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Fix overflow traps in DispatchTime/DispatchWallTime/DispatchTimeInterval #301

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Merged
merged 1 commit into from
Sep 20, 2017
Merged

Fix overflow traps in DispatchTime/DispatchWallTime/DispatchTimeInterval #301

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56 changes: 41 additions & 15 deletions src/swift/Time.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -119,6 +119,40 @@ public func ==(a: DispatchWallTime, b: DispatchWallTime) -> Bool {
return a.rawValue == b.rawValue
}

// Returns m1 * m2, clamped to the range [Int64.min, Int64.max].
// Because of the way this function is used, we can always assume
// that m2 > 0.
private func clampedInt64Product(_ m1: Int64, _ m2: Int64) -> Int64 {
assert(m2 > 0, "multiplier must be positive")
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assert is a noop in release builds. Should perhaps use _precondition?

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Not having the check in release builds is exactly what I want. The only calls to this method are internal to this file and the intent is only to document the assumption and to catch (admittedly unlikely) future changes that violate it.

let (result, overflow) = m1.multipliedReportingOverflow(by: m2)
if overflow {
return m1 > 0 ? Int64.max : Int64.min
}
return result
}

// Returns its argument clamped to the range [Int64.min, Int64.max].
private func toInt64Clamped(_ value: Double) -> Int64 {
if value.isNaN { return Int64.max }
if value >= Double(Int64.max) { return Int64.max }
if value <= Double(Int64.min) { return Int64.min }
return Int64(value)
}

/// Represents a time interval that can be used as an offset from a `DispatchTime`
/// or `DispatchWallTime`.
///
/// For example:
/// let inOneSecond = DispatchTime.now() + DispatchTimeInterval.seconds(1)
///
/// If the requested time interval is larger then the internal representation
/// permits, the result of adding it to a `DispatchTime` or `DispatchWallTime`
/// is `DispatchTime.distantFuture` and `DispatchWallTime.distantFuture`
/// respectively. Such time intervals compare as equal:
///
/// let t1 = DispatchTimeInterval.seconds(Int.max)
/// let t2 = DispatchTimeInterval.milliseconds(Int.max)
/// let result = t1 == t2 // true
public enum DispatchTimeInterval {
case seconds(Int)
case milliseconds(Int)
Expand All @@ -129,9 +163,9 @@ public enum DispatchTimeInterval {

internal var rawValue: Int64 {
switch self {
case .seconds(let s): return Int64(s) * Int64(NSEC_PER_SEC)
case .milliseconds(let ms): return Int64(ms) * Int64(NSEC_PER_MSEC)
case .microseconds(let us): return Int64(us) * Int64(NSEC_PER_USEC)
case .seconds(let s): return clampedInt64Product(Int64(s), Int64(NSEC_PER_SEC))
case .milliseconds(let ms): return clampedInt64Product(Int64(ms), Int64(NSEC_PER_MSEC))
case .microseconds(let us): return clampedInt64Product(Int64(us), Int64(NSEC_PER_USEC))
case .nanoseconds(let ns): return Int64(ns)
case .never: return Int64.max
}
Expand All @@ -158,16 +192,12 @@ public func -(time: DispatchTime, interval: DispatchTimeInterval) -> DispatchTim
}

public func +(time: DispatchTime, seconds: Double) -> DispatchTime {
let interval = seconds * Double(NSEC_PER_SEC)
let t = CDispatch.dispatch_time(time.rawValue,
interval.isInfinite || interval.isNaN ? Int64.max : Int64(interval))
let t = CDispatch.dispatch_time(time.rawValue, toInt64Clamped(seconds * Double(NSEC_PER_SEC)));
return DispatchTime(rawValue: t)
}

public func -(time: DispatchTime, seconds: Double) -> DispatchTime {
let interval = -seconds * Double(NSEC_PER_SEC)
let t = CDispatch.dispatch_time(time.rawValue,
interval.isInfinite || interval.isNaN ? Int64.min : Int64(interval))
let t = CDispatch.dispatch_time(time.rawValue, toInt64Clamped(-seconds * Double(NSEC_PER_SEC)));
return DispatchTime(rawValue: t)
}

Expand All @@ -182,15 +212,11 @@ public func -(time: DispatchWallTime, interval: DispatchTimeInterval) -> Dispatc
}

public func +(time: DispatchWallTime, seconds: Double) -> DispatchWallTime {
let interval = seconds * Double(NSEC_PER_SEC)
let t = CDispatch.dispatch_time(time.rawValue,
interval.isInfinite || interval.isNaN ? Int64.max : Int64(interval))
let t = CDispatch.dispatch_time(time.rawValue, toInt64Clamped(seconds * Double(NSEC_PER_SEC)));
return DispatchWallTime(rawValue: t)
}

public func -(time: DispatchWallTime, seconds: Double) -> DispatchWallTime {
let interval = -seconds * Double(NSEC_PER_SEC)
let t = CDispatch.dispatch_time(time.rawValue,
interval.isInfinite || interval.isNaN ? Int64.min : Int64(interval))
let t = CDispatch.dispatch_time(time.rawValue, toInt64Clamped(-seconds * Double(NSEC_PER_SEC)));
return DispatchWallTime(rawValue: t)
}