From: Matthias Kruk <m@m10k.eu>
Date: Sat, 20 Mar 2021 23:55:16 +0000 (+0900)
Subject: include/sem: Add POSIX-like semaphore implementation
X-Git-Url: https://git.corax.cc/?a=commitdiff_plain;h=6400b90ade0dad53333074fc8383f15cbd9c7d72;p=toolbox

include/sem: Add POSIX-like semaphore implementation

In certain situations, mutex are not enough for synchronization. This
commit adds a POSIX-like semaphore implementation that can be used to
implement things like queues or singleton scripts.
---

diff --git a/include/sem.sh b/include/sem.sh
new file mode 100644
index 0000000..1e53ff9
--- /dev/null
+++ b/include/sem.sh
@@ -0,0 +1,226 @@
+#!/bin/bash
+
+#
+# sem - POSIX-like semaphores for bash scripts
+# Copyright (C) 2021 - Matthias Kruk <m@m10k.eu>
+#
+
+__init() {
+	if ! include "mutex"; then
+		return 1
+	fi
+
+	declare -gr __sem_path="$TOOLBOX_HOME/sem"
+
+	return 0
+}
+
+
+_sem_mutexpath() {
+	local sem
+
+	sem="$1"
+
+	echo "$__sem_path/$sem.mutex"
+}
+
+_sem_ownerpath() {
+	local sem
+
+	sem="$1"
+
+	echo "$__sem_path/$sem.owner"
+}
+
+_sem_sempath() {
+	local sem
+
+	sem="$1"
+
+	echo "$__sem_path/$sem"
+}
+
+_sem_inc() {
+	local sem
+	local value
+
+	sem="$1"
+
+	if ! value=$(cat "$sem"); then
+		return 1
+	fi
+
+	((value++))
+
+	if ! echo "$value" > "$sem"; then
+		return 1
+	fi
+
+	return 0
+}
+
+_sem_dec() {
+	local sem
+	local value
+
+	sem="$1"
+
+	if ! value=$(cat "$sem"); then
+		return 1
+	fi
+
+	if (( value == 0 )); then
+		return 1
+	fi
+
+	((value--))
+
+	if ! echo "$value" > "$sem"; then
+		return 1
+	fi
+
+	return 0
+}
+
+sem_init() {
+	local name
+	local value
+
+	local mutex
+	local sem
+	local owner
+	local err
+
+	name="$1"
+	value="$2"
+	err=0
+
+	mutex=$(_sem_mutexpath "$name")
+	sem=$(_sem_sempath "$name")
+	owner=$(_sem_ownerpath "$name")
+
+	if ! [[ "$value" =~ ^[0-9]+$ ]]; then
+		return 1
+	fi
+
+	if ! mkdir -p "$__sem_path"; then
+		return 1
+	fi
+
+	# If the semaphore is new, locking must succeed,
+	# otherwise it was not a new semaphore
+	if ! mutex_trylock "$mutex"; then
+		error "Could not acquire $mutex"
+		return 1
+	fi
+
+	if ! mutex_trylock "$owner"; then
+		error "Could not acquire $mutex"
+		err=1
+	elif ! echo "$value" > "$sem"; then
+		err=1
+	fi
+
+	mutex_unlock "$mutex"
+	return "$err"
+}
+
+sem_destroy() {
+	local name
+
+	local mutex
+	local sem
+	local owner
+
+	name="$1"
+
+	mutex=$(_sem_mutexpath "$name")
+	sem=$(_sem_sempath "$name")
+	owner=$(_sem_ownerpath "$name")
+
+	# Make sure only the owner can destroy the semaphore
+	if ! mutex_unlock "$owner"; then
+		return 1
+	fi
+
+	if ! rm -f "$mutex" "$sem"; then
+		return 1
+	fi
+
+	return 0
+}
+
+sem_wait() {
+	local name
+
+	local mutex
+	local sem
+	local passed
+
+	name="$1"
+
+	mutex=$(_sem_mutexpath "$name")
+	sem=$(_sem_sempath "$name")
+	passed=0
+
+	while (( passed == 0)); do
+		mutex_lock "$mutex"
+
+		if _sem_dec "$sem"; then
+			passed=1
+		fi
+
+		mutex_unlock "$mutex"
+	done
+
+	return 0
+}
+
+sem_trywait() {
+	local name
+
+	local mutex
+	local sem
+	local res
+
+	name="$1"
+
+	mutex=$(_sem_mutexpath "$name")
+	sem=$(_sem_sempath "$name")
+	res=1
+
+	mutex_lock "$mutex"
+
+	if _sem_dec "$sem"; then
+		res=0
+	fi
+
+	mutex_unlock "$mutex"
+
+	return "$res"
+}
+
+sem_post() {
+	local name
+
+	local mutex
+	local sem
+	local err
+	local value
+
+	name="$1"
+
+	mutex=$(_sem_mutexpath "$name")
+	sem=$(_sem_sempath "$name")
+	err=0
+
+	mutex_lock "$mutex"
+
+	if ! _sem_inc "$sem"; then
+		err=1
+	fi
+
+	mutex_unlock "$mutex"
+
+	return "$err"
+}