feat: Hook up tx_arr_len host function

crates/xrpl-host-functions/src/lib.rs

@@ -180,6 +180,13 @@ host_functions! {
         out: &mut [u8],
     ) -> HostResult<usize>;
 
+    /// The number of elements in an array field of the transaction, selected by its
+    /// `SField` code. Answers the count directly, or a negative error (`NoArray` if
+    /// the field is not an array). Reads and writes no memory.
+    #[gas = 40]
+    #[wasm_name = "tx_arr_len"]
+    fn get_tx_array_len(&self, field: i32) -> HostResult<i32>;
+
     /// The XRPL `sha512Half` of `data`: the first [`HASH_LEN`] bytes of its SHA-512.
     #[gas = 2000]
     #[wasm_name = "sha512_half"]

crates/xrpl-host-functions/tests/generated_abi.rs

@@ -115,6 +115,14 @@ impl HostFunctions for FakeHost {
         put(out, &[cache_idx as u8, locator[0]])
     }
 
+    /// A scalar-in, scalar-out count; `NoArray` on a negative selector.
+    fn get_tx_array_len(&self, field: i32) -> HostResult<i32> {
+        if field < 0 {
+            return Err(HostError::NoArray);
+        }
+        Ok(field)
+    }
+
     fn sha512_half(&self, data: &[u8], out: &mut [u8]) -> HostResult<usize> {
         let mut digest = [0; HASH_LEN];
         digest[0] = data.len() as u8;
@@ -169,6 +177,8 @@ fn the_trait_is_implementable() {
         Ok(2)
     );
     assert_eq!(out[..2], [3, 9]);
+    assert_eq!(host.get_tx_array_len(3), Ok(3));
+    assert_eq!(host.get_tx_array_len(-1), Err(HostError::NoArray));
     assert_eq!(host.sha512_half(b"abc", &mut out), Ok(HASH_LEN));
     assert_eq!(out[0], 3);
     assert_eq!(host.trace("hello", b"xy", true), Ok(()));
@@ -248,6 +258,7 @@ fn the_spec_table_matches_the_declarations() {
             ("tx_inner", 110),
             ("home_le_inner", 110),
             ("le_inner", 110),
+            ("tx_arr_len", 40),
             ("sha512_half", 2000),
             ("trace", 500),
             ("trace_num", 500),

crates/xrpl-wasm-vm-ffi/src/lib.rs

@@ -221,6 +221,11 @@ mod ffi {
             out: &mut [u8],
         ) -> i32;
 
+        /// Answers the array's element count directly, or a negative `HostError` code.
+        #[namespace = "xrpl"]
+        #[cxx_name = "getTxArrayLen"]
+        fn get_tx_array_len(self: &HostContext, field: i32) -> i32;
+
         #[namespace = "xrpl"]
         #[cxx_name = "sha512Half"]
         fn sha512_half(self: &HostContext, data: &[u8], out: &mut [u8]) -> i32;
@@ -340,6 +345,10 @@ impl HostFunctions for CxxHost<'_> {
         )
     }
 
+    fn get_tx_array_len(&self, field: i32) -> HostResult<i32> {
+        scalar(self.ctx.get_tx_array_len(field))
+    }
+
     fn sha512_half(&self, data: &[u8], out: &mut [u8]) -> HostResult<usize> {
         bytes_written(self.ctx.sha512_half(data, out))
     }

crates/xrpl-wasm-vm/src/abi.rs

@@ -235,6 +235,9 @@ mod tests {
         ) -> HostResult<usize> {
             unreachable!("no unit test in this module calls the host")
         }
+        fn get_tx_array_len(&self, _field: i32) -> HostResult<i32> {
+            unreachable!("no unit test in this module calls the host")
+        }
         fn sha512_half(&self, _data: &[u8], _out: &mut [u8]) -> HostResult<usize> {
             unreachable!("no unit test in this module calls the host")
         }

crates/xrpl-wasm-vm/src/register.rs

@@ -221,6 +221,15 @@ pub(crate) fn register_host_functions(
                     )
                 },
             ),
+            HostFunctionSpec::GetTxArrayLen => linker.func_wrap(
+                HOST_MODULE,
+                op.wasm_name(),
+                |mut caller: Caller<'_, VmState<'_>>, field: i32| -> Result<i32, wasmi::Error> {
+                    charged(&mut caller, HostFunctionSpec::GetTxArrayLen, |c| {
+                        c.data().host.get_tx_array_len(field)
+                    })
+                },
+            ),
             HostFunctionSpec::Sha512Half => linker.func_wrap(
                 HOST_MODULE,
                 op.wasm_name(),

crates/xrpl-wasm-vm/tests/budgets.rs

@@ -111,6 +111,9 @@ fn call_for(op: HostFunctionSpec) -> Call {
             "(call $le_inner (i32.const 1) (i32.const 0) (i32.const 4) (i32.const 8) (i32.const 4))",
             5,
         ),
+        HostFunctionSpec::GetTxArrayLen => {
+            (import::TX_ARR_LEN, "(call $tx_arr_len (i32.const 1))", 1)
+        }
         HostFunctionSpec::Sha512Half => (
             import::SHA512_HALF,
             "(call $sha512_half (i32.const 0) (i32.const 4) (i32.const 0) (i32.const 32))",

crates/xrpl-wasm-vm/tests/host_calls.rs

@@ -243,6 +243,20 @@ fn le_inner_reads_the_slot_and_locator_and_writes_the_field() {
     assert_eq!(*host.le_nested_asked.borrow(), vec![(3, locator)]);
 }
 
+/// A scalar-in, scalar-out call — no memory regions at all: the field selector
+/// reaches the host and the array length comes back as the status.
+#[test]
+fn tx_arr_len_passes_the_selector_and_returns_the_count() {
+    let host = FakeHost::new().answering_tx_arr_len(17, 5);
+
+    let wat = module(
+        &[import::TX_ARR_LEN, ONE_PAGE],
+        "(call $tx_arr_len (i32.const 17))",
+    );
+    assert_eq!(status(&wat, &host), 5, "the array length");
+    assert_eq!(*host.tx_arr_lens_asked.borrow(), vec![17]);
+}
+
 /// A leading scalar parameter reaches the host as declared.
 #[test]
 fn home_le_field_passes_the_field_selector_through() {

crates/xrpl-wasm-vm/tests/preflight.rs

@@ -98,7 +98,7 @@ fn a_disabled_feature_does_not_pass() {
 
 /// Every host function the ABI declares, spelled as a guest imports it. The count
 /// is asserted against the ABI so a function added to it cannot be left out here.
-const ALL_IMPORTS: [&str; 15] = [
+const ALL_IMPORTS: [&str; 16] = [
     import::LDGR_INDEX,
     import::PARENT_LDGR_TIME,
     import::PARENT_LDGR_HASH,
@@ -111,6 +111,7 @@ const ALL_IMPORTS: [&str; 15] = [
     import::TX_INNER,
     import::HOME_LE_INNER,
     import::LE_INNER,
+    import::TX_ARR_LEN,
     import::SHA512_HALF,
     import::TRACE,
     import::TRACE_NUM,

crates/xrpl-wasm-vm/tests/support/mod.rs

@@ -145,6 +145,11 @@ pub struct FakeHost {
     pub le_nested: HashMap<(i32, Vec<u8>), Answer>,
     /// Every (cache slot, locator) `get_ledger_obj_nested_field` was asked for.
     pub le_nested_asked: RefCell<Vec<(i32, Vec<u8>)>>,
+    /// What `get_tx_array_len` answers, by field selector. An unlisted selector
+    /// answers `NoArray`.
+    pub tx_arr_lens: HashMap<i32, i32>,
+    /// Every field selector `get_tx_array_len` was asked for.
+    pub tx_arr_lens_asked: RefCell<Vec<i32>>,
     /// What `sha512_half` answers, whatever it is given.
     pub digest: Answer,
     /// Every field selector `get_current_ledger_obj_field` was asked for.
@@ -184,6 +189,8 @@ impl Default for FakeHost {
             home_le_nested_asked: RefCell::new(Vec::new()),
             le_nested: HashMap::new(),
             le_nested_asked: RefCell::new(Vec::new()),
+            tx_arr_lens: HashMap::new(),
+            tx_arr_lens_asked: RefCell::new(Vec::new()),
             digest: Answer::filler(32),
             fields_asked: RefCell::new(Vec::new()),
             digested: RefCell::new(Vec::new()),
@@ -262,6 +269,11 @@ impl FakeHost {
         self
     }
 
+    pub fn answering_tx_arr_len(mut self, field: i32, len: i32) -> FakeHost {
+        self.tx_arr_lens.insert(field, len);
+        self
+    }
+
     pub fn answering_digest(mut self, answer: Answer) -> FakeHost {
         self.digest = answer;
         self
@@ -365,6 +377,14 @@ impl HostFunctions for FakeHost {
         }
     }
 
+    fn get_tx_array_len(&self, field: i32) -> HostResult<i32> {
+        self.tx_arr_lens_asked.borrow_mut().push(field);
+        match self.tx_arr_lens.get(&field) {
+            Some(&len) => Ok(len),
+            None => Err(HostError::NoArray),
+        }
+    }
+
     fn sha512_half(&self, data: &[u8], out: &mut [u8]) -> HostResult<usize> {
         self.digested.borrow_mut().push(data.to_vec());
         self.digest.fill(out)
@@ -414,6 +434,8 @@ pub mod import {
         r#"(import "host_lib" "tx_inner" (func $tx_inner (param i32 i32 i32 i32) (result i32)))"#;
     pub const HOME_LE_INNER: &str = r#"(import "host_lib" "home_le_inner" (func $home_le_inner (param i32 i32 i32 i32) (result i32)))"#;
     pub const LE_INNER: &str = r#"(import "host_lib" "le_inner" (func $le_inner (param i32 i32 i32 i32 i32) (result i32)))"#;
+    pub const TX_ARR_LEN: &str =
+        r#"(import "host_lib" "tx_arr_len" (func $tx_arr_len (param i32) (result i32)))"#;
     pub const SHA512_HALF: &str = r#"(import "host_lib" "sha512_half" (func $sha512_half (param i32 i32 i32 i32) (result i32)))"#;
     pub const TRACE: &str =
         r#"(import "host_lib" "trace" (func $trace (param i32 i32 i32 i32 i32) (result i32)))"#;

include/xrpl/tx/wasm/HostContext.h

@@ -92,6 +92,11 @@ public:
         rust::Slice<std::uint8_t const> locator,
         rust::Slice<std::uint8_t> out) const noexcept;
 
+    // Answers the array's element count directly, or a negative `HostFunctionError`
+    // code (`NoArray` if the field is not an array).
+    [[nodiscard]] std::int32_t
+    getTxArrayLen(std::int32_t field) const noexcept;
+
     [[nodiscard]] std::int32_t
     sha512Half(rust::Slice<std::uint8_t const> data, rust::Slice<std::uint8_t> out) const noexcept;
 

src/libxrpl/tx/wasm/HostContext.cpp

@@ -279,6 +279,23 @@ HostContext::getLedgerObjNestedField(
     });
 }
 
+std::int32_t
+HostContext::getTxArrayLen(std::int32_t field) const noexcept
+{
+    return guarded(hostFunctions_.getJournal(), kHostInternal, [&] {
+        auto const& knownSFields = SField::getKnownCodeToField();
+        auto const it = knownSFields.find(field);
+        if (it == knownSFields.end())
+            return hfErrorToInt(HostFunctionError::InvalidField);
+
+        auto const len = hostFunctions_.getTxArrayLen(*it->second);
+        if (!len)
+            return hfErrorToInt(len.error());
+
+        return *len;
+    });
+}
+
 std::int32_t
 HostContext::sha512Half(rust::Slice<std::uint8_t const> data, rust::Slice<std::uint8_t> out)
     const noexcept