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.
#!/bin/sh
# Script to get stable diff output on any platform.
#
# Output of this script is almost equivalent to GNU diff with "-Nru".
#
# Use this script as "hg pdiff" via extdiff extension with preparation
# below in test scripts:
#
# $ cat >> $HGRCPATH <<EOF
# > [extdiff]
# > pdiff = sh "$RUNTESTDIR/pdiff"
# > EOF
filediff(){
# USAGE: filediff file1 file2 [header]
# compare with /dev/null if file doesn't exist (as "-N" option)
file1="$1"
if test ! -f "$file1"; then
file1=/dev/null
fi
file2="$2"
if test ! -f "$file2"; then
file2=/dev/null
fi
if cmp -s "$file1" "$file2" 2> /dev/null; then
# Return immediately, because comparison isn't needed. This
# also avoids redundant message of diff like "No differences
# encountered" (on Solaris)
return
fi
if test -n "$3"; then
# show header only in recursive case
echo "$3"
fi
# replace "/dev/null" by corresponded filename (as "-N" option)
diff -u "$file1" "$file2" |
sed "s@^--- /dev/null\(.*\)\$@--- $1\1@" |
sed "s@^\+\+\+ /dev/null\(.*\)\$@+++ $2\1@"
# in this case, files differ from each other
return 1
}
if test -d "$1" -o -d "$2"; then
# ensure comparison in dictionary order
(
if test -d "$1"; then (cd "$1" && find . -type f); fi
if test -d "$2"; then (cd "$2" && find . -type f); fi
) |
sed 's@^\./@@g' | sort | uniq |
while read file; do
filediff "$1/$file" "$2/$file" "diff -Nru $1/$file $2/$file"
done
# TODO: there is no portable way for current while-read based
# implementation to return 1 at detecting changes.
#
# On bash and dash, assignment to variable inside while-block
# doesn't affect outside, because inside while-block is executed
# in sub-shell. BTW, it affects outside while-block on ksh (as sh
# on Solaris).
else
filediff "$1" "$2"
fi