doc: add standardness policy documentation and improve -datacarriersize help

Add doc/policy/standardness.md documenting the transaction standardness
rules enforced during relay and mining. This covers transaction-level,
output-level, and input-level checks, along with a table of configurable
relay options (-datacarrier, -datacarriersize, -permitbaremultisig,
-dustrelayfee, -bytespersigop).

Also update the -datacarriersize help text to mention that setting it to
83 restores the pre-v30.0 behavior.

Author: Bortlesboat

doc/policy/README.md

@@ -9,6 +9,7 @@ contents. Policy is *not* applied to transactions in blocks.
 
 This documentation is not an exhaustive list of all policy rules.
 
+- [Standardness Rules](standardness.md)
 - [Mempool Design and Limits](mempool-design.md)
 - [Mempool Replacements](mempool-replacements.md)
 - [Packages](packages.md)

doc/policy/standardness.md

@@ -0,0 +1,60 @@
+# Standardness Rules
+
+A transaction is considered "standard" if it passes a set of relay policy checks
+in addition to consensus rules. Nodes will not relay or mine non-standard
+transactions by default. These rules are local to the node and can be partially
+configured via command-line options.
+
+Most standardness checks are applied by `IsStandardTx()`, `AreInputsStandard()`,
+and `IsWitnessStandard()` in `src/policy/policy.cpp`. Additional checks (such as
+the minimum transaction size) are applied during mempool acceptance in
+`src/validation.cpp`.
+
+## Transaction-level checks
+
+- **Version**: Must be between 1 and 3 (inclusive).
+- **Weight**: Must not exceed 400,000 weight units (`MAX_STANDARD_TX_WEIGHT`).
+- **Non-witness size**: Must be at least 65 bytes (`MIN_STANDARD_TX_NONWITNESS_SIZE`),
+  to prevent confusion with 64-byte Merkle tree inner nodes.
+- **ScriptSig**: Each input's scriptSig must be at most 1,650 bytes and contain
+  only push operations.
+
+## Output-level checks
+
+- **Script type**: Each output's scriptPubKey must be a recognized standard type
+  (P2PK, P2PKH, P2SH, P2WPKH, P2WSH, P2TR, multisig, `NULL_DATA`, or
+  `ANCHOR`). Unknown or malformed scripts are rejected as `NONSTANDARD`.
+- **Data carrier (OP_RETURN)**: Outputs with `OP_RETURN` scripts
+  (`NULL_DATA` type) are subject to a configurable size limit (see below).
+  Multiple data carrier outputs are allowed; the limit applies to their
+  aggregate scriptPubKey size.
+- **Bare multisig**: Bare multisig outputs (not wrapped in P2SH) can be
+  disabled with `-permitbaremultisig=0`.
+- **Dust**: At most one output may be below the dust threshold. The dust
+  threshold is derived from `-dustrelayfee` (default: 3,000 sat/kvB).
+
+## Input-level checks
+
+- **P2SH sigops**: A P2SH redeem script must not contain more than 15 sigops.
+- **BIP 54 sigops**: The total number of potentially executed legacy signature
+  operations across the transaction must not exceed 2,500.
+- **Unknown witness programs**: Spending an output with an unknown witness
+  version or unknown script type is non-standard.
+- **P2WSH limits**: Witness scripts are limited to 3,600 bytes, 100 stack
+  items, and 80 bytes per stack item.
+- **Tapscript limits**: Stack items are limited to 80 bytes. Annexes are
+  non-standard.
+
+## Configurable relay options
+
+| Option | Default | Description |
+|--------|---------|-------------|
+| `-datacarrier` | `1` | Relay and mine transactions with data carrier (`OP_RETURN`) outputs. Set to `0` to reject all data carrier transactions. |
+| `-datacarriersize` | `100000` | Maximum aggregate size (in bytes) of data-carrying `OP_RETURN` scriptPubKeys per transaction. Prior to v30.0, the default was `83`, which limited each `OP_RETURN` output to 80 bytes of payload plus the 3-byte script overhead. To restore the previous behavior, set `-datacarriersize=83`. |
+| `-permitbaremultisig` | `1` | Relay and mine transactions with bare (non-P2SH) multisig outputs. Set to `0` to reject them. |
+| `-dustrelayfee` | `3000` (sat/kvB) | Fee rate used to calculate the dust threshold. Outputs worth less than the cost to spend them at this fee rate are considered dust. |
+| `-bytespersigop` | `20` | Equivalent bytes per sigop in transactions for relay and mining. Affects the virtual size calculation. |
+
+These options affect only the local node's relay and mining policy. They do not
+affect consensus validation, and transactions rejected by these rules may still
+be included in blocks by miners with different policies.

src/init.cpp

@@ -677,7 +677,9 @@ void SetupServerArgs(ArgsManager& argsman, bool can_listen_ipc)
     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("Relay and mine transactions whose data-carrying raw scriptPubKeys in aggregate "
-                             "are of this size or less, allowing multiple outputs (default: %u)",
+                             "are of this size or less, allowing multiple outputs (default: %u). "
+                             "Set to 83 to restore the pre-v30.0 behavior of a single OP_RETURN "
+                             "output with at most 80 bytes of payload.",
                              MAX_OP_RETURN_RELAY),
                    ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-permitbaremultisig", strprintf("Relay transactions creating non-P2SH multisig outputs (default: %u)", DEFAULT_PERMIT_BAREMULTISIG), ArgsManager::ALLOW_ANY,