Add -q option to fsck to check for zero-byte cache files quickly (cvm…

…fs#3726)

cvmfs/cvmfs_config

@@ -145,7 +145,7 @@ cvmfs_config_usage() {
   echo "  stat [-v | <repository>]"
   echo "  status"
   echo "  probe [<repository list>]"
-  echo "  fsck [<cvmfs_fsck options>]"
+  echo "  fsck [-q(uick check for zero byte files only) | <cvmfs_fsck options>]"
   echo "  fuser <repository>"
   echo "  reload [-c | <repository>]"
   echo "  umount"
@@ -1294,9 +1294,19 @@ cvmfs_config_fsck() {
       continue
     fi
 
-    echo "CernVM-FS fsck on $dir"
-    cvmfs_fsck $@ $dir
-    retval=$(($? + $retval))
+    if [ "x$1" = "x-q" ]; then
+      # this hardcodes the hash for the (legitimately) empty file - if anything about compression or hashing changes,
+      # this needs to be updated
+      for zeroed_file in $(find $dir -empty -wholename "$dir/[a-z0-9][a-z0-9]/*" ! -name ec3d88b62ebf526e4e5a4ff6162a3aa48a6b78); do
+        echo "WARNING: unexpected zero-byte file $zeroed_file -  Moving it to quarantaine."
+        mv $zeroed_file $dir/quarantaine
+        retval=1
+      done
+    else
+      echo "CernVM-FS fsck on $dir"
+      cvmfs_fsck $@ $dir
+      retval=$(($? + $retval))
+    fi
   done
 
   return $retval