feat: add a new VM syscall to allow printing debug infos from contract

This could be very handy when debugging a contract

script/src/syscalls/debugger.rs

@@ -0,0 +1,41 @@
+use syscalls::DEBUG_PRINT_SYSCALL_NUMBER;
+use vm::{CoreMachine, Error as VMError, Memory, Register, Syscalls, A0, A7};
+
+pub struct Debugger<'a> {
+    prefix: &'a str,
+}
+
+impl<'a> Debugger<'a> {
+    pub fn new(prefix: &'a str) -> Debugger<'a> {
+        Debugger { prefix }
+    }
+}
+
+impl<'a, R: Register, M: Memory> Syscalls<R, M> for Debugger<'a> {
+    fn initialize(&mut self, _machine: &mut CoreMachine<R, M>) -> Result<(), VMError> {
+        Ok(())
+    }
+
+    fn ecall(&mut self, machine: &mut CoreMachine<R, M>) -> Result<bool, VMError> {
+        let number = machine.registers()[A7].to_u64();
+        if number != DEBUG_PRINT_SYSCALL_NUMBER {
+            return Ok(false);
+        }
+
+        let mut addr = machine.registers()[A0].to_usize();
+        let mut buffer = Vec::new();
+
+        loop {
+            let byte = machine.memory_mut().load8(addr)?;
+            if byte == 0 {
+                break;
+            }
+            buffer.push(byte);
+            addr += 1;
+        }
+
+        let s = String::from_utf8(buffer).map_err(|_| VMError::ParseError)?;
+        debug!(target: "script", "{} DEBUG OUTPUT: {}", self.prefix, s);
+        Ok(true)
+    }
+}

script/src/syscalls/mod.rs

@@ -1,9 +1,11 @@
 mod builder;
+mod debugger;
 mod fetch_script_hash;
 mod mmap_cell;
 mod mmap_tx;
 
 pub use self::builder::build_tx;
+pub use self::debugger::Debugger;
 pub use self::fetch_script_hash::FetchScriptHash;
 pub use self::mmap_cell::MmapCell;
 pub use self::mmap_tx::MmapTx;
@@ -17,6 +19,7 @@ pub const ITEM_MISSING: u8 = 2;
 pub const MMAP_TX_SYSCALL_NUMBER: u64 = 2049;
 pub const MMAP_CELL_SYSCALL_NUMBER: u64 = 2050;
 pub const FETCH_SCRIPT_HASH_SYSCALL_NUMBER: u64 = 2051;
+pub const DEBUG_PRINT_SYSCALL_NUMBER: u64 = 2177;
 
 #[derive(Debug, PartialEq, Clone, Copy, Eq)]
 pub enum Mode {

script/src/verify.rs

@@ -5,7 +5,7 @@ use core::script::Script;
 use core::transaction::{CellInput, CellOutput};
 use flatbuffers::FlatBufferBuilder;
 use fnv::FnvHashMap;
-use syscalls::{build_tx, FetchScriptHash, MmapCell, MmapTx};
+use syscalls::{build_tx, Debugger, FetchScriptHash, MmapCell, MmapTx};
 use vm::{DefaultMachine, SparseMemory};
 
 // This struct leverages CKB VM to verify transaction inputs.
@@ -85,7 +85,7 @@ impl<'a> TransactionScriptsVerifier<'a> {
         Err(ScriptError::NoScript)
     }
 
-    pub fn verify_script(&self, script: &Script) -> Result<(), ScriptError> {
+    pub fn verify_script(&self, script: &Script, prefix: &str) -> Result<(), ScriptError> {
         self.extract_script(script).and_then(|script_binary| {
             let mut args = vec![b"verify".to_vec()];
             args.extend_from_slice(&script.signed_args.as_slice());
@@ -95,6 +95,7 @@ impl<'a> TransactionScriptsVerifier<'a> {
             machine.add_syscall_module(Box::new(self.build_mmap_tx()));
             machine.add_syscall_module(Box::new(self.build_mmap_cell()));
             machine.add_syscall_module(Box::new(self.build_fetch_script_hash()));
+            machine.add_syscall_module(Box::new(Debugger::new(prefix)));
             machine
                 .run(script_binary, &args)
                 .map_err(ScriptError::VMError)
@@ -110,14 +111,16 @@ impl<'a> TransactionScriptsVerifier<'a> {
 
     pub fn verify(&self) -> Result<(), ScriptError> {
         for (i, input) in self.inputs.iter().enumerate() {
-            self.verify_script(&input.unlock).map_err(|e| {
+            let prefix = format!("Transaction {}, input {}", self.hash, i);
+            self.verify_script(&input.unlock, &prefix).map_err(|e| {
                 info!(target: "script", "Error validating input {} of transaction {}: {:?}", i, self.hash, e);
                 e
             })?;
         }
         for (i, output) in self.outputs.iter().enumerate() {
             if let Some(ref contract) = output.contract {
-                self.verify_script(contract).map_err(|e| {
+                let prefix = format!("Transaction {}, output {}", self.hash, i);
+                self.verify_script(contract, &prefix).map_err(|e| {
                     info!(target: "script", "Error validating output {} of transaction {}: {:?}", i, self.hash, e);
                     e
                 })?;