Prevent crash when instrumenting background URLSession tasks

Sources/Instrumentation/URLSession/URLSessionInstrumentation.swift

@@ -209,11 +209,6 @@ public class URLSessionInstrumentation {
           }
         }
         self.setIdKey(value: sessionTaskId, for: task)
-
-        // We want to identify background tasks
-        if session.configuration.identifier == nil {
-          objc_setAssociatedObject(task, "IsBackground", true, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
-        }
         return task
       }
       let swizzledIMP = imp_implementationWithBlock(
@@ -743,15 +738,12 @@ public class URLSessionInstrumentation {
       return
     }
 
-    // We cannot instrument async background tasks because they crash if you assign a delegate
-    if #available(OSX 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *) {
-      if objc_getAssociatedObject(task, "IsBackground") is Bool {
-        guard Task.basePriority == nil else {
-          return
-        }
-      }
+    // Background tasks cannot be instrumented because they crash if you assign a delegate
+    // They require the delegate to be set when creating the session
+    guard !task.isBackground else {
+      return
     }
-
+    
     let taskId = idKeyForTask(task)
     if let request = task.currentRequest {
       queue.sync {
@@ -878,3 +870,18 @@ class AsyncTaskDelegate: NSObject, URLSessionTaskDelegate {
     }
   }
 }
+
+extension URLSessionTask {
+  var isBackground: Bool {
+    [
+      "__NSCFBackgroundAVAggregateAssetDownloadTask",
+      "__NSCFBackgroundAVAggregateAssetDownloadTaskNoChildTask",
+      "__NSCFBackgroundAVAssetDownloadTask",
+      "__NSCFBackgroundDataTask",
+      "__NSCFBackgroundDownloadTask",
+      "__NSCFBackgroundSessionTask",
+      "__NSCFBackgroundUploadTask"
+    ]
+    .contains(String(describing: type(of: self)))
+  }
+}

Tests/InstrumentationTests/URLSessionTests/URLSessionInstrumentationTests.swift

@@ -1057,4 +1057,28 @@ class URLSessionInstrumentationTests: XCTestCase {
     XCTAssertEqual(attributes["server.port"]?.description, "8080")
     XCTAssertEqual(attributes["url.scheme"]?.description, "http")
   }
+
+  // MARK: - Background session tests
+  @available(macOS 10.15, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+  public func testBackgroundSession_ShouldNotCrashWhenAssigningDelegate() async throws {
+    let request = URLRequest(url: URL(string: "http://localhost:33333/success")!)
+
+    // Background sessions require a delegate to be set when creating the session.
+    // However, for this test, we set to nil so it tries to do it on resume (where the crash happens)
+    let session = URLSession(
+      configuration: URLSessionConfiguration.background(
+        withIdentifier: UUID().uuidString
+      ),
+      delegate: nil,
+      delegateQueue: .main
+    )
+
+    // Background sessions cannot use async/await or completion handlers
+    // Must use dataTask(with:) and call resume()
+    let task = session.dataTask(with: request)
+    task.resume()
+
+    // The test passes if tasks completes without crashing.
+    wait { task.state == .completed }
+  }
 }

Tests/Shared/TestUtils/XCTestCase+Wait.swift

@@ -0,0 +1,24 @@
+//
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+// 
+
+import XCTest
+
+extension XCTestCase {
+  public func wait(timeout: TimeInterval = 3, interval: TimeInterval = 0.1, until block: @escaping () throws -> Bool) {
+    let expectation = expectation(description: "wait for block to pass")
+    let timer = Timer.scheduledTimer(withTimeInterval: interval, repeats: true) { _ in
+      do {
+        if try block() {
+          expectation.fulfill()
+        }
+      } catch {
+        fatalError("Waiting for operation that threw an error: \(error)")
+      }
+    }
+
+    wait(for: [expectation], timeout: timeout)
+    timer.invalidate()
+  }
+}