Question 11:

Write a text file problems-with-locking.txt in which you answer the
following questions: What are the limitations of the file locking
system that you have implemented in question 10? Explain how it can
it go wrong. What is the technical term for this limitation?


What are the limitations of the file locking system that you have
implemented in question 10?


Explain how it can it go wrong.

In the following code from Question 10,

if [ -e "$lockfile" ]
then
    echo "lockfile exists"
    exit 1
else
    trap signal_handler INT TERM HUP QUIT
    touch $lockfile
    [ -e $lockfile ] || { echo "$prog: cannot create $lockfile"; exit 1; }
fi

there could be two copies of this process running at the same time.
One executes the code -e "$lockfile" and decides that it is the only
process that accesses the data at one time.  Before the code

touch $lockfile

is executed, the second process executes -e "$lockfile" and it  too
decides it has the lock.  Then both processes execute touch $lockfile,
and then they may write to the same web server data at the same time.

The idea is to limit the number of processes from this script to only
one, but it is possible for two or more to execute at the same time if
they execute -e $lockfile just after another process has checked
for the existence of the lock file, but before the second process has
had the chance to create the lockfile.

What is the technical term for this limitation?

A race condition.