8370715: JFR: Races are possible when dumping recordings

Reviewed-by: egahlin, stuefe

Author: roberttoyonaga

src/jdk.jfr/share/classes/jdk/jfr/internal/PlatformRecording.java

@@ -34,6 +34,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.channels.FileChannel;
+import java.nio.channels.FileLock;
 import java.nio.file.NoSuchFileException;
 import java.nio.file.Path;
 import java.nio.file.StandardOpenOption;
@@ -744,7 +745,14 @@ private void transferChunksWithRetry(WriteablePath path) throws IOException {
     }
 
     private void transferChunks(WriteablePath path) throws IOException {
-        try (ChunksChannel cc = new ChunksChannel(chunks); FileChannel fc = FileChannel.open(path.getReal(), StandardOpenOption.WRITE, StandardOpenOption.APPEND)) {
+        // Before writing, wipe the file if it already exists.
+        try (ChunksChannel cc = new ChunksChannel(chunks); FileChannel fc = FileChannel.open(path.getReal(), StandardOpenOption.WRITE, StandardOpenOption.TRUNCATE_EXISTING,  StandardOpenOption.CREATE)) {
+            // Mitigate races against other processes
+            FileLock l = fc.tryLock();
+            if (l == null) {
+                Logger.log(LogTag.JFR, LogLevel.INFO, "Dump operation skipped for recording \"" + name + "\" (" + id + "). File " + path.getRealPathText() + " is locked by other dump operation or activity.");
+                return;
+            }
             long bytes = cc.transferTo(fc);
             Logger.log(LogTag.JFR, LogLevel.INFO, "Transferred " + bytes + " bytes from the disk repository");
             // No need to force if no data was transferred, which avoids IOException when device is /dev/null

test/jdk/jdk/jfr/api/recording/dump/TestDumpOverwrite.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package jdk.jfr.api.recording.dump;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import jdk.jfr.Recording;
+import jdk.jfr.consumer.RecordedEvent;
+import jdk.jfr.consumer.RecordingFile;
+import jdk.test.lib.Asserts;
+import jdk.test.lib.jfr.SimpleEventHelper;
+import jdk.test.lib.jfr.Events;
+
+/**
+ * @test
+ * @summary Test that multiple dumps to the same file by ongoing recordings do not mangle data.
+ * @requires vm.flagless
+ * @requires vm.hasJFR
+ * @library /test/lib
+ * @run main/othervm jdk.jfr.api.recording.dump.TestDumpOverwrite
+ */
+public class TestDumpOverwrite {
+    private static Path DUMP_PATH = Paths.get(".", "rec_TestDumpOverwrite.jfr");
+    public static void main(String[] args) throws Exception {
+        Recording recording1 = new Recording();
+        Recording recording2 = new Recording();
+        SimpleEventHelper.enable(recording1, true);
+        SimpleEventHelper.enable(recording2, true);
+        recording1.setDestination(DUMP_PATH);
+        recording2.setDestination(DUMP_PATH);
+
+        int actualId = 0;
+        recording1.start();
+        SimpleEventHelper.createEvent(actualId++);
+        recording2.start();
+        SimpleEventHelper.createEvent(actualId++);
+        // This is results in the initial write to the dump destination
+        recording2.stop();
+        SimpleEventHelper.createEvent(actualId++);
+        recording2.close();
+        SimpleEventHelper.createEvent(actualId++);
+        // This should first wipe the data previously written by recording2.
+        recording1.stop();
+        recording1.close();
+
+        Asserts.assertTrue(Files.exists(DUMP_PATH), "Recording file does not exist: " + DUMP_PATH);
+
+        // Verify events are read in order without duplicates (otherwise chunks may be out of order).
+        // If the dump file is not being overwritten correctly, we will see event ids: 1, 0, 1, 2, 3.
+        int expectedId = 0;
+        for (RecordedEvent event : RecordingFile.readAllEvents(DUMP_PATH)) {
+            Events.assertField(event, "id").equal(expectedId++);
+        }
+        Asserts.assertTrue(expectedId == actualId, "incorrect number of events found");
+    }
+}