Fix potential arithmetic errors in CPUUsageCalculator

Author: gjcairo

Sources/SystemMetrics/SystemMetrics.swift

@@ -276,10 +276,10 @@ public enum SystemMetrics {
     /// CPU usage is calculated as the number of CPU ticks used by this process between measurements.
     /// - Note: the first measurement will be calculated since the process' start time, since there's no
     /// previous measurement to take as reference.
-    private struct CPUUsageCalculator {
+    internal struct CPUUsageCalculator {
         /// The number of ticks after system boot that the last CPU usage stat was taken.
         private var previousTicksSinceSystemBoot: Int = 0
-        /// The number of ticks the process actively used the CPU, as of the previous CPU usage measurement.
+        /// The number of ticks the process actively used the CPU for, as of the previous CPU usage measurement.
         private var previousCPUTicks: Int = 0
 
         mutating func getUsagePercentage(ticksSinceSystemBoot: Int, cpuTicks: Int) -> Double {
@@ -288,6 +288,10 @@ public enum SystemMetrics {
                 self.previousCPUTicks = cpuTicks
             }
             let ticksBetweenMeasurements = ticksSinceSystemBoot - self.previousTicksSinceSystemBoot
+            guard ticksBetweenMeasurements > 0 else {
+                return 0
+            }
+
             let cpuTicksBetweenMeasurements = cpuTicks - self.previousCPUTicks
             return Double(cpuTicksBetweenMeasurements) * 100 / Double(ticksBetweenMeasurements)
         }
@@ -371,7 +375,8 @@ public enum SystemMetrics {
         var cpuUsage: Double = 0
         if cpuTicks > 0 {
             guard let uptimeString = uptimeFileContents.split(separator: " ").first,
-                  let uptimeSeconds = Float(uptimeString)
+                  let uptimeSeconds = Float(uptimeString),
+                  uptimeSeconds.isFinite
             else { return nil }
             let uptimeTicks = Int(ceilf(uptimeSeconds)) * ticks
             cpuUsage = SystemMetrics.cpuUsageCalculator.getUsagePercentage(ticksSinceSystemBoot: uptimeTicks, cpuTicks: cpuTicks)

Tests/SystemMetricsTests/SystemMetricsTests+XCTest.swift

@@ -28,6 +28,7 @@ extension SystemMetricsTest {
             ("testSystemMetricsGeneration", testSystemMetricsGeneration),
             ("testSystemMetricsLabels", testSystemMetricsLabels),
             ("testSystemMetricsConfiguration", testSystemMetricsConfiguration),
+            ("testCPUUsageCalculator", testCPUUsageCalculator),
         ]
     }
 }

Tests/SystemMetricsTests/SystemMetricsTests.swift

@@ -21,10 +21,6 @@ class SystemMetricsTest: XCTestCase {
     func testSystemMetricsGeneration() throws {
         #if os(Linux)
         let _metrics = SystemMetrics.linuxSystemMetrics()
-        #else
-        let _metrics = SystemMetrics.noopSystemMetrics()
-        throw XCTSkip()
-        #endif
         XCTAssertNotNil(_metrics)
         let metrics = _metrics!
         XCTAssertNotNil(metrics.virtualMemoryBytes)
@@ -34,6 +30,9 @@ class SystemMetricsTest: XCTestCase {
         XCTAssertNotNil(metrics.maxFileDescriptors)
         XCTAssertNotNil(metrics.openFileDescriptors)
         XCTAssertNotNil(metrics.cpuUsage)
+        #else
+        throw XCTSkip()
+        #endif
     }
 
     func testSystemMetricsLabels() throws {
@@ -89,4 +88,19 @@ class SystemMetricsTest: XCTestCase {
         XCTAssertFalse(configuration.dimensions.contains(where: { $0 == ("environment", "staging") }))
         XCTAssertFalse(configuration.dimensions.contains(where: { $0 == ("process", "example") }))
     }
+
+    func testCPUUsageCalculator() throws {
+        #if os(Linux)
+        var calculator = SystemMetrics.CPUUsageCalculator()
+        var usage = calculator.getUsagePercentage(ticksSinceSystemBoot: 0, cpuTicks: 0)
+        XCTAssertFalse(usage.isNaN)
+        XCTAssertEqual(usage, 0)
+
+        usage = calculator.getUsagePercentage(ticksSinceSystemBoot: 20, cpuTicks: 10)
+        XCTAssertFalse(usage.isNaN)
+        XCTAssertEqual(usage, 50)
+        #else
+        throw XCTSkip()
+        #endif
+    }
 }