test(core): test Drop implementation of CPInner

Check that dropping `CheckPoint` does not cause nasty behavior
by creating a large linked list in memory and then destroying it.

Author: ValuedMammal

crates/core/src/checkpoint.rs

@@ -38,7 +38,7 @@ impl Drop for CPInner {
             // that no recursive drop calls can happen even with multiple threads.
             match Arc::try_unwrap(arc_node).ok() {
                 Some(mut node) => {
-                    // Keep goign backwards
+                    // Keep going backwards
                     current = node.prev.take();
                     // Don't call `drop` on `CPInner` since that risks it becoming recursive.
                     core::mem::forget(node);

crates/core/tests/test_checkpoint.rs

@@ -1,5 +1,5 @@
-use bdk_core::CheckPoint;
-use bdk_testenv::block_id;
+use bdk_core::{BlockId, CheckPoint};
+use bdk_testenv::{block_id, hash};
 
 /// Inserting a block that already exists in the checkpoint chain must always succeed.
 #[test]
@@ -32,3 +32,22 @@ fn checkpoint_insert_existing() {
         }
     }
 }
+
+#[test]
+fn checkpoint_destruction_is_sound() {
+    // Prior to the fix in https://github.com/bitcoindevkit/bdk/pull/1731
+    // this could have caused a stack overflow due to drop recursion in Arc.
+    // We test that a long linked list can clean itself up without blowing
+    // out the stack.
+    let mut cp = CheckPoint::new(BlockId {
+        height: 0,
+        hash: hash!("g"),
+    });
+    let end = 10_000;
+    for height in 1u32..end {
+        let hash = bitcoin::hashes::Hash::hash(height.to_be_bytes().as_slice());
+        let block = BlockId { height, hash };
+        cp = cp.push(block).unwrap();
+    }
+    assert_eq!(cp.iter().count() as u32, end);
+}