Merge bitcoin/bitcoin#27832: doc: Clarify -datacarriersize, add -datacarriersize=2 tests

faafc35a779745d59fdb0e88698b579215f42b08 doc: Clarify that -datacarriersize applies to the full raw scriptPubKey, not the data push (MarcoFalke)
55550e7fe7e4ffe14637a901b568d1d7e1c716d4 test: Add -datacarriersize=2 tests (MarcoFalke)

Pull request description:

  Clarify with a test that `-datacarriersize` applies to the serialized size of the scriptPubKey, not the size of the pushed data. So for example,

  * `-datacarriersize=2` will reject a `raw(6a01aa)`, even though only one byte is pushed
  * `-datacarriersize=0` (or `-datacarrier=0`) will reject a `raw(6a)`, even though no byte is pushed
  * `-datacarriersize=0` (or `-datacarrier=0`) will reject a `raw(6a00)`, even though zero bytes are pushed

ACKs for top commit:
  ajtowns:
    ACK faafc35a779745d59fdb0e88698b579215f42b08
  instagibbs:
    ACK https://github.com/bitcoin/bitcoin/pull/27832/commits/faafc35a779745d59fdb0e88698b579215f42b08

Tree-SHA512: f01ace02798f596ac2a02461e9f2a6ef91b3b37c976ea0b3bc860e2d3efb0ace0fd8b779dd18249cee7f84ebbe5fd21d8506afd3a15edadc00b843ff3b4aacc7

2 files changed, 31 insertions, 5 deletions

diff --git a/src/init.cpp b/src/init.cpp
index c11f100139..4d526bd0de 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -590,7 +590,11 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-acceptstalefeeestimates", strprintf("Read fee estimates even if they are stale (%sdefault: %u) fee estimates are considered stale if they are %s hours old", "regtest only; ", DEFAULT_ACCEPT_STALE_FEE_ESTIMATES, Ticks<std::chrono::hours>(MAX_FILE_AGE)), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
     argsman.AddArg("-bytespersigop", strprintf("Equivalent bytes per sigop in transactions for relay and mining (default: %u)", DEFAULT_BYTES_PER_SIGOP), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarrier", strprintf("Relay and mine data carrier transactions (default: %u)", DEFAULT_ACCEPT_DATACARRIER), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
-    argsman.AddArg("-datacarriersize", strprintf("Maximum size of data in data carrier transactions we relay and mine (default: %u)", MAX_OP_RETURN_RELAY), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-datacarriersize",
+                   strprintf("Relay and mine transactions whose data-carrying raw scriptPubKey "
+                             "is of this size or less (default: %u)",
+                             MAX_OP_RETURN_RELAY),
+                   ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-mempoolfullrbf", strprintf("Accept transaction replace-by-fee without requiring replaceability signaling (default: %u)", DEFAULT_MEMPOOL_FULL_RBF), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-permitbaremultisig", strprintf("Relay non-P2SH multisig (default: %u)", DEFAULT_PERMIT_BAREMULTISIG), ArgsManager::ALLOW_ANY,
                    OptionsCategory::NODE_RELAY);
diff --git a/test/functional/mempool_datacarrier.py b/test/functional/mempool_datacarrier.py
index c370d8fa91..951bf37ae8 100755
--- a/test/functional/mempool_datacarrier.py
+++ b/test/functional/mempool_datacarrier.py
@@ -22,16 +22,18 @@ from test_framework.wallet import MiniWallet
 
 class DataCarrierTest(BitcoinTestFramework):
     def set_test_params(self):
-        self.num_nodes = 3
+        self.num_nodes = 4
         self.extra_args = [
             [],
             ["-datacarrier=0"],
-            ["-datacarrier=1", f"-datacarriersize={MAX_OP_RETURN_RELAY - 1}"]
+            ["-datacarrier=1", f"-datacarriersize={MAX_OP_RETURN_RELAY - 1}"],
+            ["-datacarrier=1", f"-datacarriersize=2"],
         ]
 
-    def test_null_data_transaction(self, node: TestNode, data: bytes, success: bool) -> None:
+    def test_null_data_transaction(self, node: TestNode, data, success: bool) -> None:
         tx = self.wallet.create_self_transfer(fee_rate=0)["tx"]
-        tx.vout.append(CTxOut(nValue=0, scriptPubKey=CScript([OP_RETURN, data])))
+        data = [] if data is None else [data]
+        tx.vout.append(CTxOut(nValue=0, scriptPubKey=CScript([OP_RETURN] + data)))
         tx.vout[0].nValue -= tx.get_vsize()  # simply pay 1sat/vbyte fee
 
         tx_hex = tx.serialize().hex()
@@ -49,6 +51,8 @@ class DataCarrierTest(BitcoinTestFramework):
         default_size_data = random_bytes(MAX_OP_RETURN_RELAY - 3)
         too_long_data = random_bytes(MAX_OP_RETURN_RELAY - 2)
         small_data = random_bytes(MAX_OP_RETURN_RELAY - 4)
+        one_byte = random_bytes(1)
+        zero_bytes = random_bytes(0)
 
         self.log.info("Testing null data transaction with default -datacarrier and -datacarriersize values.")
         self.test_null_data_transaction(node=self.nodes[0], data=default_size_data, success=True)
@@ -65,6 +69,24 @@ class DataCarrierTest(BitcoinTestFramework):
         self.log.info("Testing a null data transaction with a size smaller than accepted by -datacarriersize.")
         self.test_null_data_transaction(node=self.nodes[2], data=small_data, success=True)
 
+        self.log.info("Testing a null data transaction with no data.")
+        self.test_null_data_transaction(node=self.nodes[0], data=None, success=True)
+        self.test_null_data_transaction(node=self.nodes[1], data=None, success=False)
+        self.test_null_data_transaction(node=self.nodes[2], data=None, success=True)
+        self.test_null_data_transaction(node=self.nodes[3], data=None, success=True)
+
+        self.log.info("Testing a null data transaction with zero bytes of data.")
+        self.test_null_data_transaction(node=self.nodes[0], data=zero_bytes, success=True)
+        self.test_null_data_transaction(node=self.nodes[1], data=zero_bytes, success=False)
+        self.test_null_data_transaction(node=self.nodes[2], data=zero_bytes, success=True)
+        self.test_null_data_transaction(node=self.nodes[3], data=zero_bytes, success=True)
+
+        self.log.info("Testing a null data transaction with one byte of data.")
+        self.test_null_data_transaction(node=self.nodes[0], data=one_byte, success=True)
+        self.test_null_data_transaction(node=self.nodes[1], data=one_byte, success=False)
+        self.test_null_data_transaction(node=self.nodes[2], data=one_byte, success=True)
+        self.test_null_data_transaction(node=self.nodes[3], data=one_byte, success=False)
+
 
 if __name__ == '__main__':
     DataCarrierTest().main()