fix: Make SentryScope.useSpan non-blocking

Instead of calling the callback of useSpan within the spanLock, get a
local copy of the span and pass this to the callback. This way, when the
callback is a blocking call, useSpan doesn't block other invocations of
itself and prevents potential deadlocks.

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 - Fix typo in BUILD_LIBRARY_FOR_DISTRIBUTION variable in Makefile (#3488)
 - Remove dispatch queue metadata collection to fix crash (#3522)
+- Make SentryScope.useSpan non-blocking (#3568)
 
 ## 8.18.0
 

Sources/Sentry/SentryScope.m

@@ -141,9 +141,11 @@ - (void)setSpan:(nullable id<SentrySpan>)span
 
 - (void)useSpan:(SentrySpanCallback)callback
 {
+    id<SentrySpan> localSpan = nil;
     @synchronized(_spanLock) {
-        callback(_span);
+        localSpan = _span;
     }
+    callback(localSpan);
 }
 
 - (void)clear

Tests/SentryTests/SentryScopeSwiftTests.swift

@@ -260,6 +260,49 @@ class SentryScopeSwiftTests: XCTestCase {
         }
     }
 
+    func testUseSpanLock_DoesNotBlock_WithBlockingCallback() {
+        let scope = fixture.scope
+        let queue = DispatchQueue(label: "test-queue", attributes: [.initiallyInactive, .concurrent])
+        let expect = expectation(description: "useSpan callback is non-blocking")
+
+        let condition = NSCondition()
+        var useSpanCalled = false
+
+        queue.async {
+            scope.useSpan { _ in
+                condition.lock()
+                while !useSpanCalled {
+                    condition.wait()
+                }
+                condition.unlock()
+            }
+        }
+
+        queue.async {
+            scope.useSpan { _ in
+                useSpanCalled = true
+                condition.broadcast()
+                expect.fulfill()
+            }
+        }
+
+        queue.activate()
+
+        wait(for: [expect], timeout: 0.1)
+    }
+
+    func testUseSpanLock_IsReentrant() {
+        let expect = expectation(description: "finish on time")
+        let scope = fixture.scope
+        scope.useSpan { _ in
+            scope.useSpan { _ in
+                expect.fulfill()
+            }
+
+        }
+        wait(for: [expect], timeout: 0.1)
+    }
+
     func testUseSpanForClear() {
         fixture.scope.span = fixture.transaction
         fixture.scope.useSpan { (_) in