Support round-tripping non trivial glue

class-inject/src/main/java/datadog/instrument/classinject/ClassInjector.java

@@ -290,7 +290,8 @@ public void visitCode() {
         mv.visitLdcInsn(DefineClassGlue.V8);
       }
 
-      // unpack the UTF-16BE encoded string back into bytecode
+      // unpack the UTF-16BE encoded glue string back into bytecode with a single JDK call
+      // note: this approach is safe because we know the string has no unpaired surrogates
       mv.visitFieldInsn(
           GETSTATIC, "java/nio/charset/StandardCharsets", "UTF_16BE", "Ljava/nio/charset/Charset;");
       mv.visitMethodInsn(

class-inject/src/test/java/datadog/instrument/glue/DefineClassGlueGeneratorTest.java

@@ -1,5 +1,6 @@
 package datadog.instrument.glue;
 
+import static java.nio.charset.StandardCharsets.UTF_16BE;
 import static org.assertj.core.api.Assertions.assertThat;
 
 import java.io.PrintWriter;
@@ -22,4 +23,21 @@ void generatedBytecodePassesVerification(String unsafeNamespace) {
 
     assertThat(errors.toString()).isEmpty();
   }
+
+  /**
+   * ClassInjector decodes this glue via a raw {@code getBytes(UTF_16BE)} call; confirm the
+   * generated bytecode packs cleanly into surrogate pairs it can safely round-trip.
+   */
+  @ParameterizedTest
+  @ValueSource(strings = {"sun/misc", "jdk/internal/misc"})
+  void generatedBytecodeSurvivesRawUtf16beDecode(String unsafeNamespace) {
+    byte[] originalBytecode = DefineClassGlueGenerator.generateBytecode(unsafeNamespace);
+    String packedBytecode =
+        "sun/misc".equals(unsafeNamespace) ? DefineClassGlue.V8 : DefineClassGlue.V9;
+
+    // mirrors exactly what ClassInjector's generated bytecode does at runtime
+    byte[] decodedBytecode = packedBytecode.getBytes(UTF_16BE);
+
+    assertThat(decodedBytecode).isEqualTo(originalBytecode);
+  }
 }

utils/src/main/java/datadog/instrument/glue/Glue.java

@@ -6,7 +6,6 @@
 
 package datadog.instrument.glue;
 
-import static java.nio.charset.StandardCharsets.UTF_16BE;
 import static java.util.Objects.requireNonNull;
 
 import datadog.instrument.utils.JVM;
@@ -26,7 +25,14 @@ private Glue() {}
    * @return the unpacked bytecode
    */
   public static byte[] unpackBytecode(String bytecode) {
-    return bytecode.getBytes(UTF_16BE);
+    int len = bytecode.length();
+    byte[] unpacked = new byte[len << 1];
+    for (int i = 0, j = 0; i < len; i++) {
+      char c = bytecode.charAt(i);
+      unpacked[j++] = (byte) (c >>> 8);
+      unpacked[j++] = (byte) c;
+    }
+    return unpacked;
   }
 
   /**

utils/src/test/java/datadog/instrument/glue/GlueTest.java

@@ -27,4 +27,13 @@ void missingGlueResource() {
     assertThrows(
         MissingResourceException.class, () -> Glue.loadBytecode(GlueTest.class, "missing.glue"));
   }
+
+  @Test
+  void unpackBytecodePreservesUnpairedSurrogates() {
+    // getBytes(UTF_16BE) would replace this lone surrogate with U+FFFD instead of preserving it
+    String packed = new String(new char[] {0x0102, 0xD800, 0x0304});
+    byte[] expected = {0x01, 0x02, (byte) 0xD8, 0x00, 0x03, 0x04};
+
+    assertArrayEquals(expected, Glue.unpackBytecode(packed));
+  }
 }