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.
Test hg log changeset printer external hook
-------------------------------------------
$ cat > $TESTTMP/logexthook.py <<EOF
> from __future__ import absolute_import
> import codecs
> from mercurial import (
> commands,
> logcmdutil,
> repair,
> )
> def brot13(b):
> return codecs.encode(b.decode('utf8'), 'rot-13').encode('utf8')
> def rot13description(self, ctx):
> description = ctx.description().strip().splitlines()[0]
> self.ui.write(b"%s: %s\n" % (brot13(b"summary"),
> brot13(description)))
> def reposetup(ui, repo):
> logcmdutil.changesetprinter._exthook = rot13description
> EOF
Prepare the repository
$ hg init empty
$ cd empty
$ touch ROOT
$ hg commit -A -m "Root" ROOT
$ touch a b c
$ hg commit -A -m "Add A, B, C" a b c
Check the log
$ hg log --config extensions.t=$TESTTMP/logexthook.py
changeset: 1:70fc82b23320
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
fhzznel: Nqq N, O, P
summary: Add A, B, C
changeset: 0:b00443a54871
user: test
date: Thu Jan 01 00:00:00 1970 +0000
fhzznel: Ebbg
summary: Root
Check that exthook is working with graph log too
$ hg log -G --config extensions.t=$TESTTMP/logexthook.py
@ changeset: 1:70fc82b23320
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| fhzznel: Nqq N, O, P
| summary: Add A, B, C
|
o changeset: 0:b00443a54871
user: test
date: Thu Jan 01 00:00:00 1970 +0000
fhzznel: Ebbg
summary: Root