Add functions for operating with file locks

Signed-off-by: Alexey Gladkov <legion@altlinux.org>

docs/shell-locks.md

@@ -0,0 +1,42 @@
+shell-locks(3)
+
+# NAME
+
+fd_rlock, fd_lock, fd_trylock, fd_unlock, fd_is_locked - Functions for operating
+with file locks.
+
+# SYNOPSIS
+
+- fd_rlock $num $filename
+- fd_lock $num $filename
+- fd_trylock $num $filename
+- fd_unlock $fd_num
+- fd_is_locked $filename
+
+# DESCRIPTION
+
+## fd_rlock
+The function opens the file to the specified descriptor and takes a read lock.
+
+## fd_lock
+The function opens the file to the specified descriptor and takes a write lock.
+
+## fd_trylock
+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.
+
+## fd_unlock
+Closes the descriptor and releases the lock.
+
+## fd_is_locked
+Checks whether a lock is present on the specified file.
+
+# ENVIRONMENT
+
+# AUTHOR
+Authors and contributors of the programs included in the *libshell* package are listed
+in the COPYING file.
+
+# BUGS
+Report bugs to the authors.
+

shell-locks

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+#!/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