Introduce NamedSemaphore

app/src/main/kotlin/moe/tarsin/coroutines/NamedSemaphore.kt

@@ -0,0 +1,41 @@
+package moe.tarsin.coroutines
+
+import androidx.collection.mutableScatterMapOf
+import io.ktor.utils.io.pool.DefaultPool
+import kotlinx.coroutines.sync.Semaphore
+import kotlinx.coroutines.sync.withPermit
+
+class SemaphoreTracker(semaphore: Semaphore, private var count: Int = 0) : Semaphore by semaphore {
+    operator fun inc() = apply { count++ }
+    operator fun dec() = apply { count-- }
+    val isFree
+        get() = count == 0
+}
+
+class SemaphorePool(val permits: Int) : DefaultPool<SemaphoreTracker>(capacity = 32) {
+    override fun produceInstance() = SemaphoreTracker(semaphore = Semaphore(permits = permits))
+    override fun validateInstance(semaphore: SemaphoreTracker) {
+        check(semaphore.availablePermits == permits)
+        check(semaphore.isFree)
+    }
+}
+
+class NamedSemaphore<K>(val permits: Int) {
+    val pool = SemaphorePool(permits = permits)
+    val active = mutableScatterMapOf<K, SemaphoreTracker>()
+}
+
+suspend inline fun <K, R> NamedSemaphore<K>.withPermit(key: K, action: () -> R): R {
+    val mutex = synchronized(active) { active.getOrPut(key) { pool.borrow() }.inc() }
+    return try {
+        mutex.withPermit(action)
+    } finally {
+        synchronized(active) {
+            mutex.dec()
+            if (mutex.isFree) {
+                active.remove(key)
+                pool.recycle(mutex)
+            }
+        }
+    }
+}