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Add a Transformer iterator to apply a limiter to jittering #6331

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merged 1 commit into from
Jun 10, 2024
Merged

Add a Transformer iterator to apply a limiter to jittering #6331

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6 changes: 3 additions & 3 deletions ios/MullvadREST/RetryStrategy/ExponentialBackoff.swift
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Expand Up @@ -12,9 +12,9 @@ import MullvadTypes
struct ExponentialBackoff: IteratorProtocol {
private var _next: Duration
private let multiplier: UInt64
private let maxDelay: Duration?
private let maxDelay: Duration

init(initial: Duration, multiplier: UInt64, maxDelay: Duration? = nil) {
init(initial: Duration, multiplier: UInt64, maxDelay: Duration) {
_next = initial
self.multiplier = multiplier
self.maxDelay = maxDelay
Expand All @@ -23,7 +23,7 @@ struct ExponentialBackoff: IteratorProtocol {
mutating func next() -> Duration? {
let next = _next

if let maxDelay, next > maxDelay {
if next > maxDelay {
return maxDelay
}

Expand Down
16 changes: 16 additions & 0 deletions ios/MullvadREST/RetryStrategy/Jittered.swift
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Expand Up @@ -27,3 +27,19 @@ struct Jittered<InnerIterator: IteratorProtocol>: IteratorProtocol
return .milliseconds(millisWithJitter)
}
}

/// Iterator that applies a transform function to the result of another iterator.
struct Transformer<Inner: IteratorProtocol>: IteratorProtocol {
typealias Element = Inner.Element
private var inner: Inner
private let transformer: (Inner.Element?) -> Inner.Element?

init(inner: Inner, transform: @escaping (Inner.Element?) -> Inner.Element?) {
self.inner = inner
self.transformer = transform
}

mutating func next() -> Inner.Element? {
transformer(inner.next())
}
}
14 changes: 12 additions & 2 deletions ios/MullvadREST/RetryStrategy/RetryStrategy.swift
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Expand Up @@ -25,7 +25,17 @@ extension REST {
let inner = delay.makeIterator()

if applyJitter {
return AnyIterator(Jittered(inner))
return switch delay {
case .never:
AnyIterator(inner)
case .constant:
AnyIterator(Jittered(inner))
case let .exponentialBackoff(_, _, maxDelay):
AnyIterator(Transformer(inner: Jittered(inner)) { nextValue in
guard let nextValue else { return maxDelay }
return nextValue >= maxDelay ? maxDelay : nextValue
})
}
} else {
return AnyIterator(inner)
}
Expand Down Expand Up @@ -68,7 +78,7 @@ extension REST {
case constant(Duration)

/// Exponential backoff.
case exponentialBackoff(initial: Duration, multiplier: UInt64, maxDelay: Duration?)
case exponentialBackoff(initial: Duration, multiplier: UInt64, maxDelay: Duration)

func makeIterator() -> AnyIterator<Duration> {
switch self {
Expand Down
12 changes: 6 additions & 6 deletions ios/MullvadRESTTests/ExponentialBackoffTests.swift
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Expand Up @@ -12,15 +12,15 @@ import XCTest

final class ExponentialBackoffTests: XCTestCase {
func testExponentialBackoff() {
var backoff = ExponentialBackoff(initial: .seconds(2), multiplier: 3)
var backoff = ExponentialBackoff(initial: .seconds(2), multiplier: 3, maxDelay: .seconds(18))

XCTAssertEqual(backoff.next(), .seconds(2))
XCTAssertEqual(backoff.next(), .seconds(6))
XCTAssertEqual(backoff.next(), .seconds(18))
}

func testAtMaximumValue() {
var backoff = ExponentialBackoff(initial: .milliseconds(.max - 1), multiplier: 2)
var backoff = ExponentialBackoff(initial: .milliseconds(.max - 1), multiplier: 2, maxDelay: .seconds(.max - 1))

XCTAssertEqual(backoff.next(), .milliseconds(.max - 1))
XCTAssertEqual(backoff.next(), .milliseconds(.max))
Expand All @@ -40,20 +40,20 @@ final class ExponentialBackoffTests: XCTestCase {
}

func testMinimumValue() {
var backoff = ExponentialBackoff(initial: .milliseconds(0), multiplier: 10)
var backoff = ExponentialBackoff(initial: .milliseconds(0), multiplier: 10, maxDelay: .milliseconds(0))

XCTAssertEqual(backoff.next(), .milliseconds(0))
XCTAssertEqual(backoff.next(), .milliseconds(0))

backoff = ExponentialBackoff(initial: .milliseconds(1), multiplier: 0)
backoff = ExponentialBackoff(initial: .milliseconds(1), multiplier: 0, maxDelay: .zero)

XCTAssertEqual(backoff.next(), .milliseconds(1))
XCTAssertEqual(backoff.next(), .milliseconds(0))
XCTAssertEqual(backoff.next(), .milliseconds(0))
}

func testJitter() {
let initial: Duration = .milliseconds(500)
var iterator = Jittered(ExponentialBackoff(initial: initial, multiplier: 3))
var iterator = Jittered(ExponentialBackoff(initial: initial, multiplier: 3, maxDelay: .milliseconds(1500)))

XCTAssertGreaterThanOrEqual(iterator.next()!, initial)
}
Expand Down
52 changes: 52 additions & 0 deletions ios/MullvadRESTTests/RetryStrategyTests.swift
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Original file line number Diff line number Diff line change
@@ -0,0 +1,52 @@
//
// RetryStrategyTests.swift
// MullvadRESTTests
//
// Created by Marco Nikic on 2024-06-07.
// Copyright © 2024 Mullvad VPN AB. All rights reserved.
//

import Foundation
@testable import MullvadREST
@testable import MullvadTypes
import XCTest

class RetryStrategyTests: XCTestCase {
func testJitteredBackoffDoesNotGoBeyondMaxDelay() throws {
let maxDelay = Duration(secondsComponent: 10, attosecondsComponent: 0)
let retryDelay = REST.RetryDelay.exponentialBackoff(initial: .seconds(1), multiplier: 2, maxDelay: maxDelay)
let retry = REST.RetryStrategy(maxRetryCount: 0, delay: retryDelay, applyJitter: true)
let iterator = retry.makeDelayIterator()
var previousDelay = Duration(secondsComponent: 0, attosecondsComponent: 0)

for _ in 0 ... 10 {
let currentDelay = try XCTUnwrap(iterator.next())
XCTAssertLessThanOrEqual(previousDelay, currentDelay)
XCTAssertLessThanOrEqual(currentDelay, maxDelay)
previousDelay = currentDelay
}
}

func testJitteredConstantCannotBeMoreThanDouble() throws {
let retryDelay = REST.RetryDelay.constant(.seconds(10))
let retry = REST.RetryStrategy(maxRetryCount: 0, delay: retryDelay, applyJitter: true)
let iterator = retry.makeDelayIterator()
let minimumDelay = Duration(secondsComponent: 10, attosecondsComponent: 0)
let maximumDelay = Duration(secondsComponent: 20, attosecondsComponent: 0)

for _ in 0 ... 10 {
let currentDelay = try XCTUnwrap(iterator.next())
let maximumJitterRange = minimumDelay ... maximumDelay
print(currentDelay)
XCTAssertLessThanOrEqual(maximumJitterRange.lowerBound, currentDelay)
XCTAssertGreaterThanOrEqual(maximumJitterRange.upperBound, currentDelay)
}
}

func testCannotApplyJitterToNeverRetry() throws {
let retryDelay = REST.RetryDelay.never
let retry = REST.RetryStrategy(maxRetryCount: 0, delay: retryDelay, applyJitter: true)
let iterator = retry.makeDelayIterator()
XCTAssertNil(iterator.next())
}
}
4 changes: 4 additions & 0 deletions ios/MullvadVPN.xcodeproj/project.pbxproj
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Expand Up @@ -688,6 +688,7 @@
A917352129FAAA5200D5DCFD /* TransportStrategyTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A917352029FAAA5200D5DCFD /* TransportStrategyTests.swift */; };
A91D78E32B03BDF200FCD5D3 /* TunnelObfuscation.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 5840231F2A406BF5007B27AC /* TunnelObfuscation.framework */; };
A91D78E42B03C01600FCD5D3 /* MullvadSettings.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 58B2FDD32AA71D2A003EB5C6 /* MullvadSettings.framework */; };
A91EBEDA2C1337040004A84D /* RetryStrategyTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A91EBED92C1337040004A84D /* RetryStrategyTests.swift */; };
A93181A12B727ED700E341D2 /* TunnelSettingsV4.swift in Sources */ = {isa = PBXBuildFile; fileRef = A93181A02B727ED700E341D2 /* TunnelSettingsV4.swift */; };
A932D9EF2B5ADD0700999395 /* ProxyConfigurationTransportProvider.swift in Sources */ = {isa = PBXBuildFile; fileRef = A932D9EE2B5ADD0700999395 /* ProxyConfigurationTransportProvider.swift */; };
A932D9F32B5EB61100999395 /* HeadRequestTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A932D9F22B5EB61100999395 /* HeadRequestTests.swift */; };
Expand Down Expand Up @@ -2029,6 +2030,7 @@
A91614D02B108D1B00F416EB /* TransportLayer.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TransportLayer.swift; sourceTree = "<group>"; };
A91614D52B10B26B00F416EB /* TunnelControlViewModel.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TunnelControlViewModel.swift; sourceTree = "<group>"; };
A917352029FAAA5200D5DCFD /* TransportStrategyTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TransportStrategyTests.swift; sourceTree = "<group>"; };
A91EBED92C1337040004A84D /* RetryStrategyTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = RetryStrategyTests.swift; sourceTree = "<group>"; };
A92962582B1F4FDB00DFB93B /* PrivacyInfo.xcprivacy */ = {isa = PBXFileReference; lastKnownFileType = text.xml; path = PrivacyInfo.xcprivacy; sourceTree = "<group>"; };
A92ECC202A77FFAF0052F1B1 /* TunnelSettings.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TunnelSettings.swift; sourceTree = "<group>"; };
A92ECC232A7802520052F1B1 /* StoredAccountData.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = StoredAccountData.swift; sourceTree = "<group>"; };
Expand Down Expand Up @@ -3749,6 +3751,7 @@
A932D9F22B5EB61100999395 /* HeadRequestTests.swift */,
58BDEB9E2A98F6B400F578F2 /* Mocks */,
58B4656F2A98C53300467203 /* RequestExecutorTests.swift */,
A91EBED92C1337040004A84D /* RetryStrategyTests.swift */,
F0164EC22B4C49D30020268D /* ShadowsocksLoaderStub.swift */,
A917352029FAAA5200D5DCFD /* TransportStrategyTests.swift */,
);
Expand Down Expand Up @@ -6039,6 +6042,7 @@
58BDEB9D2A98F69E00F578F2 /* MemoryCache.swift in Sources */,
58BDEB9B2A98F58600F578F2 /* TimeServerProxy.swift in Sources */,
A932D9F52B5EBB9D00999395 /* RESTTransportStub.swift in Sources */,
A91EBEDA2C1337040004A84D /* RetryStrategyTests.swift in Sources */,
58BDEB992A98F4ED00F578F2 /* AnyTransport.swift in Sources */,
A932D9F32B5EB61100999395 /* HeadRequestTests.swift in Sources */,
);
Expand Down
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