askrene: add test for reservation leaks

diff --git a/tests/test_askrene.py b/tests/test_askrene.py
index 09a669e7..7f2c84db 100644
--- a/tests/test_askrene.py
+++ b/tests/test_askrene.py
@@ -1868,6 +1868,66 @@ def test_askrene_timeout(node_factory, bitcoind):
                      final_cltv=5)
 
 
+def test_reservations_leak(node_factory, executor):
+    l1, l2, l3, l4, l5, l6 = node_factory.get_nodes(
+        6,
+        opts=[
+            {"fee-base": 0, "fee-per-satoshi": 0},
+            {"fee-base": 0, "fee-per-satoshi": 0},
+            {
+                "fee-base": 0,
+                "fee-per-satoshi": 0,
+                "plugin": os.path.join(os.getcwd(), "tests/plugins/hold_htlcs.py"),
+            },
+            {"fee-base": 0, "fee-per-satoshi": 0},
+            {"fee-base": 0, "fee-per-satoshi": 0},
+            {"fee-base": 1000, "fee-per-satoshi": 0},
+        ],
+    )
+
+    # There must be a common non-local channel in both payment paths.
+    # With a local channel we cannot trigger the reservation leak because we
+    # reserve slightly different amounts locally due to HTLC onchain costs.
+    node_factory.join_nodes([l1, l2, l4, l6, l3], wait_for_announce=True)
+    node_factory.join_nodes([l1, l2, l4, l5], wait_for_announce=True)
+
+    # Use offers instead of bolt11 because we are going to pay through a blinded
+    # path and trigger a fake channel collision between both payments.
+    offer1 = l3.rpc.offer("any")["bolt12"]
+    offer2 = l5.rpc.offer("any")["bolt12"]
+
+    inv1 = l1.rpc.fetchinvoice(offer1, "100sat")["invoice"]
+    inv2 = l1.rpc.fetchinvoice(offer2, "101sat")["invoice"]
+
+    # Initiate the first payment that has a delay.
+    fut = executor.submit(l1.rpc.xpay, (inv1))
+
+    # Wait for the first payment to reserve the path.
+    l1.daemon.wait_for_log(r"json_askrene_reserve called")
+
+    # A second payment starts.
+    l1.rpc.xpay(inv2)
+    l1.daemon.wait_for_log(r"json_askrene_unreserve called")
+
+    l3.daemon.wait_for_log(r"Holding onto an incoming htlc for 10 seconds")
+
+    # There is a payment pending therefore we expect reservations.
+    reservations = l1.rpc.askrene_listreservations()
+    assert reservations != {"reservations": []}
+
+    l3.daemon.wait_for_log(r"htlc_accepted hook called")
+    fut.result()
+    l1.daemon.wait_for_log(r"json_askrene_unreserve called")
+
+    # The first payment has finished we expect no reservations.
+    reservations = l1.rpc.askrene_listreservations()
+    assert reservations == {"reservations": []}
+
+    # We shouldn't fail askrene-unreserve. If it does it means something went
+    # wrong.
+    assert l1.daemon.is_in_log("askrene-unreserve failed") is None
+
+
 def test_askrene_reserve_clash(node_factory, bitcoind):
     """Reserves get (erroneously) counted globally by scid, even for fake scids."""
     l1 = node_factory.get_node()