scripts: Use stat to check lock claim

Replace the perl locking check with stat(1).  Stat is able to fstat
stdin (file descriptor 0) when passed '-' as an argument.  This is now
used to check $_lockfd.  stat(1) support for '-' was introduced to
coreutils in 2009.

After A releases its lock, script B will return from flock and execute
stat.  Since the lockfile has been removed by A, stat prints an error to
stderr and exits non-zero.  Redirect stderr to /dev/null to avoid
filling /var/log/xen/xen-hotplug.log with "No such file or directory"
messages.

Placing the stat call inside the "if" condition ensures we only check
the stat output when the command completed successfully.

This change removes the only runtime dependency of the xen toolstack on
perl.

Suggested-by: Ian Jackson <ian.jackson@citrix.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/hotplug/Linux/locking.sh

@@ -41,7 +41,7 @@ claim_lock()
     # from chiark-utils, except using flock.  It has the benefit of
     # it being possible to safely remove the lockfile when done.
     # See below for a correctness proof.
-    local rightfile
+    local stat
     while true; do
         eval "exec $_lockfd<>$_lockfile"
         flock -x $_lockfd || return $?
@@ -50,14 +50,24 @@ claim_lock()
         # actually a synthetic symlink in /proc and we aren't
         # guaranteed that our stat(2) won't lose the race with an
         # rm(1) between reading the synthetic link and traversing the
-        # file system to find the inum.  Perl is very fast so use that.
-        rightfile=$( perl -e '
-            open STDIN, "<&'$_lockfd'" or die $!;
-            my $fd_inum = (stat STDIN)[1]; die $! unless defined $fd_inum;
-            my $file_inum = (stat $ARGV[0])[1];
-            print "y\n" if $fd_inum eq $file_inum;
-                             ' "$_lockfile" )
-        if [ x$rightfile = xy ]; then break; fi
+        # file system to find the inum.  stat(1) translates '-' into an
+        # fstat(2) of FD 0.  So we just need to arrange the FDs properly
+        # to get the fstat(2) we need.  stat will output two lines like:
+        # WW.XXX
+        # YY.ZZZ
+        # which need to be separated and compared.
+        if stat=$( stat -L -c '%D.%i' - $_lockfile 0<&$_lockfd 2>/dev/null )
+        then
+            local file_stat
+            local fd_stat
+
+            # match on literal newline
+            fd_stat=${stat%
+*}
+            file_stat=${stat#*
+}
+            if [ "$fd_stat" = "$file_stat" ] ; then break; fi
+        fi
         # Some versions of bash appear to be buggy if the same
         # $_lockfile is opened repeatedly. Close the current fd here.
         eval "exec $_lockfd<&-"