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Closed file descriptors
I wasn't sure whether to file this on the /oops section. It is a mistake, and I spent a lot longer chasing the bug than I should have, because it's actually a simple bug. But it isn't a really big conceptual screwup of the type I like to feature in the /oops section. It concerns a program that I'll discuss in detail tomorrow. In the meantime, here's a stripped-down summary, and a stripped-down version of the code:

        my $command = shift;
        for my $file (@ARGV) {
          if ($file =~ /\.gz$/) {
            my $fh;
            unless (open $fh, "<", $file) {
              warn "Couldn't open $file: $!; skipping\n";
              next;
            }
            my $fd = fileno $fh;
            $file = "/proc/self/fd/$fd";
          }
        }

        exec $command, @ARGV;
        die "Couldn't run command '$command': $!\n";

Having written something like this, I then ran a test, which failed:

% z cat foo.gz
cat: /proc/self/fd/3: No such file or directory

You see, after a successful exec, the kernel will automatically close all file descriptors that have the close-on-exec flag set, before the exec'ed image starts running. Perl normally sets the close-on-exec flag on all open files except for standard input, standard output, and standard error. Actually it sets it on all open files whose file descriptor is greater than the value of $^F, but $^F defaults to 2.

So there is an easy fix for the problem: I just set $^F = 100000 at the top of the program. That is not the best solution, but it can be replaced with a better one once the program is working properly. Which I expected it would be:

% z cat foo.gz
cat: /proc/self/fd/3: No such file or directory

Maybe I misspelled /proc/self/fd? No, it is there, and contains the special files that I expected to find.

Maybe $^F did not work the way I thought it did? I checked the manual, but it looked okay.

Nevertheless I put in use Fcntl and used the fcntl function to remove the close-on-exec flags explicitly. The code to do that looks something like this:

    use Fcntl;

    ....

    my $flags = fcntl($fh, F_GETFD, 0);
    fcntl($fh, F_SETFD, $flags & ~FD_CLOEXEC);

% z cat foo.gz
cat: /proc/self/fd/3: No such file or directory

I then wasted a lot of time trying to figure out an easy way to tell if the file descriptor was actually open after the exec call. (The answer turns out to be something like this: perl -MPOSIX=fstat -le 'print "file descriptor 3 is ", fstat(3) ? "open" : "closed"'.) This told me whether the error from cat meant what I thought it meant. It did: descriptor 3 was indeed closed after the exec.

Now your job is to figure out what is wrong. It took me a shockingly long time. No need to email me about it; I have it working now. I expect that you will figure it out faster than I did, but I will also post the answer on the blog tomorrow. Sometime on Friday, 21 March 2008, this link will start working and will point to the answer.

[ Addendum 20080321: I posted the answer. ]

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