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Re: [libvirt] consistency: push "update" hook vs. "make syntax-check"



Daniel P. Berrange wrote:
> On Wed, Jul 15, 2009 at 11:51:41PM +0200, Jim Meyering wrote:
>> If it matters, we can come up with a more efficient (yet still portable)
>> way to compare the last two bytes of each file to "\n\n".
>> I went ahead and wrote a nearly-minimal script to do that.
>> Rather than reading/processing all 27MB of sources,
>> this reads just the last 2 bytes of each of the 1048 files,
>> comparing those bytes to "\n\n" and printing the name when
>> there's a match:
>
> I just strace'd the 'tail' program  and that does the right
> thing too

It had better ;-)

> # strace tail -c 2 somefile
> ....
> open("somefile", O_RDONLY|O_LARGEFILE)       = 3
> fstat64(3, {st_mode=S_IFREG|0664, st_size=10600, ...}) = 0
> _llseek(3, 0, [0], SEEK_CUR)            = 0
> _llseek(3, 0, [10600], SEEK_END)        = 0
> _llseek(3, -2, [10598], SEEK_END)       = 0
> read(3, "l\n"..., 2)                    = 2
>
>> git ls-files -z \
>>   | xargs -0 perl -le '
>>   foreach my $f (@ARGV) {
>>     open F,"<",$f or (warn "failed to open $f: $!\n"), next;
>>     my $p = sysseek(F, -2, 2);
>>     # seek failure probably means file has < 2 bytes; ignore
>>     my $two;
>>     defined $p and $p = sysread F,$two,2;
>>     close F;
>>     # ignore read failure
>>     $p && $two eq "\n\n" and (print $f),$fail=1;
>>     } END {exit defined $fail ? 1 : 0}'
>
> So using 'tail' instead of this perl script would be more readable
> and efficient.

tail -c2 might be my choice, too, for a small number of files.
But in our case, using it would incur the cost of 1000+ fork+execs.


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