Fix attrs_data overflow, stale sidecar encodings, and JfrMetadata reinit

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: jbachorik

ddprof-lib/src/main/cpp/jfrMetadata.cpp

@@ -26,6 +26,16 @@ bool JfrMetadata::_initialized = false;
 
 JfrMetadata::JfrMetadata() : Element("root") {}
 
+void JfrMetadata::reset() {
+  _root._children.clear();
+  _root._attributes.clear();
+  _strings.clear();
+  _string_map.clear();
+  // Re-register "root" at ID 0 so _root._name (const 0) stays valid
+  getId("root");
+  _initialized = false;
+}
+
 void JfrMetadata::initialize(
     const std::vector<std::string> &contextAttributes) {
   if (_initialized) {

ddprof-lib/src/main/cpp/jfrMetadata.h

@@ -259,6 +259,7 @@ class JfrMetadata : Element {
   JfrMetadata();
 
   static void initialize(const std::vector<std::string> &contextAttributes);
+  static void reset();
 
   static Element *root() { return &_root; }
 

ddprof-lib/src/main/cpp/profiler.cpp

@@ -1448,6 +1448,7 @@ Error Profiler::start(Arguments &args, bool reset) {
   // Always enable library trap to catch wasmtime loading and patch its broken sigaction
   switchLibraryTrap(true);
 
+  JfrMetadata::reset();
   JfrMetadata::initialize(args._context_attributes);
   _num_context_attributes = args._context_attributes.size();
   error = _jfr.start(args, reset);

ddprof-lib/src/main/java/com/datadoghq/profiler/ThreadContext.java

@@ -100,6 +100,16 @@ public ThreadContext(ByteBuffer recordBuffer, ByteBuffer tlsPtrBuffer, ByteBuffe
             throw new UnsupportedOperationException(
                 "ByteBuffer context path requires little-endian platform");
         }
+        // Zero sidecar + record to prevent stale encodings from a previous profiler session.
+        // The native ProfiledThread survives across sessions, so the sidecar may hold
+        // old tag encodings and the record may hold old attrs_data.
+        for (int i = 0; i < MAX_CUSTOM_SLOTS; i++) {
+            this.sidecarBuffer.putInt(i * 4, 0);
+        }
+        this.sidecarBuffer.putLong(this.lrsSidecarOffset, 0);
+        this.recordBuffer.put(this.validOffset, (byte) 0);
+        this.recordBuffer.putShort(this.attrsDataSizeOffset, (short) 0);
+        this.tlsPtrBuffer.putLong(0, 0);
     }
 
     /**
@@ -219,9 +229,9 @@ private boolean setContextAttributeDirect(int keyIndex, String value) {
         // otepKeyIndex is offset by 1: index 0 is reserved for LRS
         int otepKeyIndex = keyIndex + 1;
         detach();
-        replaceOtepAttribute(otepKeyIndex, utf8);
+        boolean written = replaceOtepAttribute(otepKeyIndex, utf8);
         attach();
-        return true;
+        return written;
     }
 
     /**
@@ -305,7 +315,7 @@ private void attach() {
      * Replace or insert an attribute in attrs_data. Record must be detached.
      * Writes the pre-encoded UTF-8 bytes into the record.
      */
-    private void replaceOtepAttribute(int otepKeyIndex, byte[] utf8) {
+    private boolean replaceOtepAttribute(int otepKeyIndex, byte[] utf8) {
         int currentSize = compactOtepAttribute(otepKeyIndex);
         int valueLen = Math.min(utf8.length, 255);
         int entrySize = 2 + valueLen;
@@ -317,8 +327,11 @@ private void replaceOtepAttribute(int otepKeyIndex, byte[] utf8) {
                 recordBuffer.put(base + 2 + i, utf8[i]);
             }
             currentSize += entrySize;
+            recordBuffer.putShort(attrsDataSizeOffset, (short) currentSize);
+            return true;
         }
         recordBuffer.putShort(attrsDataSizeOffset, (short) currentSize);
+        return false;
     }
 
     /** Remove an attribute from attrs_data by compacting. Record must be detached. */

ddprof-test/src/test/java/com/datadoghq/profiler/AbstractProfilerTest.java

@@ -323,6 +323,7 @@ protected void runTests(Runnable... runnables) throws InterruptedException {
   public final void stopProfiler() {
     if (!stopped) {
       profiler.stop();
+      profiler.resetThreadContext();
       stopped = true;
       checkConfig();
     }

ddprof-test/src/test/java/com/datadoghq/profiler/context/OtelContextStorageModeTest.java

@@ -50,6 +50,7 @@ public static void setup() throws IOException {
     public void cleanup() {
         if (profilerStarted) {
             profiler.stop();
+            profiler.resetThreadContext();
             profilerStarted = false;
         }
     }
@@ -62,7 +63,7 @@ public void cleanup() {
     public void testOtelStorageModeContext() throws Exception {
         Path jfrFile = Files.createTempFile("otel-ctx-otel", ".jfr");
 
-        profiler.execute(String.format("start,cpu=1ms,jfr,file=%s", jfrFile.toAbsolutePath()));
+        profiler.execute(String.format("start,cpu=1ms,attributes=tag1;tag2;tag3,jfr,file=%s", jfrFile.toAbsolutePath()));
         profilerStarted = true;
 
         long localRootSpanId = 0x1111222233334444L;
@@ -83,7 +84,7 @@ public void testOtelStorageModeContext() throws Exception {
     public void testOtelModeStartsOnAnyPlatform() throws Exception {
         Path jfrFile = Files.createTempFile("otel-ctx-any", ".jfr");
 
-        profiler.execute(String.format("start,cpu=1ms,jfr,file=%s", jfrFile.toAbsolutePath()));
+        profiler.execute(String.format("start,cpu=1ms,attributes=tag1;tag2;tag3,jfr,file=%s", jfrFile.toAbsolutePath()));
         profilerStarted = true;
 
         // Context operations should not crash
@@ -97,7 +98,7 @@ public void testOtelModeStartsOnAnyPlatform() throws Exception {
     public void testOtelModeClearContext() throws Exception {
         Path jfrFile = Files.createTempFile("otel-ctx-clear", ".jfr");
 
-        profiler.execute(String.format("start,cpu=1ms,jfr,file=%s", jfrFile.toAbsolutePath()));
+        profiler.execute(String.format("start,cpu=1ms,attributes=tag1;tag2;tag3,jfr,file=%s", jfrFile.toAbsolutePath()));
         profilerStarted = true;
 
         profiler.setContext(0xCAFEBABEL, 0xDEADBEEFL, 0L, 0x12345678L);
@@ -116,7 +117,7 @@ public void testOtelModeClearContext() throws Exception {
     public void testOtelModeCustomAttributes() throws Exception {
         Path jfrFile = Files.createTempFile("otel-ctx-attrs", ".jfr");
 
-        profiler.execute(String.format("start,cpu=1ms,contextattribute=http.route;db.system,jfr,file=%s", jfrFile.toAbsolutePath()));
+        profiler.execute(String.format("start,cpu=1ms,attributes=http.route;db.system,jfr,file=%s", jfrFile.toAbsolutePath()));
         profilerStarted = true;
 
         // Register attribute keys
@@ -145,7 +146,7 @@ public void testOtelModeCustomAttributes() throws Exception {
     public void testSetContextAttribute() throws Exception {
         Path jfrFile = Files.createTempFile("otel-ctx-attr", ".jfr");
 
-        profiler.execute(String.format("start,cpu=1ms,contextattribute=http.route,jfr,file=%s", jfrFile.toAbsolutePath()));
+        profiler.execute(String.format("start,cpu=1ms,attributes=http.route,jfr,file=%s", jfrFile.toAbsolutePath()));
         profilerStarted = true;
 
         profiler.setContext(0x456L, 0x123L, 0L, 0x789L);
@@ -162,7 +163,7 @@ public void testSetContextAttribute() throws Exception {
     public void testOtelModeAttributeOverflow() throws Exception {
         Path jfrFile = Files.createTempFile("otel-ctx-overflow", ".jfr");
 
-        profiler.execute(String.format("start,cpu=1ms,contextattribute=k0;k1;k2;k3;k4,jfr,file=%s", jfrFile.toAbsolutePath()));
+        profiler.execute(String.format("start,cpu=1ms,attributes=k0;k1;k2;k3;k4,jfr,file=%s", jfrFile.toAbsolutePath()));
         profilerStarted = true;
 
         OTelContext.getInstance().registerAttributeKeys("k0", "k1", "k2", "k3", "k4");
@@ -171,8 +172,8 @@ public void testOtelModeAttributeOverflow() throws Exception {
 
         ThreadContext ctx = profiler.getThreadContext();
 
-        // Fill up attrs_data with long values (255 bytes each = 257 bytes per entry)
-        // 612 / 257 = 2 full entries, third should fail
+        // LRS "2" occupies 3 bytes (key=1, len=1, value=1) in attrs_data.
+        // Each 255-char attr = 257 bytes per entry. Two fit; third overflows.
         StringBuilder sb = new StringBuilder(255);
         for (int i = 0; i < 255; i++) sb.append('x');
         String longValue = sb.toString();