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[Xen-devel] [PATCH] 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 from A, stat prints an error
to stderr and exits non-zero.  '|| :' is needed to squash the non-zero
exit status - otherwise the script terminates since `set -e` is enabled.
stderr needs to be redirected to /dev/null otherwise
/var/log/xen/xen-hotplug.log will get filled with "No such file or
directory" messages.

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

While here, replace some tabs with spaces to match the rest of the file.

Suggested-by: Ian Jackson <ian.jackson@xxxxxxxxxx>
Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>
 tools/hotplug/Linux/locking.sh | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/tools/hotplug/Linux/locking.sh b/tools/hotplug/Linux/locking.sh
index c6a7e96ff9..03361f405f 100644
--- a/tools/hotplug/Linux/locking.sh
+++ b/tools/hotplug/Linux/locking.sh
@@ -41,7 +41,9 @@ 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
+    local fd_stat
+    local file_stat
     while true; do
         eval "exec $_lockfd<>$_lockfile"
         flock -x $_lockfd || return $?
@@ -50,16 +52,20 @@ claim_lock()
         # actually a synthetic symlink in /proc and we aren't
         # guaranteed that our stat(2) won't lose the race with an
         # rm(1) between reading the synthetic link and traversing the
-        # file system to find the inum.  Perl is very fast so use that.
-        rightfile=$( 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
-       # Some versions of bash appear to be buggy if the same
-       # $_lockfile is opened repeatedly. Close the current fd here.
+        # file system to find the inum.  stat(1) translates '-' into an
+        # fstat(2) of FD 0.  So we just need to arrange the FDs properly
+        # to get the fstat(2) we need.  stat will output two lines like:
+        # WW.XXX
+        # YY.ZZZ
+        # which need to be separated and compared.
+        stat=$( stat -L -c '%D.%i' - $_lockfile 0<&$_lockfd 2>/dev/null || : )
+        if [ -n "$stat" ]; then
+            fd_stat=$( echo "$stat" | sed -n '1p' )
+            file_stat=$( echo "$stat" | sed -n '2p' )
+            if [ "$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<&-"

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