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.
https://bz.mercurial-scm.org/672
# 0-2-4
# \ \ \
# 1-3-5
#
# rename in #1, content change in #4.
$ hg init
$ touch 1
$ touch 2
$ hg commit -Am init # 0
adding 1
adding 2
$ hg rename 1 1a
$ hg commit -m rename # 1
$ hg co -C 0
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo unrelated >> 2
$ hg ci -m unrelated1 # 2
created new head
$ hg merge --debug 1
searching for copies back to rev 1
unmatched files in other:
1a
all copies found (* = to merge, ! = divergent, % = renamed and deleted):
src: '1' -> dst: '1a'
checking for directory renames
resolving manifests
branchmerge: True, force: False, partial: False
ancestor: 81f4b099af3d, local: c64f439569a9+, remote: c12dcd37c90a
1: other deleted -> r
removing 1
1a: remote created -> g
getting 1a
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg ci -m merge1 # 3
$ hg co -C 2
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo hello >> 1
$ hg ci -m unrelated2 # 4
created new head
$ hg co -C 3
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg merge -y --debug 4
searching for copies back to rev 1
unmatched files in local:
1a
all copies found (* = to merge, ! = divergent, % = renamed and deleted):
src: '1' -> dst: '1a' *
checking for directory renames
resolving manifests
branchmerge: True, force: False, partial: False
ancestor: c64f439569a9, local: e327dca35ac8+, remote: 746e9549ea96
preserving 1a for resolve of 1a
starting 4 threads for background file closing (?)
1a: local copied/moved from 1 -> m (premerge)
picked tool ':merge' for 1a (binary False symlink False changedelete False)
merging 1a and 1 to 1a
my 1a@e327dca35ac8+ other 1@746e9549ea96 ancestor 1@c64f439569a9
premerge successful
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg co -C 4
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg merge -y --debug 3
searching for copies back to rev 1
unmatched files in other:
1a
all copies found (* = to merge, ! = divergent, % = renamed and deleted):
src: '1' -> dst: '1a' *
checking for directory renames
resolving manifests
branchmerge: True, force: False, partial: False
ancestor: c64f439569a9, local: 746e9549ea96+, remote: e327dca35ac8
preserving 1 for resolve of 1a
removing 1
starting 4 threads for background file closing (?)
1a: remote moved from 1 -> m (premerge)
picked tool ':merge' for 1a (binary False symlink False changedelete False)
merging 1 and 1a to 1a
my 1a@746e9549ea96+ other 1a@e327dca35ac8 ancestor 1@c64f439569a9
premerge successful
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)