fix(semaphore): release acquired permits "allocated" to a recently failed tryLock

1

Author: HugoMendes98

src/semaphore/semaphore.spec.ts

@@ -264,8 +264,10 @@ describe("Semaphore", () => {
 			}, delay);
 
 			await Promise.all([
-				// second acquire in time
-				new Promise(resolve => setTimeout(resolve, 10)).then(() => semaphore.acquire(2)),
+				// `acquire` after the `tryAcquire`
+				new Promise(resolve => setTimeout(resolve, delay / 2)).then(() =>
+					semaphore.acquire(3)
+				),
 
 				semaphore
 					.tryAcquire(delay * 2, 3)
@@ -286,7 +288,7 @@ describe("Semaphore", () => {
 					})
 			]);
 
-			// The state is reset: 2 permits released for a single successful acquire (+ the initial one)
+			// The state is reset: 3 permits released for a single successful acquire (+ the initial one)
 			expect(semaphore.permitsAvailable).toBe(1);
 			expect(semaphore.permitsRequired).toBe(0);
 			expect(semaphore.queueLength).toBe(0);

src/semaphore/semaphore.ts

@@ -142,15 +142,22 @@ export class Semaphore implements Synchronizer {
 			const { reject, resolvers } = item;
 
 			setTimeout(() => {
-				// Re-establish th permits: the previous permits + the releases that were called during the wait
-				this.permits += permitsBck + (permitsRemaining - resolvers.length);
-
-				// Removes the item form the queue
+				// Remove the item from the queue
 				const index = this.queue.findIndex(i => i === item);
 				if (index >= 0) {
 					this.queue.splice(index, 1);
 				}
 
+				// Re-establish the permits: the previous permits + the releases that were called during the wait
+				const permitsTaken = permitsBck + (permitsRemaining - resolvers.length);
+				const permitsToRelease = Math.min(permitsTaken, this.permitsRequired);
+
+				this.permits += permitsTaken - permitsToRelease;
+				if (permitsToRelease > 0) {
+					// "free" the permits for this failed `tryLock`
+					this.release(permitsToRelease);
+				}
+
 				reject(
 					new ConcurrencyExceedTimeoutException(
 						`Timeout of ${timeout}ms exceed when acquiring.`