shell-locks

#!/bin/bash -efu
# SPDX-License-Identifier: GPL-2.0-or-later

if [ -z "${__included_shell_locks-}" ]; then
__included_shell_locks=1

__shell_fd_lock()
{
	local fd="$1"; shift
	local fn="$1"; shift
	eval "exec $fd<\"$fn\""
	flock "$@" "$fd"
}

### The function opens the file to the specified descriptor and takes a read
### lock.
### Usage: fd_rlock $num $filename
fd_rlock()   { __shell_fd_lock "$1" "$2" -s; }

### The function opens the file to the specified descriptor and takes a write
### lock.
### Usage: fd_lock $num $filename
fd_lock()    { __shell_fd_lock "$1" "$2" -x; }

### The function opens the file to the specified descriptor and takes a write
### lock. If the lock could not be taken, then the function failed.
### Usage: fd_trylock $num $filename
fd_trylock() { __shell_fd_lock "$1" "$2" -n; }

### Closes the descriptor and releases the lock.
### Usage: fd_unlock $fd_num
fd_unlock()  { eval "exec $1<&-"; }

### Checks whether a lock is present on the specified file.
### Usage: fd_is_locked $filename
fd_is_locked()
{
	local fd=0
	while [ -e "/proc/self/fd/$fd" ]; do
		fd=$(( $fd + 1 ))
	done

	if fd_trylock "$fd" "$1"; then
		fd_unlock "$fd"
		return 1
	fi

	eval "exec $fd<&-"
	return 0
}

fi #__included_shell_locks