Add READ_ONLY flag to contract call function (paritytech#4418)

This PR implements the `READ_ONLY` flag to be used as a `Callflag` in
the `call` function.
The flag indicates that the callee is restricted from modifying the
state during call execution.
It is equivalent to Ethereum's
[STATICCALL](https://eips.ethereum.org/EIPS/eip-214).

---------

Co-authored-by: command-bot <>
Co-authored-by: Andrew Jones <ascjones@gmail.com>
Co-authored-by: Alexander Theißen <alex.theissen@me.com>

prdoc/pr_4418.prdoc

@@ -0,0 +1,19 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "[pallet_contracts] Add `READ_ONLY` flag to contract `call` function"
+
+doc:
+  - audience: Runtime User
+    description: |
+      This PR implements the `READ_ONLY` flag to be used as a `Callflag` in the contract `call` function.
+      The flag indicates that the callee is restricted from modifying the state during call execution.
+      It is equivalent to Ethereum's [STATICCALL](https://eips.ethereum.org/EIPS/eip-214).
+
+crates:
+  - name: pallet-contracts
+    bump: minor
+  - name: pallet-contracts-uapi
+    bump: minor
+  - name: pallet-contracts-proc-macro
+    bump: minor

substrate/frame/contracts/fixtures/contracts/call_with_flags_and_value.rs

@@ -0,0 +1,51 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! This fixture calls the account_id with the flags and value.
+#![no_std]
+#![no_main]
+
+use common::input;
+use uapi::{HostFn, HostFnImpl as api};
+
+#[no_mangle]
+#[polkavm_derive::polkavm_export]
+pub extern "C" fn deploy() {}
+
+#[no_mangle]
+#[polkavm_derive::polkavm_export]
+pub extern "C" fn call() {
+	input!(
+		256,
+		callee_addr: [u8; 32],
+		flags: u32,
+		value: u64,
+		forwarded_input: [u8],
+	);
+
+	api::call_v2(
+		uapi::CallFlags::from_bits(flags).unwrap(),
+		callee_addr,
+		0u64,                 // How much ref_time to devote for the execution. 0 = all.
+		0u64,                 // How much proof_size to devote for the execution. 0 = all.
+		None,                 // No deposit limit.
+		&value.to_le_bytes(), // Value transferred to the contract.
+		forwarded_input,
+		None,
+	)
+	.unwrap();
+}

substrate/frame/contracts/fixtures/contracts/read_only_call.rs

@@ -0,0 +1,50 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This fixture tests if read-only call works as expected.
+#![no_std]
+#![no_main]
+
+use common::input;
+use uapi::{HostFn, HostFnImpl as api};
+
+#[no_mangle]
+#[polkavm_derive::polkavm_export]
+pub extern "C" fn deploy() {}
+
+#[no_mangle]
+#[polkavm_derive::polkavm_export]
+pub extern "C" fn call() {
+	input!(
+		256,
+		callee_addr: [u8; 32],
+		callee_input: [u8],
+	);
+
+	// Call the callee
+	api::call_v2(
+		uapi::CallFlags::READ_ONLY,
+		callee_addr,
+		0u64,                // How much ref_time to devote for the execution. 0 = all.
+		0u64,                // How much proof_size to devote for the execution. 0 = all.
+		None,                // No deposit limit.
+		&0u64.to_le_bytes(), // Value transferred to the contract.
+		callee_input,
+		None,
+	)
+	.unwrap();
+}

substrate/frame/contracts/proc-macro/src/lib.rs

@@ -170,14 +170,15 @@ impl HostFn {
 
 		// process attributes
 		let msg =
-			"Only #[version(<u8>)], #[unstable], #[prefixed_alias], #[cfg] and #[deprecated] attributes are allowed.";
+			"Only #[version(<u8>)], #[unstable], #[prefixed_alias], #[cfg], #[mutating] and #[deprecated] attributes are allowed.";
 		let span = item.span();
 		let mut attrs = item.attrs.clone();
 		attrs.retain(|a| !a.path().is_ident("doc"));
 		let mut maybe_version = None;
 		let mut is_stable = true;
 		let mut alias_to = None;
 		let mut not_deprecated = true;
+		let mut mutating = false;
 		let mut cfg = None;
 		while let Some(attr) = attrs.pop() {
 			let ident = attr.path().get_ident().ok_or(err(span, msg))?.to_string();
@@ -208,6 +209,12 @@ impl HostFn {
 					}
 					not_deprecated = false;
 				},
+				"mutating" => {
+					if mutating {
+						return Err(err(span, "#[mutating] can only be specified once"))
+					}
+					mutating = true;
+				},
 				"cfg" => {
 					if cfg.is_some() {
 						return Err(err(span, "#[cfg] can only be specified once"))
@@ -217,6 +224,16 @@ impl HostFn {
 				id => return Err(err(span, &format!("Unsupported attribute \"{id}\". {msg}"))),
 			}
 		}
+
+		if mutating {
+			let stmt = syn::parse_quote! {
+				if ctx.ext().is_read_only() {
+					return Err(Error::<E::T>::StateChangeDenied.into());
+				}
+			};
+			item.block.stmts.insert(0, stmt);
+		}
+
 		let name = item.sig.ident.to_string();
 
 		if !(is_stable || not_deprecated) {

substrate/frame/contracts/src/chain_extension.rs

@@ -112,6 +112,12 @@ pub trait ChainExtension<C: Config> {
 	/// In case of `Err` the contract execution is immediately suspended and the passed error
 	/// is returned to the caller. Otherwise the value of [`RetVal`] determines the exit
 	/// behaviour.
+	///
+	/// # Note
+	///
+	/// The [`Self::call`] can be invoked within a read-only context, where any state-changing calls
+	/// are disallowed. This information can be obtained using `env.ext().is_read_only()`. It is
+	/// crucial for the implementer to handle this scenario appropriately.
 	fn call<E: Ext<T = C>>(&mut self, env: Environment<E, InitState>) -> Result<RetVal>;
 
 	/// Determines whether chain extensions are enabled for this chain.