feat: Hook up parent_ldgr_hash host function

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

@@ -111,6 +111,11 @@ host_functions! {
     #[wasm_name = "parent_ldgr_time"]
     fn get_parent_ledger_time(&self, out: &mut [u8]) -> HostResult<usize>;
 
+    /// The hash of the parent (last-closed) ledger, as 32 bytes.
+    #[gas = 60]
+    #[wasm_name = "parent_ldgr_hash"]
+    fn get_parent_ledger_hash(&self, out: &mut [u8]) -> HostResult<usize>;
+
     /// The serialized bytes of one field of the current (escrow) ledger object.
     #[gas = 70]
     #[wasm_name = "home_le_field"]

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

@@ -35,6 +35,10 @@ impl HostFunctions for FakeHost {
         put(out, &9u32.to_le_bytes())
     }
 
+    fn get_parent_ledger_hash(&self, out: &mut [u8]) -> HostResult<usize> {
+        put(out, &[0xab; HASH_LEN])
+    }
+
     /// Fails on a field it doesn't know, so the error channel is exercised too.
     fn get_current_ledger_obj_field(&self, field: i32, out: &mut [u8]) -> HostResult<usize> {
         if field < 0 {
@@ -71,6 +75,8 @@ fn the_trait_is_implementable() {
     assert_eq!(out[..4], [7, 0, 0, 0]);
     assert_eq!(host.get_parent_ledger_time(&mut out), Ok(4));
     assert_eq!(out[..4], [9, 0, 0, 0]);
+    assert_eq!(host.get_parent_ledger_hash(&mut out), Ok(HASH_LEN));
+    assert_eq!(out[0], 0xab);
     assert_eq!(host.get_current_ledger_obj_field(3, &mut out), Ok(1));
     assert_eq!(out[0], 3);
     assert_eq!(host.sha512_half(b"abc", &mut out), Ok(HASH_LEN));
@@ -142,6 +148,7 @@ fn the_spec_table_matches_the_declarations() {
         [
             ("ldgr_index", 60),
             ("parent_ldgr_time", 60),
+            ("parent_ldgr_hash", 60),
             ("home_le_field", 70),
             ("sha512_half", 2000),
             ("trace", 500),

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

@@ -163,6 +163,10 @@ mod ffi {
         #[cxx_name = "getParentLedgerTime"]
         fn get_parent_ledger_time(self: &HostContext, out: &mut [u8]) -> i32;
 
+        #[namespace = "xrpl"]
+        #[cxx_name = "getParentLedgerHash"]
+        fn get_parent_ledger_hash(self: &HostContext, out: &mut [u8]) -> i32;
+
         #[namespace = "xrpl"]
         #[cxx_name = "getCurrentLedgerObjField"]
         fn get_current_ledger_obj_field(self: &HostContext, field: i32, out: &mut [u8]) -> i32;
@@ -220,6 +224,10 @@ impl HostFunctions for CxxHost<'_> {
         bytes_written(self.ctx.get_parent_ledger_time(out))
     }
 
+    fn get_parent_ledger_hash(&self, out: &mut [u8]) -> HostResult<usize> {
+        bytes_written(self.ctx.get_parent_ledger_hash(out))
+    }
+
     fn get_current_ledger_obj_field(&self, field: i32, out: &mut [u8]) -> HostResult<usize> {
         bytes_written(self.ctx.get_current_ledger_obj_field(field, out))
     }

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

@@ -191,6 +191,9 @@ mod tests {
         fn get_parent_ledger_time(&self, _out: &mut [u8]) -> HostResult<usize> {
             unreachable!("no unit test in this module calls the host")
         }
+        fn get_parent_ledger_hash(&self, _out: &mut [u8]) -> HostResult<usize> {
+            unreachable!("no unit test in this module calls the host")
+        }
         fn get_current_ledger_obj_field(&self, _field: i32, _out: &mut [u8]) -> HostResult<usize> {
             unreachable!("no unit test in this module calls the host")
         }

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

@@ -48,6 +48,19 @@ pub(crate) fn register_host_functions(
                     })
                 },
             ),
+            HostFunctionSpec::GetParentLedgerHash => linker.func_wrap(
+                HOST_MODULE,
+                op.wasm_name(),
+                |mut caller: Caller<'_, VmState<'_>>,
+                 out_ptr: i32,
+                 out_len: i32|
+                 -> Result<i32, wasmi::Error> {
+                    charged(&mut caller, HostFunctionSpec::GetParentLedgerHash, |c| {
+                        let out = Region::new(out_ptr, out_len);
+                        write_into(c, out, |host, out| host.get_parent_ledger_hash(out))
+                    })
+                },
+            ),
             HostFunctionSpec::GetCurrentLedgerObjField => linker.func_wrap(
                 HOST_MODULE,
                 op.wasm_name(),

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

@@ -61,6 +61,11 @@ fn call_for(op: HostFunctionSpec) -> Call {
             "(call $parent_ldgr_time (i32.const 0) (i32.const 4))",
             2,
         ),
+        HostFunctionSpec::GetParentLedgerHash => (
+            import::PARENT_LDGR_HASH,
+            "(call $parent_ldgr_hash (i32.const 0) (i32.const 32))",
+            2,
+        ),
         HostFunctionSpec::GetCurrentLedgerObjField => (
             import::HOME_LE_FIELD,
             "(call $home_le_field (i32.const 1) (i32.const 0) (i32.const 4))",

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

@@ -49,6 +49,31 @@ fn parent_ldgr_time_writes_the_close_time_where_the_guest_asked() {
     assert_eq!(status(&wat, &host), 4, "the byte count");
 }
 
+/// A 32-byte value (a ledger hash) travels the same getter path as the 4-byte
+/// scalars: every byte lands where the guest asked, and the status is the length.
+#[test]
+fn parent_ldgr_hash_writes_all_32_bytes_where_the_guest_asked() {
+    let host = FakeHost::new();
+
+    let wat = module(
+        &[import::PARENT_LDGR_HASH, ONE_PAGE],
+        "(call $parent_ldgr_hash (i32.const 64) (i32.const 32))",
+    );
+    assert_eq!(status(&wat, &host), 32, "the byte count");
+
+    // The default hash is 0, 1, 2, ..., so its first four bytes load as 0x03020100.
+    let wat = module(
+        &[import::PARENT_LDGR_HASH, ONE_PAGE],
+        "(drop (call $parent_ldgr_hash (i32.const 64) (i32.const 32)))
+         (i32.load (i32.const 64))",
+    );
+    assert_eq!(
+        status(&wat, &host),
+        0x03020100,
+        "the first four bytes the host wrote"
+    );
+}
+
 /// The output region is wherever the guest points, not a fixed address.
 #[test]
 fn the_output_region_is_the_pointer_the_guest_gave() {

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

@@ -98,9 +98,10 @@ 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; 6] = [
+const ALL_IMPORTS: [&str; 7] = [
     import::LDGR_INDEX,
     import::PARENT_LDGR_TIME,
+    import::PARENT_LDGR_HASH,
     import::HOME_LE_FIELD,
     import::SHA512_HALF,
     import::TRACE,

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

@@ -105,6 +105,8 @@ pub struct FakeHost {
     pub ledger_sqn: Answer,
     /// What `get_parent_ledger_time` answers.
     pub parent_ledger_time: Answer,
+    /// What `get_parent_ledger_hash` answers.
+    pub parent_ledger_hash: Answer,
     /// What `get_current_ledger_obj_field` answers, by field selector. An
     /// unlisted selector answers `FieldNotFound`.
     pub fields: HashMap<i32, Answer>,
@@ -126,6 +128,8 @@ impl Default for FakeHost {
             // A distinct value from the sequence number, so a test cannot pass by
             // reading one where it meant the other.
             parent_ledger_time: Answer::bytes(9u32.to_le_bytes()),
+            // 32 bytes counting up from 0, the length of a real ledger hash.
+            parent_ledger_hash: Answer::filler(32),
             fields: HashMap::new(),
             digest: Answer::filler(32),
             fields_asked: RefCell::new(Vec::new()),
@@ -150,6 +154,11 @@ impl FakeHost {
         self
     }
 
+    pub fn answering_parent_ledger_hash(mut self, answer: Answer) -> FakeHost {
+        self.parent_ledger_hash = answer;
+        self
+    }
+
     pub fn answering_field(mut self, field: i32, answer: Answer) -> FakeHost {
         self.fields.insert(field, answer);
         self
@@ -174,6 +183,10 @@ impl HostFunctions for FakeHost {
         self.parent_ledger_time.fill(out)
     }
 
+    fn get_parent_ledger_hash(&self, out: &mut [u8]) -> HostResult<usize> {
+        self.parent_ledger_hash.fill(out)
+    }
+
     fn get_current_ledger_obj_field(&self, field: i32, out: &mut [u8]) -> HostResult<usize> {
         self.fields_asked.borrow_mut().push(field);
         match self.fields.get(&field) {
@@ -216,6 +229,7 @@ pub mod import {
     pub const LDGR_INDEX: &str =
         r#"(import "host_lib" "ldgr_index" (func $ldgr_index (param i32 i32) (result i32)))"#;
     pub const PARENT_LDGR_TIME: &str = r#"(import "host_lib" "parent_ldgr_time" (func $parent_ldgr_time (param i32 i32) (result i32)))"#;
+    pub const PARENT_LDGR_HASH: &str = r#"(import "host_lib" "parent_ldgr_hash" (func $parent_ldgr_hash (param i32 i32) (result i32)))"#;
     pub const HOME_LE_FIELD: &str = r#"(import "host_lib" "home_le_field" (func $home_le_field (param i32 i32 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 =

include/xrpl/tx/wasm/HostContext.h

@@ -47,6 +47,9 @@ public:
     [[nodiscard]] std::int32_t
     getParentLedgerTime(rust::Slice<std::uint8_t> out) const noexcept;
 
+    [[nodiscard]] std::int32_t
+    getParentLedgerHash(rust::Slice<std::uint8_t> out) const noexcept;
+
     [[nodiscard]] std::int32_t
     getCurrentLedgerObjField(std::int32_t field, rust::Slice<std::uint8_t> out) const noexcept;
 

src/libxrpl/tx/wasm/HostContext.cpp

@@ -83,6 +83,18 @@ HostContext::getParentLedgerTime(rust::Slice<std::uint8_t> out) const noexcept
     });
 }
 
+std::int32_t
+HostContext::getParentLedgerHash(rust::Slice<std::uint8_t> out) const noexcept
+{
+    return guarded(hostFunctions_.getJournal(), kHostInternal, [&] {
+        auto const hash = hostFunctions_.getParentLedgerHash();
+        if (!hash)
+            return hfErrorToInt(hash.error());
+
+        return answer(out, hash->data(), hash->size());
+    });
+}
+
 std::int32_t
 HostContext::getCurrentLedgerObjField(std::int32_t field, rust::Slice<std::uint8_t> out)
     const noexcept