From abc96214fa7c8d3401935d03b2415012651ad309 Mon Sep 17 00:00:00 2001
From: Nick Dowell <nick@bugsnag.com>
Date: Wed, 4 Aug 2021 17:18:34 +0100
Subject: [PATCH] Fix ThreadSanitizer data race in BugsnagBreadcrumbs

---
 Bugsnag/Breadcrumbs/BugsnagBreadcrumbs.m | 7 +++++--
 CHANGELOG.md                             | 7 +++++++
 Tests/BugsnagBreadcrumbsTest.m           | 4 ++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Bugsnag/Breadcrumbs/BugsnagBreadcrumbs.m b/Bugsnag/Breadcrumbs/BugsnagBreadcrumbs.m
index e1ade0277..1fcce3642 100644
--- a/Bugsnag/Breadcrumbs/BugsnagBreadcrumbs.m
+++ b/Bugsnag/Breadcrumbs/BugsnagBreadcrumbs.m
@@ -157,8 +157,11 @@ - (void)addBreadcrumbWithData:(NSData *)data writeToDisk:(BOOL)writeToDisk {
         dispatch_async(BSGGlobalsFileSystemQueue(), ^{
             // Avoid writing breadcrumbs that have already been deleted from the in-memory store.
             // This can occur when breadcrumbs are being added faster than they can be written.
-            unsigned int nextFileNumber = self.nextFileNumber;
-            BOOL isStale = (self.maxBreadcrumbs < nextFileNumber) && (fileNumber < (nextFileNumber - self.maxBreadcrumbs));
+            BOOL isStale;
+            @synchronized (self) {
+                unsigned int nextFileNumber = self.nextFileNumber;
+                isStale = (self.maxBreadcrumbs < nextFileNumber) && (fileNumber < (nextFileNumber - self.maxBreadcrumbs));
+            }
             
             NSError *error = nil;
             
diff --git a/CHANGELOG.md b/CHANGELOG.md
index a6462f6f1..2d2ec6144 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,13 @@
 Changelog
 =========
 
+## TBD
+
+### Bug fixes
+
+* Fix ThreadSanitizer data race in `BugsnagBreadcrumbs`.
+  [#1160](https://github.com/bugsnag/bugsnag-cocoa/pull/1160)
+
 ## 6.10.3 (2021-08-04)
 
 ### Bug fixes
diff --git a/Tests/BugsnagBreadcrumbsTest.m b/Tests/BugsnagBreadcrumbsTest.m
index efbe90fb5..52abe2ca8 100644
--- a/Tests/BugsnagBreadcrumbsTest.m
+++ b/Tests/BugsnagBreadcrumbsTest.m
@@ -451,6 +451,10 @@ - (void)testCrashReportWriter {
 }
 
 - (void)testCrashReportWriterConcurrency {
+#if defined(__has_feature) && __has_feature(thread_sanitizer)
+    NSLog(@"Skipping test because ThreadSanitizer deadlocks if other threads are suspended");
+    return;
+#endif
     //
     // The aim of this test is to ensure that BugsnagBreadcrumbsWriteCrashReport will insert only valid JSON
     // into a crash report when other threads are (paused while) updating the breadcrumbs linked list.