From 22c9ba0949551271e1325b7a9eebcd5de8c442d4 Mon Sep 17 00:00:00 2001
From: Mert Buran <mert.buran@datadoghq.com>
Date: Thu, 10 Jun 2021 10:20:05 +0200
Subject: [PATCH] RUMM-1276 PR comments addressed

---
 .../RUM/RUMVitals/VitalCPUReader.swift        | 13 +++++--
 .../RUM/RUMVitals/VitalCPUReaderTest.swift    | 36 ++++++++++++-------
 2 files changed, 34 insertions(+), 15 deletions(-)

diff --git a/Sources/Datadog/RUM/RUMVitals/VitalCPUReader.swift b/Sources/Datadog/RUM/RUMVitals/VitalCPUReader.swift
index 73448c8106..78113912dc 100644
--- a/Sources/Datadog/RUM/RUMVitals/VitalCPUReader.swift
+++ b/Sources/Datadog/RUM/RUMVitals/VitalCPUReader.swift
@@ -7,7 +7,7 @@
 import Foundation
 import UIKit.UIApplication
 
-/// A class reading the CPU ticks (_1 second = 600 ticks_) since the start of the process.
+/// A class reading the CPU ticks of the processor.
 internal class VitalCPUReader: VitalReader {
     /// host_cpu_load_info_count is 4 (tested in iOS 14.4)
     private static let host_cpu_load_info_count = MemoryLayout<host_cpu_load_info>.stride / MemoryLayout<integer_t>.stride
@@ -28,6 +28,9 @@ internal class VitalCPUReader: VitalReader {
         return nil
     }
 
+    // TODO: RUMM-1276 appWillResignActive&appDidBecomeActive are called in main thread
+    // IF readVitalData() is called from non-main threads, they must be synchronized
+
     @objc
     private func appWillResignActive() {
         utilizedTicksWhenResigningActive = readUtilizedTicks()
@@ -60,7 +63,13 @@ internal class VitalCPUReader: VitalReader {
             }
         }
         if result != KERN_SUCCESS {
-            // TODO: RUMM-1276 use sdkLogger to log errors?
+            // in case of error, refer to `kern_return.h` (Objc)
+            // as its Swift interface doesn't have integer values
+            //
+            // NOTE: RUMM-1276 consider using sdkLogger.errorOnce(...) to avoid flooding
+            InternalMonitoringFeature.instance?.monitor.sdkLogger.error(
+                "CPU Vital cannot be read! Error code: \(result)"
+            )
             return nil
         }
 
diff --git a/Tests/DatadogTests/Datadog/RUM/RUMVitals/VitalCPUReaderTest.swift b/Tests/DatadogTests/Datadog/RUM/RUMVitals/VitalCPUReaderTest.swift
index 9bac4d0a28..81bb30cc88 100644
--- a/Tests/DatadogTests/Datadog/RUM/RUMVitals/VitalCPUReaderTest.swift
+++ b/Tests/DatadogTests/Datadog/RUM/RUMVitals/VitalCPUReaderTest.swift
@@ -12,12 +12,9 @@ class VitalCPUReaderTest: XCTestCase {
     let testNotificationCenter = NotificationCenter()
     lazy var cpuReader = VitalCPUReader(notificationCenter: testNotificationCenter)
 
-    func test_whenCPUUnderHeavyLoad_itMeasuresHigherCPUTicks() throws {
+    func testWhenCPUUnderHeavyLoadItMeasuresHigherCPUTicks() throws {
         let highLoadAverage = try averageCPUTicks {
-            for _ in 0...500_000 {
-                let random = Double.random(in: Double.leastNonzeroMagnitude...Double.greatestFiniteMagnitude)
-                _ = tan(random).squareRoot()
-            }
+            heavyLoad()
         }
 
         let lowLoadAverage = try averageCPUTicks {
@@ -27,14 +24,7 @@ class VitalCPUReaderTest: XCTestCase {
         XCTAssertGreaterThan(highLoadAverage, lowLoadAverage)
     }
 
-    func test_whenInactiveAppState_itIggnoresCPUTicks() throws {
-        let heavyLoad = {
-            for _ in 0...500_000 {
-                let random = Double.random(in: Double.leastNonzeroMagnitude...Double.greatestFiniteMagnitude)
-                _ = tan(random).squareRoot()
-            }
-        }
-
+    func testWhenInactiveAppStateItIgnoresCPUTicks() throws {
         let baseline = try XCTUnwrap(cpuReader.readVitalData())
         testNotificationCenter.post(name: UIApplication.willResignActiveNotification, object: nil)
         heavyLoad()
@@ -52,7 +42,9 @@ class VitalCPUReaderTest: XCTestCase {
     private func averageCPUTicks(with block: () -> Void) throws -> Double {
         let startDate = Date()
         let startUtilization = try XCTUnwrap(cpuReader.readVitalData())
+
         block()
+
         let endUtilization = try XCTUnwrap(cpuReader.readVitalData())
         let duration = Date().timeIntervalSince(startDate)
 
@@ -62,3 +54,21 @@ class VitalCPUReaderTest: XCTestCase {
         return utilization
     }
 }
+
+fileprivate func heavyLoad() {
+    // cpuTicksResolution is measured by trial&error.
+    // most of the time `readVitalData()` returns incremented data after 0.01sec.
+    // however, sometimes it returns the same value for 1.0sec.
+    // looking at the source code, iOS should update cpu ticks at
+    // every thread scheduling and/or system->user/user->system mode changes in CPU.
+    // but empirically, it gets stuck for 1.0sec randomly.
+    let worstCaseCPUTicksResolution: TimeInterval = 1.0
+    let startDate = Date()
+
+    while Date().timeIntervalSince(startDate) <= worstCaseCPUTicksResolution {
+        for _ in 0...100_000 {
+            let random = Double.random(in: Double.leastNonzeroMagnitude...Double.greatestFiniteMagnitude)
+            _ = tan(random).squareRoot()
+        }
+    }
+}