Cleanup

README.md

@@ -19,7 +19,7 @@ Task {
 semaphore.signal()
 ```
 
-The `wait()` method has a `waitUntilTaskCancellation()` variant that throws `CancellationError` if the task is canceled before a signal occurs.
+The `wait()` method has a `waitUnlessCancelled()` variant that throws `CancellationError` if the task is cancelled before a signal occurs.
 
 For a nice introduction to semaphores, see [The Beauty of Semaphores in Swift 🚦](https://medium.com/@roykronenfeld/semaphores-in-swift-e296ea80f860). The article discusses [`DispatchSemaphore`], but it can easily be ported to Swift concurrency: see the [demo playground](Demo/SemaphorePlayground.playground/Contents.swift) of this package. 
 

Sources/Semaphore/Semaphore.swift

@@ -37,19 +37,24 @@ import Foundation
 ///
 /// - ``signal()``
 ///
-/// ### Blocking on the Semaphore
+/// ### Waiting for the Semaphore
 ///
 /// - ``wait()``
-/// - ``waitUntilTaskCancellation()``
+/// - ``waitUnlessCancelled()``
 public final class Semaphore {
-    /// The semaphore value.
-    private var value: Int
-
+    /// "Waiting for a signal" is easily said, but several possible states exist.
     private class Suspension {
         enum State {
+            /// Initial state. Next is suspended, or cancelled.
             case pending
-            case suspendedUntilTaskCancellation(UnsafeContinuation<Void, Error>)
+
+            /// Waiting for a signal, with support for cancellation.
+            case suspendedUnlessCancelled(UnsafeContinuation<Void, Error>)
+
+            /// Waiting for a signal, with no support for cancellation.
             case suspended(UnsafeContinuation<Void, Never>)
+
+            /// Cancelled before we have started waiting.
             case cancelled
         }
 
@@ -64,15 +69,25 @@ public final class Semaphore {
         }
     }
 
+    // MARK: - Internal State
+
+    /// The semaphore value.
+    private var value: Int
+
+    /// As many elements as there are suspended tasks waiting for a signal.
+    /// We store `Suspension` instances instead of `UnsafeContinuation`, because
+    /// we support cancellation by removing `Suspension` instances from
+    /// this array.
     private var suspensions: [Suspension] = []
 
-    /// This lock would be required even if ``Semaphore`` were made an actor,
-    /// because `withUnsafeContinuation` suspends before it runs its closure
-    /// argument. Also, by making ``Semaphore`` a plain class, we can expose a
-    /// non-async ``signal()`` method. The lock is recursive in order to handle
-    /// cancellation (see the implementation of ``wait()``).
+    /// The lock that protects `value` and `suspensions`.
+    ///
+    /// It is recursive in order to handle cancellation (see the implementation
+    /// of ``waitUnlessCancelled()``).
     private let lock = NSRecursiveLock()
 
+    // MARK: - Creating a Semaphore
+
     /// Creates a semaphore.
     ///
     /// - parameter value: The starting value for the semaphore. Do not pass a
@@ -86,6 +101,8 @@ public final class Semaphore {
         precondition(suspensions.isEmpty, "Semaphore is deallocated while some task(s) are suspended waiting for a signal.")
     }
 
+    // MARK: - Waiting for the Semaphore
+
     /// Waits for, or decrements, a semaphore.
     ///
     /// Decrement the counting semaphore. If the resulting value is less than
@@ -119,7 +136,7 @@ public final class Semaphore {
     ///
     /// - Throws: If the task is canceled before a signal occurs, this function
     ///   throws `CancellationError`.
-    public func waitUntilTaskCancellation() async throws {
+    public func waitUnlessCancelled() async throws {
         lock.lock()
 
         value -= 1
@@ -148,7 +165,7 @@ public final class Semaphore {
                     // The first suspended task will be the first task resumed by `signal`.
                     // This is not intended to be a strong fifo guarantee, but just
                     // an attempt at some fairness.
-                    suspension.state = .suspendedUntilTaskCancellation(continuation)
+                    suspension.state = .suspendedUnlessCancelled(continuation)
                     suspensions.insert(suspension, at: 0)
                     lock.unlock()
                 }
@@ -168,7 +185,7 @@ public final class Semaphore {
                 suspensions.remove(at: index)
             }
 
-            if case let .suspendedUntilTaskCancellation(continuation) = suspension.state {
+            if case let .suspendedUnlessCancelled(continuation) = suspension.state {
                 // Task is cancelled while suspended: resume with a CancellationError.
                 continuation.resume(throwing: CancellationError())
             } else {
@@ -179,6 +196,8 @@ public final class Semaphore {
         }
     }
 
+    // MARK: - Signaling the Semaphore
+
     /// Signals (increments) a semaphore.
     ///
     /// Increment the counting semaphore. If the previous value was less than
@@ -194,16 +213,14 @@ public final class Semaphore {
         value += 1
 
         switch suspensions.popLast()?.state {
-        case let .suspendedUntilTaskCancellation(continuation):
+        case let .suspendedUnlessCancelled(continuation):
             continuation.resume()
             return true
         case let .suspended(continuation):
             continuation.resume()
             return true
         default:
-            break
+            return false
         }
-
-        return false
     }
 }

Tests/SemaphoreTests/SemaphoreTests.swift

@@ -124,7 +124,7 @@ final class SemaphoreTests: XCTestCase {
         let ex = expectation(description: "cancellation")
         let task = Task {
             do {
-                try await sem.waitUntilTaskCancellation()
+                try await sem.waitUnlessCancelled()
                 XCTFail("Expected CancellationError")
             } catch is CancellationError {
             } catch {
@@ -148,7 +148,7 @@ final class SemaphoreTests: XCTestCase {
                 }
             }
             do {
-                try await sem.waitUntilTaskCancellation()
+                try await sem.waitUnlessCancelled()
                 XCTFail("Expected CancellationError")
             } catch is CancellationError {
             } catch {
@@ -164,7 +164,7 @@ final class SemaphoreTests: XCTestCase {
         // Given a task cancelled while suspended on a semaphore,
         let sem = Semaphore(value: 0)
         let task = Task {
-            try await sem.waitUntilTaskCancellation()
+            try await sem.waitUnlessCancelled()
         }
         try await Task.sleep(nanoseconds: 100_000_000)
         task.cancel()
@@ -197,7 +197,7 @@ final class SemaphoreTests: XCTestCase {
                     continuation.resume()
                 }
             }
-            try await sem.waitUntilTaskCancellation()
+            try await sem.waitUnlessCancelled()
         }
         task.cancel()
 
@@ -279,7 +279,7 @@ final class SemaphoreTests: XCTestCase {
             await withThrowingTaskGroup(of: Void.self) { group in
                 for _ in 0..<(maxCount * 2) {
                     group.addTask {
-                        try await sem.waitUntilTaskCancellation()
+                        try await sem.waitUnlessCancelled()
                         await runner.run()
                         sem.signal()
                     }