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 (file)
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"
+}