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.
# debugshell extension
"""a python shell with repo, changelog & manifest objects"""
from __future__ import absolute_import
import code
import mercurial
import sys
from mercurial import (
demandimport,
pycompat,
registrar,
)
cmdtable = {}
command = registrar.command(cmdtable)
def pdb(ui, repo, msg, **opts):
objects = {
'mercurial': mercurial,
'repo': repo,
'cl': repo.changelog,
'mf': repo.manifestlog,
}
code.interact(msg, local=objects)
def ipdb(ui, repo, msg, **opts):
import IPython
cl = repo.changelog
mf = repo.manifestlog
cl, mf # use variables to appease pyflakes
IPython.embed()
@command(b'debugshell|dbsh', [])
def debugshell(ui, repo, **opts):
bannermsg = ("loaded repo : %s\n"
"using source: %s" % (pycompat.sysstr(repo.root),
mercurial.__path__[0]))
pdbmap = {
'pdb' : 'code',
'ipdb' : 'IPython'
}
debugger = ui.config(b"ui", b"debugger")
if not debugger:
debugger = 'pdb'
else:
debugger = pycompat.sysstr(debugger)
# if IPython doesn't exist, fallback to code.interact
try:
with demandimport.deactivated():
__import__(pdbmap[debugger])
except ImportError:
ui.warn((b"%s debugger specified but %s module was not found\n")
% (debugger, pdbmap[debugger]))
debugger = b'pdb'
getattr(sys.modules[__name__], debugger)(ui, repo, bannermsg, **opts)