fix: Potential deadlock when starting the SDK (getsentry#3970)

Fix a deadlock when two threads access SentrySDK.options and
SentrySDK.currentHub, which used the same object in synchronized.
This problem is fixed now by using two independent locks for
SentrySDK.options and SentrySDK.currentHub.

Fixes getsentryGH-3956, Fixes getsentryGH-3899

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?