Fix TraceFileObserver NPE when trace file already exists

modules/nextflow/src/main/groovy/nextflow/trace/TraceFileObserver.groovy

@@ -171,7 +171,13 @@ class TraceFileObserver implements TraceObserverV2 {
             Files.createDirectories(parent)
 
         // create a new trace file
-        traceFile = new PrintWriter(TraceHelper.newFileWriter(tracePath,overwrite, 'Trace'))
+        try {
+            traceFile = new PrintWriter(TraceHelper.newFileWriter(tracePath, overwrite, 'Trace'))
+        }
+        catch (Exception e) {
+            log.warn "Failed to create trace file: ${tracePath.toUriString()} -- ${e.message}"
+            return
+        }
 
         // launch the agent
         writer = new Agent<PrintWriter>(traceFile)
@@ -186,12 +192,14 @@ class TraceFileObserver implements TraceObserverV2 {
         log.debug "Workflow completed -- saving trace file"
 
         // wait for termination and flush the agent content
-        writer.await()
+        writer?.await()
 
         // write the remaining records
-        current.values().each { record -> traceFile.println(render(record)) }
-        traceFile.flush()
-        traceFile.close()
+        if( traceFile ) {
+            current.values().each { record -> traceFile.println(render(record)) }
+            traceFile.flush()
+            traceFile.close()
+        }
     }
 
     @Override
@@ -216,7 +224,7 @@ class TraceFileObserver implements TraceObserverV2 {
         current.remove(taskId)
 
         // save to the file
-        writer.send { PrintWriter it ->
+        writer?.send { PrintWriter it ->
             it.println(render(event.trace))
             it.flush()
         }
@@ -230,7 +238,7 @@ class TraceFileObserver implements TraceObserverV2 {
         }
 
         // save to the file
-        writer.send { PrintWriter it ->
+        writer?.send { PrintWriter it ->
             it.println(render(event.trace))
             it.flush()
         }

modules/nextflow/src/test/groovy/nextflow/trace/TraceFileObserverTest.groovy

@@ -240,6 +240,49 @@ class TraceFileObserverTest extends Specification {
 
     }
 
+    def 'should degrade gracefully when trace file already exists'() {
+
+        given:
+        def testFolder = Files.createTempDirectory('trace-dir')
+        def file = testFolder.resolve('trace')
+        file.text = 'existing content'  // pre-create the file so newFileWriter fails
+
+        // the handler
+        def task = new TaskRun(id:TaskId.of(1), name:'test_task', hash: CacheHelper.hasher(1).hash(), config: new TaskConfig())
+        task.processor = Mock(TaskProcessor)
+        task.processor.getSession() >> new Session()
+        task.processor.getName() >> 'x'
+        task.processor.getExecutor() >> Mock(Executor)
+        task.processor.getProcessEnvironment() >> [:]
+
+        def handler = new NopeTaskHandler(task)
+        handler.status = TaskStatus.COMPLETED
+
+        // observer with overwrite=false (the default)
+        def config = new TraceConfig(file: file.toString())
+        def observer = new TraceFileObserver(config)
+
+        when: 'onFlowCreate fails to create the file'
+        observer.onFlowCreate(null)
+        then: 'writer and traceFile remain null'
+        observer.@writer == null
+        observer.@traceFile == null
+
+        when: 'task events and flow completion should not throw NPE'
+        observer.onTaskSubmit( new TaskEvent(handler, handler.getTraceRecord()) )
+        observer.onTaskComplete( new TaskEvent(handler, handler.getTraceRecord()) )
+        observer.onTaskCached( new TaskEvent(handler, handler.getTraceRecord()) )
+        observer.onFlowComplete()
+        then:
+        noExceptionThrown()
+
+        and: 'the original file content is unchanged'
+        file.text == 'existing content'
+
+        cleanup:
+        testFolder.deleteDir()
+    }
+
     def 'should create a record in the trace file'() {
 
         given: