C API tweaks for wasmtime-py (#2029)

* wasmtime-c-api: Only drop non-null `*mut wasm_ref_t`s

* wasmtime-c-api: Handle null refs in `wasm_val_t` to `Val` conversion

* wasmtime-c-api: Don't unwrap and rewrap `Option`s

The `unwrap` can panic, and there isn't any point to this unwrap+rewrap.

* wasmtime-c-api: Add conversions between `funcref` and `wasm_func_t`

* wasmtime-c-api: More ownership documentation for `wasmtime.h`

Author: fitzgen

crates/c-api/include/wasmtime.h

@@ -515,8 +515,7 @@ typedef own wasm_trap_t* (*wasmtime_func_callback_t)(const wasmtime_caller_t* ca
  *
  * This function is the same as #wasm_func_callback_with_env_t except that its
  * first argument is a #wasmtime_caller_t which allows learning information
- * about the
- * caller.
+ * about the caller.
  */
 typedef own wasm_trap_t* (*wasmtime_func_callback_with_env_t)(const wasmtime_caller_t* caller, void* env, const wasm_val_t args[], wasm_val_t results[]);
 
@@ -544,6 +543,28 @@ WASM_API_EXTERN own wasm_func_t* wasmtime_func_new_with_env(
   void (*finalizer)(void*)
 );
 
+/**
+ * \brief Creates a new `funcref` value referencing `func`.
+ *
+ * Create a `funcref` value that references `func` and writes it to `funcrefp`.
+ *
+ * Gives ownership fo the `funcref` value written to `funcrefp`.
+ *
+ * Both `func` and `funcrefp` must not be NULL.
+ */
+WASM_API_EXTERN void wasmtime_func_as_funcref(const wasm_func_t* func, wasm_val_t* funcrefp);
+
+/**
+ * \brief Get the `wasm_func_t*` referenced by the given `funcref` value.
+ *
+ * Gets an owning handle to the `wasm_func_t*` that the given `funcref` value is
+ * referencing. Returns NULL if the value is not a `funcref`, or if the value is
+ * a null function reference.
+ *
+ * The `val` pointer must not be NULL.
+ */
+WASM_API_EXTERN own wasm_func_t* wasmtime_funcref_as_func(const wasm_val_t* val);
+
 /**
  * \brief Loads a #wasm_extern_t from the caller's context
  *
@@ -845,8 +866,10 @@ WASM_API_EXTERN wasmtime_error_t *wasmtime_funcref_table_grow(
  * 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.
+ *
+ * Gives ownership of the newly created `externref` value.
  */
-WASM_API_EXTERN void wasmtime_externref_new(void *data, wasm_val_t *valp);
+WASM_API_EXTERN void wasmtime_externref_new(own void *data, wasm_val_t *valp);
 
 /**
  * \brief A finalizer for an `externref`'s wrapped data.
@@ -866,9 +889,11 @@ typedef void (*wasmtime_externref_finalizer_t)(void*);
  * 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.
+ *
+ * Gives ownership of the newly created `externref` value.
  */
 WASM_API_EXTERN void wasmtime_externref_new_with_finalizer(
-    void *data,
+    own void *data,
     wasmtime_externref_finalizer_t finalizer,
     wasm_val_t *valp
 );
@@ -887,7 +912,8 @@ WASM_API_EXTERN void wasmtime_externref_new_with_finalizer(
  * If the given value is not an `externref`, returns `false` and leaves `datap`
  * unmodified.
  *
- * Does not take ownership of `val`.
+ * Does not take ownership of `val`. Does not give up ownership of the `void*`
+ * data written to `datap`.
  *
  * Both `val` and `datap` must not be `NULL`.
  */

crates/c-api/src/func.rs

@@ -6,7 +6,7 @@ use std::mem::MaybeUninit;
 use std::panic::{self, AssertUnwindSafe};
 use std::ptr;
 use std::str;
-use wasmtime::{Caller, Extern, Func, Trap};
+use wasmtime::{Caller, Extern, Func, Trap, Val};
 
 #[derive(Clone)]
 #[repr(transparent)]
@@ -275,3 +275,21 @@ pub extern "C" fn wasmtime_caller_export_get(
     let which = caller.caller.get_export(name)?;
     Some(Box::new(wasm_extern_t { which }))
 }
+
+#[no_mangle]
+pub extern "C" fn wasmtime_func_as_funcref(
+    func: &wasm_func_t,
+    funcrefp: &mut MaybeUninit<wasm_val_t>,
+) {
+    let funcref = wasm_val_t::from_val(Val::FuncRef(Some(func.func().clone())));
+    crate::initialize(funcrefp, funcref);
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_funcref_as_func(val: &wasm_val_t) -> Option<Box<wasm_func_t>> {
+    if let Val::FuncRef(Some(f)) = val.val() {
+        Some(Box::new(f.into()))
+    } else {
+        None
+    }
+}

crates/c-api/src/table.rs

@@ -91,7 +91,7 @@ pub extern "C" fn wasm_table_get(
     index: wasm_table_size_t,
 ) -> Option<Box<wasm_ref_t>> {
     let val = t.table().get(index)?;
-    Some(val_into_ref(val).unwrap())
+    val_into_ref(val)
 }
 
 #[no_mangle]

crates/c-api/src/val.rs

@@ -26,7 +26,9 @@ impl Drop for wasm_val_t {
     fn drop(&mut self) {
         match into_valtype(self.kind) {
             ValType::ExternRef => unsafe {
-                drop(Box::from_raw(self.of.ref_));
+                if !self.of.ref_.is_null() {
+                    drop(Box::from_raw(self.of.ref_));
+                }
             },
             _ => {}
         }
@@ -116,7 +118,20 @@ impl wasm_val_t {
             ValType::I64 => Val::from(unsafe { self.of.i64 }),
             ValType::F32 => Val::from(unsafe { self.of.f32 }),
             ValType::F64 => Val::from(unsafe { self.of.f64 }),
-            ValType::ExternRef | ValType::FuncRef => ref_to_val(unsafe { &*self.of.ref_ }),
+            ValType::ExternRef => unsafe {
+                if self.of.ref_.is_null() {
+                    Val::ExternRef(None)
+                } else {
+                    ref_to_val(&*self.of.ref_)
+                }
+            },
+            ValType::FuncRef => unsafe {
+                if self.of.ref_.is_null() {
+                    Val::FuncRef(None)
+                } else {
+                    ref_to_val(&*self.of.ref_)
+                }
+            },
             _ => unimplemented!("wasm_val_t::val {:?}", self.kind),
         }
     }