tracing: pass if replaced by tx/pkg to tracepoint

The mempool:replaced tracepoint now reports either a txid or a
package hash (previously it always was a txid). To let users know
if a txid or package hash is passed, a boolean argument is added
the the tracepoint.

In the functional test, a ctypes.Structure class for MempoolReplaced
is introduced as Python warns the following when not explcitly
casting it to a ctype:

  Type: 'bool' not recognized. Please define the data with ctypes manually.

doc/tracing.md

@@ -245,14 +245,15 @@ Arguments passed:
 2. Replaced transaction virtual size as `int32`
 3. Replaced transaction fee as `int64`
 4. Replaced transaction mempool entry time (epoch) as `uint64`
-5. Replacement transaction ID (hash) as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
+5. Replacement transaction ID or package hash as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
 6. Replacement transaction virtual size as `int32`
 7. Replacement transaction fee as `int64`
+8. `bool` indicating if the argument 5. is a transaction ID or package hash (true if it's a transaction ID)
 
-Note: In cases where a single replacement transaction replaces multiple
+Note: In cases where a replacement transaction or package replaces multiple
 existing transactions in the mempool, the tracepoint is called once for each
-replaced transaction, with data of the replacement transaction being the same
-in each call.
+replaced transaction, with data of the replacement transaction or package
+being the same in each call.
 
 #### Tracepoint `mempool:rejected`
 

src/validation.cpp

@@ -1299,7 +1299,8 @@ void MemPoolAccept::FinalizeSubpackage(const ATMPArgs& args)
                                       it->GetTxSize());
         FeeFrac feerate{m_subpackage.m_total_modified_fees, int32_t(m_subpackage.m_total_vsize)};
         uint256 tx_or_package_hash{};
-        if (m_subpackage.m_changeset->GetTxCount() == 1) {
+        const bool replaced_with_tx{m_subpackage.m_changeset->GetTxCount() == 1};
+        if (replaced_with_tx) {
             const CTransaction& tx = m_subpackage.m_changeset->GetAddedTxn(0);
             tx_or_package_hash = tx.GetHash();
             log_string += strprintf("New tx %s (wtxid=%s, fees=%s, vsize=%s)",
@@ -1324,7 +1325,8 @@ void MemPoolAccept::FinalizeSubpackage(const ATMPArgs& args)
                 std::chrono::duration_cast<std::chrono::duration<std::uint64_t>>(it->GetTime()).count(),
                 tx_or_package_hash.data(),
                 feerate.size,
-                feerate.fee
+                feerate.fee,
+                replaced_with_tx
         );
         m_subpackage.m_replaced_transactions.push_back(it->GetSharedTx());
     }

test/functional/interface_usdt_mempool.py

@@ -7,6 +7,7 @@
      See https://github.com/bitcoin/bitcoin/blob/master/doc/tracing.md#context-mempool
 """
 
+import ctypes
 from decimal import Decimal
 
 # Test will be skipped if we don't have bcc installed
@@ -63,6 +64,7 @@ struct replaced_event
   u8    replacement_hash[HASH_LENGTH];
   s32   replacement_vsize;
   s64   replacement_fee;
+  bool  replaced_by_transaction;
 };
 
 // BPF perf buffer to push the data to user space.
@@ -115,6 +117,7 @@ int trace_replaced(struct pt_regs *ctx) {
   bpf_usdt_readarg_p(5, ctx, &replaced.replacement_hash, HASH_LENGTH);
   bpf_usdt_readarg(6, ctx, &replaced.replacement_vsize);
   bpf_usdt_readarg(7, ctx, &replaced.replacement_fee);
+  bpf_usdt_readarg(8, ctx, &replaced.replaced_by_transaction);
 
   replaced_events.perf_submit(ctx, &replaced, sizeof(replaced));
   return 0;
@@ -123,6 +126,19 @@ int trace_replaced(struct pt_regs *ctx) {
 """
 
 
+class MempoolReplaced(ctypes.Structure):
+    _fields_ = [
+        ("replaced_hash", ctypes.c_ubyte * 32),
+        ("replaced_vsize", ctypes.c_int32),
+        ("replaced_fee", ctypes.c_int64),
+        ("replaced_entry_time", ctypes.c_uint64),
+        ("replacement_hash", ctypes.c_ubyte * 32),
+        ("replacement_vsize", ctypes.c_int32),
+        ("replacement_fee", ctypes.c_int64),
+        ("replaced_by_transaction", ctypes.c_bool),
+    ]
+
+
 class MempoolTracepointTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
@@ -230,7 +246,8 @@ class MempoolTracepointTest(BitcoinTestFramework):
         bpf = BPF(text=MEMPOOL_TRACEPOINTS_PROGRAM, usdt_contexts=[ctx], debug=0, cflags=["-Wno-error=implicit-function-declaration"])
 
         def handle_replaced_event(_, data, __):
-            events.append(bpf["replaced_events"].event(data))
+            event = ctypes.cast(data, ctypes.POINTER(MempoolReplaced)).contents
+            events.append(event)
 
         bpf["replaced_events"].open_perf_buffer(handle_replaced_event)
 
@@ -261,6 +278,7 @@ class MempoolTracepointTest(BitcoinTestFramework):
         assert_equal(bytes(event.replacement_hash)[::-1].hex(), replacement_tx["txid"])
         assert_equal(event.replacement_vsize, replacement_tx["tx"].get_vsize())
         assert_equal(event.replacement_fee, replacement_fee)
+        assert_equal(event.replaced_by_transaction, True)
 
         bpf.cleanup()
         self.generate(self.wallet, 1)