[Xen-changelog] [xen staging] scripts: Use stat to check lock claim

[patchbot@xxxxxxx]

commit b72682c602b8d1aaadca439d49cc79c79dbc17bc
Author:     Jason Andryuk <jandryuk@xxxxxxxxx>
AuthorDate: Thu Mar 12 10:54:17 2020 -0400
Commit:     Wei Liu <wl@xxxxxxx>
CommitDate: Mon Mar 30 14:35:53 2020 +0100

    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@xxxxxxxxxx>
    Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>
    Reviewed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
 tools/hotplug/Linux/locking.sh | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/tools/hotplug/Linux/locking.sh b/tools/hotplug/Linux/locking.sh
index baaaf73508..69aefae6f6 100644
--- a/tools/hotplug/Linux/locking.sh
+++ b/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<&-"
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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