From a1d650a2b9903937bd90f3e2b1d5f7abf4993020 Mon Sep 17 00:00:00 2001
From: Stuart McCulloch <stuart.mcculloch@datadoghq.com>
Date: Sat, 17 Oct 2026 15:15:57 +0100
Subject: [PATCH] Generate a zero-branch root for empty tries

Encode the empty root sentinel when generating Java source from an empty
mapping, so nonempty lookups return no match instead of reading past an
empty array.

Generate and compile an empty trie in a regression test and exercise
both lookup overloads.
---
 .../instrument/utils/ClassNameTrie.java       |  3 +++
 .../instrument/utils/ClassNameTrieTest.java   | 22 +++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/utils/src/main/java/datadog/instrument/utils/ClassNameTrie.java b/utils/src/main/java/datadog/instrument/utils/ClassNameTrie.java
index dc1bccf..5e3f617 100644
--- a/utils/src/main/java/datadog/instrument/utils/ClassNameTrie.java
+++ b/utils/src/main/java/datadog/instrument/utils/ClassNameTrie.java
@@ -1058,6 +1058,9 @@ public static boolean generateJavaTrie(
       int chunkSize = 0;
       StringBuilder buf = new StringBuilder();
       buf.append("      \"");
+      if (trie.isEmpty()) {
+        buf.append("\\000"); // encode an empty root node with zero branches
+      }
       for (int i = 0; i < trie.trieLength; i++) {
         char c = trie.trieData[i];
         if (++chunkSize > 10_000) {
diff --git a/utils/src/test/java/datadog/instrument/utils/ClassNameTrieTest.java b/utils/src/test/java/datadog/instrument/utils/ClassNameTrieTest.java
index 722202c..43e48be 100644
--- a/utils/src/test/java/datadog/instrument/utils/ClassNameTrieTest.java
+++ b/utils/src/test/java/datadog/instrument/utils/ClassNameTrieTest.java
@@ -166,6 +166,28 @@ void generateJavaSource(@TempDir File classesDir) throws Exception {
     assertEquals(5, trieApply.invoke(null, "com.foobar.Five"));
   }
 
+  @Test
+  void generateEmptyJavaSource(@TempDir File classesDir) throws Exception {
+    Files.createFile(new File(classesDir, "empty.trie").toPath());
+    ClassNameTrie.JavaGenerator.main(
+        new String[] {classesDir.toString(), classesDir.toString(), "empty.trie"});
+
+    assertEquals(
+        0,
+        getSystemJavaCompiler()
+            .run(null, null, null, new File(classesDir, "EmptyTrie.java").toString()));
+
+    try (URLClassLoader cl = URLClassLoader.newInstance(new URL[] {classesDir.toURI().toURL()})) {
+      Class<?> trieClass = cl.loadClass("EmptyTrie");
+      Method apply = trieClass.getMethod("apply", String.class);
+      Method applyFromIndex = trieClass.getMethod("apply", String.class, int.class);
+      for (String key : new String[] {"", "a", "com.example.Type"}) {
+        assertEquals(-1, apply.invoke(null, key));
+        assertEquals(-1, applyFromIndex.invoke(null, "prefix/" + key, 7));
+      }
+    }
+  }
+
   @Test
   @SuppressWarnings("UnnecessaryUnicodeEscape")
   void manual() throws IOException {