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.
Set up repo
$ cat << EOF >> $HGRCPATH
> [ui]
> origbackuppath=.hg/origbackups
> [merge]
> checkunknown=warn
> EOF
$ hg init repo
$ cd repo
$ echo base > base
$ hg add base
$ hg commit -m "base"
Make a dir named b that contains a file, and a file named d
$ mkdir -p b
$ echo c1 > b/c
$ echo d1 > d
$ hg add b/c d
$ hg commit -m "c1"
$ hg bookmark c1
Peform an update that causes b/c to be backed up
$ hg up -q 0
$ mkdir -p b
$ echo c2 > b/c
$ hg up --verbose c1
resolving manifests
b/c: replacing untracked file
getting b/c
creating directory: $TESTTMP/repo/.hg/origbackups/b
getting d
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
(activating bookmark c1)
$ test -f .hg/origbackups/b/c
Make files named b and d
$ hg up -q 0
$ echo b1 > b
$ echo d2 > d
$ hg add b d
$ hg commit -m b1
created new head
$ hg bookmark b1
Perform an update that causes b to be backed up - it should replace the backup b dir
$ hg up -q 0
$ echo b2 > b
$ hg up --verbose b1
resolving manifests
b: replacing untracked file
getting b
removing conflicting directory: $TESTTMP/repo/.hg/origbackups/b
getting d
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
(activating bookmark b1)
$ test -f .hg/origbackups/b
Perform an update the causes b/c to be backed up again - it should replace the backup b file
$ hg up -q 0
$ mkdir b
$ echo c3 > b/c
$ hg up --verbose c1
resolving manifests
b/c: replacing untracked file
getting b/c
creating directory: $TESTTMP/repo/.hg/origbackups/b
removing conflicting file: $TESTTMP/repo/.hg/origbackups/b
getting d
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
(activating bookmark c1)
$ test -d .hg/origbackups/b
Cause two symlinks to be backed up that points to a valid location from the backup dir
$ hg up -q 0
$ mkdir ../sym-link-target
#if symlink
$ ln -s ../../../sym-link-target b
$ ln -s ../../../sym-link-target d
#else
$ touch b d
#endif
$ hg up b1
b: replacing untracked file
d: replacing untracked file
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
(activating bookmark b1)
#if symlink
$ readlink.py .hg/origbackups/b
.hg/origbackups/b -> ../../../sym-link-target
#endif
Perform an update that causes b/c and d to be backed up again - b/c should not go into the target dir
$ hg up -q 0
$ mkdir b
$ echo c4 > b/c
$ echo d3 > d
$ hg up --verbose c1
resolving manifests
b/c: replacing untracked file
d: replacing untracked file
getting b/c
creating directory: $TESTTMP/repo/.hg/origbackups/b
removing conflicting file: $TESTTMP/repo/.hg/origbackups/b
getting d
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
(activating bookmark c1)
$ cat .hg/origbackups/b/c
c4
$ cat .hg/origbackups/d
d3
$ ls ../sym-link-target
Incorrectly configure origbackuppath to be under a file
$ echo data > .hg/badorigbackups
$ hg up -q 0
$ mkdir b
$ echo c5 > b/c
$ hg up --verbose c1 --config ui.origbackuppath=.hg/badorigbackups
resolving manifests
b/c: replacing untracked file
getting b/c
creating directory: $TESTTMP/repo/.hg/badorigbackups/b
abort: $ENOTDIR$: *$TESTTMP/repo/.hg/badorigbackups/b* (glob)
[255]
$ cat .hg/badorigbackups
data