Replace the entire observers list when adding/removing observers in o…

Bugsnag/Metadata/BugsnagMetadata.m

@@ -31,7 +31,7 @@
 #import "BugsnagStateEvent.h"
 
 @interface BugsnagMetadata ()
-@property(nonatomic, readwrite, strong) NSMutableArray *stateEventBlocks;
+@property(atomic, readwrite, strong) NSMutableArray *stateEventBlocks;
 @end
 
 @implementation BugsnagMetadata
@@ -102,11 +102,17 @@ - (void)notifyObservers {
 }
 
 - (void)addObserverWithBlock:(BugsnagObserverBlock _Nonnull)block {
-    [self.stateEventBlocks addObject:[block copy]];
+    // Make a copy to avoid concurrency issues
+    NSMutableArray *newStateEventBlocks = [self.stateEventBlocks mutableCopy];
+    [newStateEventBlocks addObject:[block copy]];
+    self.stateEventBlocks = newStateEventBlocks;
 }
 
 - (void)removeObserverWithBlock:(BugsnagObserverBlock _Nonnull)block {
-    [self.stateEventBlocks removeObject:block];
+    // Make a copy to avoid concurrency issues
+    NSMutableArray *newStateEventBlocks = [self.stateEventBlocks mutableCopy];
+    [newStateEventBlocks removeObject:block];
+    self.stateEventBlocks = newStateEventBlocks;
 }
 
 // MARK: - <NSMutableCopying>

CHANGELOG.md

@@ -5,6 +5,9 @@ Changelog
 
 ### Bug fixes
 
+* Copy the metadata observer list rather than mutating it directly.
+  [796](https://github.com/bugsnag/bugsnag-cocoa/pull/796)
+
 * Reorganized the project file
   [793](https://github.com/bugsnag/bugsnag-cocoa/pull/793)
 

Tests/BugsnagMetadataTests.m

@@ -328,4 +328,38 @@ - (void)testSanitizeNestedArrayDict {
     XCTAssertEqual(0, [nestedDict count]);
 }
 
+- (void)testConcurrentObserverAccess {
+    BugsnagMetadata *metadata = [[BugsnagMetadata alloc] initWithDictionary:@{
+            @"foo": @{
+                    @"bar": @[
+                            @[
+                                    @{ @"custom": [NSNull null] }
+                            ]
+                    ]
+            }
+    }];
+
+    BugsnagObserverBlock firstObserver = ^(BugsnagStateEvent *_Nonnull event){};
+    BugsnagObserverBlock secondObserver = ^(BugsnagStateEvent *_Nonnull event){};
+
+    [metadata addObserverWithBlock:firstObserver];
+
+    bool threadShouldQuit = false;
+    if (@available(iOS 10.0, tvOS 10.0, *)) {
+        [NSThread detachNewThreadWithBlock:^{
+            while(!threadShouldQuit) {
+                [metadata addObserverWithBlock:secondObserver];
+                [metadata removeObserverWithBlock:secondObserver];
+            }
+        }];
+    } else {
+        // Fallback on earlier versions
+    }
+
+    for(int i = 0; i < 10000; i++) {
+        [metadata addMetadata:[NSNumber numberWithInt:i] withKey:@"bar" toSection:@"foo"];
+    }
+    threadShouldQuit = true;
+}
+
 @end