test(session-replay): Add deterministic sample rate to fix flakiness (#6012)

Author: philprime

Tests/SentryTests/Integrations/SessionReplay/SentrySessionReplayIntegrationTests.swift

@@ -272,24 +272,46 @@ class SentrySessionReplayIntegrationTests: XCTestCase {
     }
 
     func testBufferReplayIgnoredBecauseSampleRateForCrash() throws {
-        startSDK(sessionSampleRate: 1, errorSampleRate: 1)
+        // -- Arrange --
+        // Use deterministic random number to avoid flaky test behavior.
+        // CRITICAL: Set random value to 1.0 to ensure shouldReplayFullSession(sessionSampleRate) returns false,
+        // preventing the session from starting as a full session. Buffer replay sample rate checks
+        // only apply to non-full sessions. The sample rate check uses: random >= errorSampleRate
+        // With errorSampleRate=0 and random=1.0: 1.0 >= 0 = true → replay dropped
+        SentryDependencyContainer.sharedInstance().random = TestRandom(value: 1.0)
+
+        // Start current session with 0% session sample rate to ensure it's NOT a full session
+        // (shouldReplayFullSession: 1.0 < 0 = false), but 100% error sample rate would normally 
+        // capture all error replays if this were not a buffer replay from previous session
+        startSDK(sessionSampleRate: 0, errorSampleRate: 1)
 
         let client = SentryClient(options: try XCTUnwrap(SentrySDKInternal.options))
         let scope = Scope()
         let hub = TestHub(client: client, andScope: scope)
         SentrySDKInternal.setCurrentHub(hub)
         let expectation = expectation(description: "Replay to be captured")
-        expectation.isInverted = true
-        hub.onReplayCapture = {
+        expectation.isInverted = true // We expect NO replay to be captured
+        hub.onReplayCapture = { 
             expectation.fulfill()
         }
-        
+
+        // -- Act --
+        // Create a previous session replay file with 0% error sample rate.
+        // This simulates a previous session that crashed and had error replay disabled.
+        // The key insight: replay capture decision uses the PREVIOUS session's sample rate,
+        // not the current session's sample rate, because the replay frames were recorded
+        // during the previous session with its own sampling configuration.
         try createLastSessionReplay(writeSessionInfo: false, errorSampleRate: 0)
         let crash = Event(error: NSError(domain: "Error", code: 1))
         crash.context = [:]
         crash.isFatalEvent = true
-        globalEventProcessor.reportAll(crash)
-        
+        globalEventProcessor.reportAll(crash) // This triggers resumePreviousSessionReplay
+
+        // -- Assert --
+        // The replay should be dropped because:
+        // 1. Previous session had errorSampleRate = 0 (no error replays wanted)
+        // 2. Sample rate check: 1.0 >= 0 = true → drop replay
+        // 3. Current session's errorSampleRate = 1 is irrelevant for previous session data
         wait(for: [expectation], timeout: 1)
         XCTAssertEqual(hub.capturedReplayRecordingVideo.count, 0)
     }