving backwards
$ init <<'EOS'
> C
> |\
> A B
> |
> Z
> EOS
$ hg debugbruterebase 'all()-Z' Z
B: B':Z
A:
BA: B':Z
C: ABORT: cannot rebase 3:b8d7149b562b without moving at least one of its parents
BC: B':Z C':B'A
AC:
BAC: B':Z C':B'A