fix: Always start SDK on the main thread

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Fixes
 
+- Always start SDK on the main thread (#3291)
 - App hang with race condition for tick counter (#3290)
 - Remove "duplicate library" warning (#3312)
 - Remove unnecessary build settings (#3325)

Sources/Sentry/Public/SentrySDK.h

@@ -27,12 +27,18 @@ SENTRY_NO_INIT
 /**
  * Inits and configures Sentry (SentryHub, SentryClient) and sets up all integrations. Make sure to
  * set a valid DSN.
+ *
+ * @discussion Call this method on the main thread. When calling it from a background thread, the
+ * SDK starts on the main thread async.
  */
 + (void)startWithOptions:(SentryOptions *)options NS_SWIFT_NAME(start(options:));
 
 /**
  * Inits and configures Sentry (SentryHub, SentryClient) and sets up all integrations. Make sure to
  * set a valid DSN.
+ *
+ * @discussion Call this method on the main thread. When calling it from a background thread, the
+ * SDK starts on the main thread async.
  */
 + (void)startWithConfigureOptions:(void (^)(SentryOptions *options))configureOptions
     NS_SWIFT_NAME(start(configureOptions:));

Sources/Sentry/SentryReachability.m

@@ -98,6 +98,13 @@
     }
 }
 
+void
+SentryConnectivityReset(void)
+{
+    [sentry_reachability_change_blocks removeAllObjects];
+    sentry_current_reachability_state = kSCNetworkReachabilityFlagsUninitialized;
+}
+
 @implementation SentryReachability
 
 + (void)initialize
@@ -109,9 +116,8 @@ + (void)initialize
 
 - (void)dealloc
 {
-    for (id<SentryReachabilityObserver> observer in sentry_reachability_change_blocks.allKeys) {
-        [self removeObserver:observer];
-    }
+    sentry_reachability_change_blocks = [NSMutableDictionary dictionary];
+    [self removeReachabilityNotification];
 }
 
 - (void)addObserver:(id<SentryReachabilityObserver>)observer
@@ -147,6 +153,11 @@ - (void)removeObserver:(id<SentryReachabilityObserver>)observer
         return;
     }
 
+    [self removeReachabilityNotification];
+}
+
+- (void)removeReachabilityNotification
+{
     sentry_current_reachability_state = kSCNetworkReachabilityFlagsUninitialized;
 
     if (_sentry_reachability_ref == nil) {

Sources/Sentry/SentrySDK.m

@@ -8,12 +8,14 @@
 #import "SentryCrash.h"
 #import "SentryCrashWrapper.h"
 #import "SentryDependencyContainer.h"
+#import "SentryDispatchQueueWrapper.h"
 #import "SentryFileManager.h"
 #import "SentryHub+Private.h"
 #import "SentryLog.h"
 #import "SentryMeta.h"
 #import "SentryOptions+Private.h"
 #import "SentryScope.h"
+#import "SentryThreadWrapper.h"
 #import "SentryUIDeviceWrapper.h"
 
 @interface
@@ -135,27 +137,32 @@ + (void)setStartInvocations:(NSUInteger)value
 
 + (void)startWithOptions:(SentryOptions *)options
 {
-    startInvocations++;
+    // We accept the tradeoff that the SDK might not be fully initialized directly after
+    // initializing it on a background thread because scheduling the init synchronously on the main
+    // thread could lead to deadlocks.
+    [SentryThreadWrapper onMainThread:^{
+        startInvocations++;
 
-    [SentryLog configure:options.debug diagnosticLevel:options.diagnosticLevel];
+        [SentryLog configure:options.debug diagnosticLevel:options.diagnosticLevel];
 
-    SentryClient *newClient = [[SentryClient alloc] initWithOptions:options];
-    [newClient.fileManager moveAppStateToPreviousAppState];
-    [newClient.fileManager moveBreadcrumbsToPreviousBreadcrumbs];
+        SentryClient *newClient = [[SentryClient alloc] initWithOptions:options];
+        [newClient.fileManager moveAppStateToPreviousAppState];
+        [newClient.fileManager moveBreadcrumbsToPreviousBreadcrumbs];
 
-    SentryScope *scope
-        = options.initialScope([[SentryScope alloc] initWithMaxBreadcrumbs:options.maxBreadcrumbs]);
-    // The Hub needs to be initialized with a client so that closing a session
-    // can happen.
-    [SentrySDK setCurrentHub:[[SentryHub alloc] initWithClient:newClient andScope:scope]];
-    SENTRY_LOG_DEBUG(@"SDK initialized! Version: %@", SentryMeta.versionString);
-    [SentrySDK installIntegrations];
+        SentryScope *scope = options.initialScope(
+            [[SentryScope alloc] initWithMaxBreadcrumbs:options.maxBreadcrumbs]);
+        // The Hub needs to be initialized with a client so that closing a session
+        // can happen.
+        [SentrySDK setCurrentHub:[[SentryHub alloc] initWithClient:newClient andScope:scope]];
+        SENTRY_LOG_DEBUG(@"SDK initialized! Version: %@", SentryMeta.versionString);
+        [SentrySDK installIntegrations];
 
-    [SentryCrashWrapper.sharedInstance startBinaryImageCache];
-    [SentryDependencyContainer.sharedInstance.binaryImageCache start];
+        [SentryCrashWrapper.sharedInstance startBinaryImageCache];
+        [SentryDependencyContainer.sharedInstance.binaryImageCache start];
 #if TARGET_OS_IOS
-    [SentryDependencyContainer.sharedInstance.uiDeviceWrapper start];
+        [SentryDependencyContainer.sharedInstance.uiDeviceWrapper start];
 #endif
+    }];
 }
 
 + (void)startWithConfigureOptions:(void (^)(SentryOptions *options))configureOptions

Tests/SentryTests/Networking/SentryReachabilityTests.m

@@ -2,6 +2,8 @@
 #import "SentryReachability.h"
 #import <XCTest/XCTest.h>
 
+void SentryConnectivityReset(void);
+
 @interface TestSentryReachabilityObserver : NSObject <SentryReachabilityObserver>
 @end
 @implementation TestSentryReachabilityObserver
@@ -22,6 +24,7 @@ - (void)setUp
 - (void)tearDown
 {
     self.reachability = nil;
+    SentryConnectivityReset();
 }
 
 - (void)testConnectivityRepresentations

Tests/SentryTests/SentrySDKTests.swift

@@ -653,7 +653,25 @@ class SentrySDKTests: XCTestCase {
 
         XCTAssertEqual(flushTimeout, transport.flushInvocations.first)
     }
-
+
+    func testStartOnTheMainThread() {
+
+        let expectation = expectation(description: "MainThreadTestIntegration install called")
+        MainThreadTestIntegration.expectation = expectation
+
+        DispatchQueue.global(qos: .background).async {
+            SentrySDK.start { options in
+                options.integrations = [ NSStringFromClass(MainThreadTestIntegration.self) ]
+            }
+        }
+
+        wait(for: [expectation], timeout: 1.0)
+
+        let mainThreadIntegration = SentrySDK.currentHub().installedIntegrations().first { integration in integration is MainThreadTestIntegration } as? MainThreadTestIntegration
+        XCTAssertEqual(mainThreadIntegration?.installedInTheMainThread, true, "SDK is not being initialized in the main thread")
+
+    }
+
 #if SENTRY_HAS_UIKIT
 
     func testSetAppStartMeasurementConcurrently() {
@@ -773,3 +791,17 @@ class SentrySDKTests: XCTestCase {
         fixture.currentDate.setDate(date: SentryDependencyContainer.sharedInstance().dateProvider.date().addingTimeInterval(bySeconds))
     }
 }
+
+public class MainThreadTestIntegration: NSObject, SentryIntegrationProtocol {
+
+    static var expectation: XCTestExpectation?
+
+    public var installedInTheMainThread = false
+
+    public func install(with options: Options) -> Bool {
+        installedInTheMainThread = Thread.isMainThread
+        MainThreadTestIntegration.expectation?.fulfill()
+        MainThreadTestIntegration.expectation = nil
+        return true
+    }
+}

develop-docs/DECISIONS.md

@@ -127,3 +127,16 @@ This way, if these files are changed, we will be reminded to test the changes wi
 Additionally, two new 'make' commands(test-alamofire, test-homekit) are being added to the project to assist in testing the Sentry SDK in third-party projects.
 
 Related to [GH-2916](https://github.com/getsentry/sentry-cocoa/pull/2916)
+
+## Async SDK init on main thread
+
+Date: October 11th 2023
+Contributors: @philipphofmann, @brustolin
+
+We decided to initialize the SDK on the main thread async when being initialized from a background thread.
+We accept the tradeoff that the SDK might not be fully initialized directly after initializing it on a background
+thread because scheduling the init synchronously on the main thread could lead to deadlocks, such as https://github.com/getsentry/sentry-cocoa/issues/3277.
+
+Related links:
+
+- https://github.com/getsentry/sentry-cocoa/pull/3291