Use atomic mkdir rather than pidof

[raphaelcohn]

mkdir -m 0700 folder is atomic (with -p it is not).

Thus one can use it to make singleton instances of processes, eg:-

singleton_instance_lock()
{
local temporary_folder_path=...
local program_name=...
	single_instance_lock_folder_path=''

	local potential_single_instance_lock_folder_path="$temporary_folder_path"/"$program_name"
	local lock_holder_pid_file_path="$potential_single_instance_lock_folder_path"/lock_holder.pid
	local loop_count=0
	
	local exit_code
	while true
	do
		set +e
			mkdir -m 0700 "$potential_single_instance_lock_folder_path" 1>/dev/null 2>/dev/null
			exit_code=$?
		set -e
		
		if [ $exit_code -eq 0 ]; then
			printf '%s\n' $$ >"$lock_holder_pid_file_path"
			single_instance_lock_folder_path="$potential_single_instance_lock_folder_path"
			break
		fi
		
		local loop_division=$((loop_count % 5))
		if [ $loop_division -eq 0 ]; then
			local lock_holder_pid=''
			set +e
# Uses cat rather than $(<X) as the later echos failures to stderr irrespective.
				lock_holder_pid="$(cat "$lock_holder_pid_file_path" 2>/dev/null)"
			set -e
			
			if [ -z "$lock_holder_pid" ]; then
				lock_holder_pid='(unknown)'
			fi
			
			printf '%s:%s\n' "$_program_name" "Still waiting for single instance lock $potential_single_instance_lock_folder_path held by process $lock_holder_pid"
		fi
		loop_count=$((loop_count + 1))
		
		# Non-portable fractional sleep
		set +e
			sleep 0.1 1>/dev/null 2>/dev/null
			exit_code=$?
		set -e
		
		if [ $exit_code -ne 0 ]; then
			sleep 1 1>/dev/null 2>/dev/null
		fi
		
	done
	
	remove_single_instance_lock_folder_path()
	{
		if [ -n "$single_instance_lock_folder_path" ]; then
			rm -rf "$single_instance_lock_folder_path"
			single_instance_lock_folder_path=''
		fi
	}
	
	trap remove_single_instance_lock_folder_path EXIT
}