wasmtime-c-api: Add Wasmtime-specific APIs for externrefs

This commit adds APIs to create new `externref` values (with and without
finalizers) and to get an `externref`'s wrapped data.

crates/c-api/include/wasmtime.h

@@ -836,6 +836,60 @@ WASM_API_EXTERN wasmtime_error_t *wasmtime_funcref_table_grow(
     wasm_table_size_t *prev_size
 );
 
+/**
+ * \brief Create a new `externref` value.
+ *
+ * Creates a new `externref` value wrapping the provided data.
+ *
+ * This function does not take an associated finalizer to clean up the data when
+ * the reference is reclaimed. If you need a finalizer to clean up the data,
+ * then use #wasmtime_externref_new_with_finalizer.
+ */
+WASM_API_EXTERN wasm_val_t wasmtime_externref_new(void *data);
+
+/**
+ * \brief A finalizer for an `externref`'s wrapped data.
+ *
+ * A finalizer callback to clean up an `externref`'s wrapped data after the
+ * `externref` has been reclaimed. This is an opportunity to run destructors,
+ * free dynamically allocated memory, close file handles, etc.
+ */
+typedef void (*wasmtime_externref_finalizer_t)(void*);
+
+/**
+ * \brief Create a new `externref` value with a finalizer.
+ *
+ * Creates a new `externref` value wrapping the provided data.
+ *
+ * When the reference is reclaimed, the wrapped data is cleaned up with the
+ * provided finalizer. If you do not need to clean up the wrapped data, then use
+ * #wasmtime_externref_new.
+ */
+WASM_API_EXTERN wasm_val_t wasmtime_externref_new_with_finalizer(
+    void *data,
+    wasmtime_externref_finalizer_t finalizer
+);
+
+/**
+ * \brief Get an `externref`'s wrapped data
+ *
+ * If the given value is a reference to a non-null `externref`, writes the
+ * wrapped data that was passed into #wasmtime_externref_new or
+ * #wasmtime_externref_new_with_finalizer when creating the given `externref` to
+ * `datap`, and returns `true`.
+ *
+ * If the value is a reference to a null `externref`, writes `NULL` to `datap`
+ * and returns `true`.
+ *
+ * If the given value is not an `externref`, returns `false` and leaves `datap`
+ * unmodified.
+ *
+ * Does not take ownership of `val`.
+ *
+ * Both `val` and `datap` must not be `NULL`.
+ */
+WASM_API_EXTERN bool wasmtime_externref_data(wasm_val_t* val, void** datap);
+
 #undef own
 
 #ifdef __cplusplus

crates/c-api/src/ref.rs

@@ -1,4 +1,7 @@
+use crate::wasm_val_t;
+use std::any::Any;
 use std::os::raw::c_void;
+use std::ptr;
 use wasmtime::{ExternRef, Func, Val};
 
 /// `*mut wasm_ref_t` is a reference type (`externref` or `funcref`), as seen by
@@ -92,3 +95,60 @@ pub extern "C" fn wasm_ref_set_host_info_with_finalizer(
     eprintln!("`wasm_ref_set_host_info_with_finalizer` is not implemented");
     std::process::abort();
 }
+
+type wasmtime_externref_finalizer_t = extern "C" fn(*mut c_void);
+
+struct CExternRef {
+    data: *mut c_void,
+    finalizer: Option<wasmtime_externref_finalizer_t>,
+}
+
+impl Drop for CExternRef {
+    fn drop(&mut self) {
+        if let Some(f) = self.finalizer {
+            f(self.data);
+        }
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_externref_new(data: *mut c_void) -> wasm_val_t {
+    wasm_val_t::from_val(Val::ExternRef(Some(ExternRef::new(CExternRef {
+        data,
+        finalizer: None,
+    }))))
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_externref_new_with_finalizer(
+    data: *mut c_void,
+    finalizer: Option<wasmtime_externref_finalizer_t>,
+) -> wasm_val_t {
+    wasm_val_t::from_val(Val::ExternRef(Some(ExternRef::new(CExternRef {
+        data,
+        finalizer,
+    }))))
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_externref_data(val: &wasm_val_t, datap: *mut *mut c_void) -> bool {
+    match val.val() {
+        Val::ExternRef(None) => {
+            unsafe {
+                ptr::write(datap, ptr::null_mut());
+            }
+            true
+        }
+        Val::ExternRef(Some(x)) => {
+            let data = match x.data().downcast_ref::<CExternRef>() {
+                Some(r) => r.data,
+                None => x.data() as *const dyn Any as *mut c_void,
+            };
+            unsafe {
+                ptr::write(datap, data);
+            }
+            true
+        }
+        _ => false,
+    }
+}