Using Flock in Bash so Request is Made Only Once - bash

Question

I'm trying to configure my script in such a way that
If some data isn't available, try to fetch it
If another process is already fetching it, wait for that process to finish
Use the data
And from here I found this very nice example of flock:
exec 200>$pidfile
flock -n 200 || exit 1
pid=$$
echo $pid 1>&200
And this fails if it can't aquire the lock (-n flag).
Can I assume that this means another file has locked the $pidfile, and how can I detect that the lock has been released in a different process?
I understand that wait $pid would wait until that process is complete, and so if there's some way to record which process currently holds the lock or just detect the unlocking so that other processes know once the data is available, then I think this will work.
Any ideas?

Answer 1

As per the flock (1) man page,
if the lock cannot be immediately acquired, [in the absence of a -w timeout], flock waits until the lock is available
You can use fuser to see which process is holding a file handle.

Answer 2

My solution uses two files, pid.temp and data.temp:
backgroundGetData() {
local data=$1
# if global is null, check file.
if [ -z "$data" ]; then
data=$( cat $DATA_TEMP_FILE 2>/dev/null )
fi
# if file is empty, check process is making the request
if [ -z "$data" ]; then
for i in {1..5}; do
echo "INFO - Process: $BASHPID - Attempting to lock data temp file" >&2
local request_pid=$( cat $PID_FILE 2>/dev/null )
if [ -z "$request_pid" ]; then request_pid=0; fi
local exit_code=1
if [ "$request_pid" -eq 0 ]; then
( flock -n 200 || exit 1
echo "INFO - Process: $BASHPID - Fetching data." >&2
echo "$BASHPID">"$PID_FILE"
getData > $DATA_TEMP_FILE
) 200>$DATA_TEMP_FILE
exit_code=$?
fi
echo "INFO - Process: $BASHPID - returned $exit_code from lock attempt.">&2
[ $request_pid -ne 0 ] && echo "INFO - Process: $BASHPID - $request_pid is possibly locking">&2
if [ $exit_code -ne 0 ] && [ $request_pid -ne 0 ]; then
echo "INFO - Process: $BASHPID - waiting on $request_pid to complete">&2
tail --pid=${request_pid} -f /dev/null
echo "INFO - Process: $BASHPID - finished waiting.">&2
break
elif [ $exit_code -eq 0 ]; then break;
else
sleep 2
fi
done
data=$( cat $DATA_TEMP_FILE )
if [ -z "$data" ]; then
echo "WARN - Process: $BASHPID - Failed to retrieve data.">&2
fi
fi
echo "$least_loaded"
}
And it can be used like so:
DATA=""
DATA_TEMP_FILE="data.temp"
PID_FILE="pid.temp"
$( backgroundGetData $DATA ) & ## Begin making request
doThing() {
if [ -z $DATA ]; then
# Redirect 3 to stdout, then use sterr in backgroundGetData to 3 so that
# logging messages can be shown and output can also be captued in variable.
exec 3>&1
DATA=$( backgroundGetData $DATA 2>&3)
fi
}
for job in "$jobs"; do
doThing &
done
It's working for me, though I'm not 100% sure on how safe it is.