Mock out time itself in the elapsed timer and rate limiter tests

These occasionally fail nondeterministically; remove the property which allows them to do so (timing).

This also fixes some "off by one" errors that relying on a mock clock revealed -- there was an implicit assumption that sleep() would take a bit more time than requested, and environments with extreme timing sensitivity sometimes failed to do so.

Author: jakepetroules

Sources/SWBBuildService/Messages.swift

@@ -683,7 +683,7 @@ private struct GetIndexingHeaderInfoMsg: MessageHandler {
 }
 
 extension MessageHandler {
-    fileprivate func handleIndexingInfoRequest<T: IndexingInfoRequest>(serializationQueue: ActorLock, request: Request, message: T, _ transformResponse: @escaping @Sendable (T, WorkspaceContext, BuildRequest, BuildRequestContext, BuildDescription, ConfiguredTarget, ElapsedTimer) -> any SWBProtocol.Message) async throws -> VoidResponse {
+    fileprivate func handleIndexingInfoRequest<T: IndexingInfoRequest>(serializationQueue: ActorLock, request: Request, message: T, _ transformResponse: @escaping @Sendable (T, WorkspaceContext, BuildRequest, BuildRequestContext, BuildDescription, ConfiguredTarget, ElapsedTimer<ContinuousClock>) -> any SWBProtocol.Message) async throws -> VoidResponse {
         let elapsedTimer = ElapsedTimer()
 
         // FIXME: Move this to use ActiveBuild.

Sources/SWBTestSupport/MockClock.swift

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SWBUtil
+
+/// A mock clock whose current time is controllable.
+public struct MockClock: Clock {
+    public typealias Instant = ContinuousClock.Instant
+
+    private final class State: Sendable {
+        let now: SWBMutex<Instant>
+
+        init(now: Instant) {
+            self.now = .init(now)
+        }
+    }
+
+    private let state: State
+
+    public init(now: Instant = .now) {
+        self.state = .init(now: now)
+    }
+
+    public var now: Instant {
+        state.now.withLock({ $0 })
+    }
+
+    public var minimumResolution: Duration {
+        ContinuousClock.continuous.minimumResolution
+    }
+
+    public func sleep(until deadline: Instant, tolerance: Duration?) async throws {
+        state.now.withLock { now in
+            if now < deadline {
+                now = deadline
+            }
+        }
+    }
+}

Sources/SWBUtil/ElapsedTimer.swift

@@ -15,47 +15,51 @@ public import struct Foundation.TimeInterval
 /// Provides a simple nanosecond-precision elapsed timer using a monotonic clock.
 ///
 /// This is a completely immutable object and thus is thread safe.
-public struct ElapsedTimer: Sendable {
+public struct ElapsedTimer<ClockType: Clock>: Sendable where ClockType.Duration == Duration {
+    private let clock: ClockType
+
     /// The beginning of the time interval, measured when the `ElapsedTimer` was initialized.
-    private let start = ContinuousClock.now
+    private let start: ClockType.Instant
 
     /// Initializes a new timer.
     ///
     /// The `ElapsedTimer` object captures the current time at initialization and uses this as the starting point of an elapsed time measurement.
     ///
     /// - note: The starting point of the timer is fixed once it is initialized and the object provides no facilities to "restart" the clock (simply create a new instance to do so).
-    public init() {
+    public init(clock: ClockType = ContinuousClock.continuous) {
+        self.clock = clock
+        self.start = clock.now
     }
 
     /// Computes the length of the time interval, that is, the length of the time interval between now and the point in time when the `ElapsedTimer` was initialized.
     ///
     /// This value is guaranteed to be positive.
     public func elapsedTime() -> ElapsedTimerInterval {
-        return ElapsedTimerInterval(duration: ContinuousClock.now - start)
+        return ElapsedTimerInterval(duration: start.duration(to: clock.now))
     }
 
     /// Time the given closure, returning the elapsed time and the result.
-    public static func measure<T>(_ body: () throws -> T) rethrows -> (elapsedTime: ElapsedTimerInterval, result: T) {
-        let timer = ElapsedTimer()
+    public static func measure<T>(clock: ClockType = ContinuousClock.continuous, _ body: () throws -> T) rethrows -> (elapsedTime: ElapsedTimerInterval, result: T) {
+        let timer = ElapsedTimer(clock: clock)
         let result = try body()
         return (timer.elapsedTime(), result)
     }
 
     /// Time the given closure, returning the elapsed time and the result.
-    public static func measure<T>(_ body: () async throws -> T) async rethrows -> (elapsedTime: ElapsedTimerInterval, result: T) {
-        let timer = ElapsedTimer()
+    public static func measure<T>(clock: ClockType = ContinuousClock.continuous, _ body: () async throws -> T) async rethrows -> (elapsedTime: ElapsedTimerInterval, result: T) {
+        let timer = ElapsedTimer(clock: clock)
         let result = try await body()
         return (timer.elapsedTime(), result)
     }
 
     /// Time the given closure, returning the elapsed time.
-    public static func measure(_ body: () throws -> Void) rethrows -> ElapsedTimerInterval {
-        return try measure(body).elapsedTime
+    public static func measure(clock: ClockType = ContinuousClock.continuous, _ body: () throws -> Void) rethrows -> ElapsedTimerInterval {
+        return try measure(clock: clock, body).elapsedTime
     }
 
     /// Time the given closure, returning the elapsed time.
-    public static func measure(_ body: () async throws -> Void) async rethrows -> ElapsedTimerInterval {
-        return try await measure(body).elapsedTime
+    public static func measure(clock: ClockType = ContinuousClock.continuous, _ body: () async throws -> Void) async rethrows -> ElapsedTimerInterval {
+        return try await measure(clock: clock, body).elapsedTime
     }
 }
 

Sources/SWBUtil/RateLimiter.swift

@@ -13,15 +13,17 @@
 /// Provides a simple utility for implementing rate-limiting mechanisms.
 ///
 /// This object is NOT thread-safe.
-public struct RateLimiter: ~Copyable {
-    private let start: ContinuousClock.Instant
-    private var last: ContinuousClock.Instant
+public struct RateLimiter<ClockType: Clock>: ~Copyable {
+    private let clock: ClockType
+    private let start: ClockType.Instant
+    private var last: ClockType.Instant
 
     /// The length of the time interval to which updates are rate-limited.
-    public let interval: Duration
+    public let interval: ClockType.Duration
 
-    public init(interval: Duration) {
-        let now = ContinuousClock.now
+    public init(interval: ClockType.Duration, clock: ClockType = ContinuousClock.continuous) {
+        self.clock = clock
+        let now = clock.now
         self.interval = interval
         self.start = now
         self.last = now
@@ -31,8 +33,8 @@ public struct RateLimiter: ~Copyable {
     /// time this function returned `true`, is greater than the time interval
     /// with which this object was initialized.
     public mutating func hasNextIntervalPassed() -> Bool {
-        let now = ContinuousClock.now
-        let elapsed = now - last
+        let now = clock.now
+        let elapsed = last.duration(to: now)
         if elapsed >= interval {
             last = now
             return true

Tests/SWBUtilTests/ElapsedTimerTests.swift

@@ -19,16 +19,18 @@ import SWBTestSupport
     @Test(.skipHostOS(.freebsd, "Currently hangs on FreeBSD"))
     func time() async throws {
         do {
-            let delta = try await ElapsedTimer.measure {
-                try await Task.sleep(for: .microseconds(1000))
+            let clock = MockClock()
+            let delta = try await ElapsedTimer.measure(clock: clock) {
+                try await clock.sleep(for: .microseconds(1001))
                 return ()
             }
             #expect(delta.seconds > 1.0 / 1000.0)
         }
 
         do {
-            let (delta, result) = try await ElapsedTimer.measure { () -> Int in
-                try await Task.sleep(for: .microseconds(1000))
+            let clock = MockClock()
+            let (delta, result) = try await ElapsedTimer.measure(clock: clock) { () -> Int in
+                try await clock.sleep(for: .microseconds(1001))
                 return 22
             }
             #expect(delta.seconds > 1.0 / 1000.0)

Tests/SWBUtilTests/RateLimiterTests.swift

@@ -19,16 +19,17 @@ import SWBTestSupport
 fileprivate struct RateLimiterTests {
     @Test
     func rateLimiterSeconds() async throws {
-        let timer = ElapsedTimer()
-        var limiter = RateLimiter(interval: .seconds(1))
+        let clock = MockClock()
+        let timer = ElapsedTimer(clock: clock)
+        var limiter = RateLimiter(interval: .seconds(1), clock: clock)
         #expect(limiter.interval == .nanoseconds(1_000_000_000))
 
         var count = 0
         for _ in 0..<3 {
             if limiter.hasNextIntervalPassed() {
                 count += 1
             }
-            try await Task.sleep(for: .seconds(1))
+            try await clock.sleep(for: .seconds(1))
         }
 
         #expect(count > 1)
@@ -37,16 +38,17 @@ fileprivate struct RateLimiterTests {
 
     @Test
     func rateLimiterTwoSeconds() async throws {
-        let timer = ElapsedTimer()
-        var limiter = RateLimiter(interval: .seconds(2))
+        let clock = MockClock()
+        let timer = ElapsedTimer(clock: clock)
+        var limiter = RateLimiter(interval: .seconds(2), clock: clock)
         #expect(limiter.interval == .nanoseconds(2_000_000_000))
 
         var count = 0
         for _ in 0..<3 {
             if limiter.hasNextIntervalPassed() {
                 count += 1
             }
-            try await Task.sleep(for: .seconds(1))
+            try await clock.sleep(for: .seconds(1))
         }
 
         #expect(count > 0)
@@ -55,16 +57,17 @@ fileprivate struct RateLimiterTests {
 
     @Test
     func rateLimiterMilliseconds() async throws {
-        let timer = ElapsedTimer()
-        var limiter = RateLimiter(interval: .milliseconds(100))
+        let clock = MockClock()
+        let timer = ElapsedTimer(clock: clock)
+        var limiter = RateLimiter(interval: .milliseconds(100), clock: clock)
         #expect(limiter.interval == .nanoseconds(100_000_000))
 
         var count = 0
-        for _ in 0..<100 {
+        for _ in 0..<101 {
             if limiter.hasNextIntervalPassed() {
                 count += 1
             }
-            try await Task.sleep(for: .microseconds(2001))
+            try await clock.sleep(for: .microseconds(2001))
         }
 
         #expect(count > 1)
@@ -73,16 +76,17 @@ fileprivate struct RateLimiterTests {
 
     @Test
     func rateLimiterMicroseconds() async throws {
-        let timer = ElapsedTimer()
-        var limiter = RateLimiter(interval: .microseconds(100000))
+        let clock = MockClock()
+        let timer = ElapsedTimer(clock: clock)
+        var limiter = RateLimiter(interval: .microseconds(100000), clock: clock)
         #expect(limiter.interval == .nanoseconds(100_000_000))
 
         var count = 0
-        for _ in 0..<100 {
+        for _ in 0..<101 {
             if limiter.hasNextIntervalPassed() {
                 count += 1
             }
-            try await Task.sleep(for: .microseconds(1001))
+            try await clock.sleep(for: .microseconds(1001))
         }
 
         #expect(count > 0)
@@ -91,16 +95,17 @@ fileprivate struct RateLimiterTests {
 
     @Test
     func rateLimiterNanoseconds() async throws {
-        let timer = ElapsedTimer()
-        var limiter = RateLimiter(interval: .nanoseconds(100_000_000))
+        let clock = MockClock()
+        let timer = ElapsedTimer(clock: clock)
+        var limiter = RateLimiter(interval: .nanoseconds(100_000_000), clock: clock)
         #expect(limiter.interval == .nanoseconds(100_000_000))
 
         var count = 0
-        for _ in 0..<100 {
+        for _ in 0..<101 {
             if limiter.hasNextIntervalPassed() {
                 count += 1
             }
-            try await Task.sleep(for: .microseconds(1001))
+            try await clock.sleep(for: .microseconds(1001))
         }
 
         #expect(count > 0)
@@ -109,12 +114,13 @@ fileprivate struct RateLimiterTests {
 
     @Test
     func rateLimiterNoCrashNever() async throws {
-        var limiter = RateLimiter(interval: .nanoseconds(UInt64.max))
+        let clock = MockClock()
+        var limiter = RateLimiter(interval: .nanoseconds(UInt64.max), clock: clock)
 
         for _ in 0..<2 {
             let nextIntervalPassed = limiter.hasNextIntervalPassed()
             #expect(!nextIntervalPassed)
-            try await Task.sleep(for: .seconds(1))
+            try await clock.sleep(for: .seconds(1))
         }
     }
 }