Ensure instructions belong only to one list

Fail early when adding an instruction that already belongs to a
different instruction list.

asm-tree/src/main/java/org/objectweb/asm/tree/InsnList.java

@@ -222,6 +222,7 @@ public class InsnList {
    * @param insnNode an instruction, <i>which must not belong to any {@link InsnList}</i>.
    */
   public void add(final AbstractInsnNode insnNode) {
+    ensureNotOwned(insnNode);
     ++size;
     if (lastInsn == null) {
       firstInsn = insnNode;
@@ -475,6 +476,20 @@ public class InsnList {
     }
   }
 
+  /** Ensures that the given node does not already belong to an <tt>InsnList</tt> */
+  private void ensureNotOwned(AbstractInsnNode insnNode) {
+    if (insnNode.previousInsn != null || insnNode.nextInsn != null || insnNode.index != -1) {
+      // Adding an instruction that still refers to others (in the same or another InsnList)
+      // leads to hard to debug bugs.
+      // Initially everything may look ok (e.g. iteration follows `next` thus a stale `prev`
+      // isn't noticed). However, a stale link brings the doubly-linked into disarray
+      // e.g. upon removing an element, which results in the `next` of a stale `prev` being
+      // updated, among other failure scenarios. Better fail early.
+      throw new IllegalArgumentException(
+              "Instruction " + insnNode + " belongs to another InsnList.");
+    }
+  }
+
   // Note: this class is not generified because it would create bridges.
   @SuppressWarnings("rawtypes")
   private final class InsnListIterator implements ListIterator {

asm-tree/src/test/java/org/objectweb/asm/tree/InsnListTest.java

@@ -54,6 +54,8 @@ public class InsnListTest {
 
   private InsnList list2;
 
+  private InsnList list3Unchecked;
+
   private InsnNode insn1;
 
   private InsnNode insn2;
@@ -66,6 +68,7 @@ public class InsnListTest {
     insn2 = new InsnNode(0);
     list2.add(insn1);
     list2.add(insn2);
+    list3Unchecked = new InsnList();
   }
 
   void assertEqualInsnArrays(final AbstractInsnNode[] expected, final AbstractInsnNode[] value) {
@@ -747,6 +750,11 @@ public class InsnListTest {
     assertNotSame(label, labelNode.getLabel());
   }
 
+  @Test
+  public void testAddNodeAssociatedWithAnotherList() {
+    assertThrows(IllegalArgumentException.class, () -> list3Unchecked.add(insn1));
+  }
+
   /** An InsnList which checks that its methods are properly used. */
   static class CheckedInsnList extends InsnList {