fix: Potential deadlock when starting the SDK

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 - Fix data race when calling reportFullyDisplayed from a background thread (#3926)
 - Ensure flushing envelopes directly after capturing them (#3915)
+- Potential deadlock when starting the SDK (#3970)
 
 ### Improvements
 

Sources/Sentry/SentrySDK.m

@@ -44,11 +44,13 @@
 @implementation SentrySDK
 
 static SentryHub *_Nullable currentHub;
+static NSObject *currentHubLock;
 static BOOL crashedLastRunCalled;
 static SentryAppStartMeasurement *sentrySDKappStartMeasurement;
 static NSObject *sentrySDKappStartMeasurementLock;
 static BOOL _detectedStartUpCrash;
 static SentryOptions *_Nullable startOption;
+static NSObject *startOptionsLock;
 
 /**
  * @brief We need to keep track of the number of times @c +[startWith...] is called, because our OOM
@@ -65,14 +67,16 @@ + (void)initialize
 {
     if (self == [SentrySDK class]) {
         sentrySDKappStartMeasurementLock = [[NSObject alloc] init];
+        currentHubLock = [[NSObject alloc] init];
+        startOptionsLock = [[NSObject alloc] init];
         startInvocations = 0;
         _detectedStartUpCrash = NO;
     }
 }
 
 + (SentryHub *)currentHub
 {
-    @synchronized(self) {
+    @synchronized(currentHubLock) {
         if (nil == currentHub) {
             currentHub = [[SentryHub alloc] initWithClient:nil andScope:nil];
         }
@@ -82,22 +86,22 @@ + (SentryHub *)currentHub
 
 + (nullable SentryOptions *)options
 {
-    @synchronized(self) {
+    @synchronized(startOptionsLock) {
         return startOption;
     }
 }
 
 /** Internal, only needed for testing. */
 + (void)setCurrentHub:(nullable SentryHub *)hub
 {
-    @synchronized(self) {
+    @synchronized(currentHubLock) {
         currentHub = hub;
     }
 }
 /** Internal, only needed for testing. */
 + (void)setStartOptions:(nullable SentryOptions *)options
 {
-    @synchronized(self) {
+    @synchronized(startOptionsLock) {
         startOption = options;
     }
 }

Tests/SentryTests/SentrySDKTests.swift

@@ -887,6 +887,41 @@ class SentrySDKTests: XCTestCase {
     }
 }
 
+/// Tests in this class aren't part of SentrySDKTests because we need would need to undo a bunch of operations 
+/// that are done in the setup.
+class SentrySDKWithSetupTests: XCTestCase {
+
+    func testAccessingHubAndOptions_NoDeadlock() {
+        SentryLog.withOutLogs {
+
+            let concurrentQueue = DispatchQueue(label: "concurrent", attributes: .concurrent)
+
+            let expectation = expectation(description: "no deadlock")
+            expectation.expectedFulfillmentCount = 20
+
+            SentrySDK.setStart(Options())
+
+            for _ in 0..<10 {
+                concurrentQueue.async {
+                    SentrySDK.currentHub().capture(message: "mess")
+                    SentrySDK.setCurrentHub(nil)
+
+                    expectation.fulfill()
+                }
+
+                concurrentQueue.async {
+                    let hub = SentryHub(client: nil, andScope: nil)
+                    expect(hub) != nil
+
+                    expectation.fulfill()
+                }
+            }
+
+            wait(for: [expectation], timeout: 5.0)
+        }
+    }
+}
+
 public class MainThreadTestIntegration: NSObject, SentryIntegrationProtocol {
 
     static var expectation: XCTestExpectation?