Add a wasmtime_linker_define_func C API function (#3122)

This exposes the functionality of the `Linker` type where a
store-independent function can be created and inserted, allowing a
linker's functions to be used across many stores (instead of requiring
one linker-per-store).

Closes #3110

crates/c-api/include/wasmtime/linker.h

@@ -80,6 +80,41 @@ WASM_API_EXTERN wasmtime_error_t* wasmtime_linker_define(
     const wasmtime_extern_t *item
 );
 
+/**
+ * \brief Defines a new function in this linker.
+ *
+ * \param linker the linker the name is being defined in.
+ * \param module the module name the item is defined under.
+ * \param module_len the byte length of `module`
+ * \param name the field name the item is defined under
+ * \param name_len the byte length of `name`
+ * \param ty the type of the function that's being defined
+ * \param cb the host callback to invoke when the function is called
+ * \param data the host-provided data to provide as the first argument to the callback
+ * \param finalizer an optional finalizer for the `data` argument.
+ *
+ * \return On success `NULL` is returned, otherwise an error is returned which
+ * describes why the definition failed.
+ *
+ * For more information about name resolution consult the [Rust
+ * documentation](https://bytecodealliance.github.io/wasmtime/api/wasmtime/struct.Linker.html#name-resolution).
+ *
+ * Note that this function does not create a #wasmtime_func_t. This creates a
+ * store-independent function within the linker, allowing this function
+ * definition to be used with multiple stores.
+ */
+WASM_API_EXTERN wasmtime_error_t* wasmtime_linker_define_func(
+    wasmtime_linker_t *linker,
+    const char *module,
+    size_t module_len,
+    const char *name,
+    size_t name_len,
+    const wasm_functype_t *ty,
+    wasmtime_func_callback_t cb,
+    void *data,
+    void (*finalizer)(void*)
+);
+
 /**
  * \brief Defines WASI functions in this linker.
  *

crates/c-api/src/func.rs

@@ -9,7 +9,7 @@ use std::mem::MaybeUninit;
 use std::panic::{self, AssertUnwindSafe};
 use std::ptr;
 use std::str;
-use wasmtime::{AsContextMut, Caller, Extern, Func, Trap};
+use wasmtime::{AsContextMut, Caller, Extern, Func, Trap, Val};
 
 #[derive(Clone)]
 #[repr(transparent)]
@@ -187,25 +187,37 @@ pub struct wasmtime_caller_t<'a> {
     caller: Caller<'a, crate::StoreData>,
 }
 
-#[no_mangle]
-pub unsafe extern "C" fn wasmtime_func_new(
-    store: CStoreContextMut<'_>,
-    ty: &wasm_functype_t,
-    callback: extern "C" fn(
+pub type wasmtime_func_callback_t = extern "C" fn(
     *mut c_void,
     *mut wasmtime_caller_t,
     *const wasmtime_val_t,
     usize,
     *mut wasmtime_val_t,
     usize,
-    ) -> Option<Box<wasm_trap_t>>,
+) -> Option<Box<wasm_trap_t>>;
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_func_new(
+    store: CStoreContextMut<'_>,
+    ty: &wasm_functype_t,
+    callback: wasmtime_func_callback_t,
     data: *mut c_void,
     finalizer: Option<extern "C" fn(*mut std::ffi::c_void)>,
     func: &mut Func,
 ) {
-    let foreign = crate::ForeignData { data, finalizer };
     let ty = ty.ty().ty.clone();
-    let f = Func::new(store, ty, move |caller, params, results| {
+    let cb = c_callback_to_rust_fn(callback, data, finalizer);
+    let f = Func::new(store, ty, cb);
+    *func = f;
+}
+
+pub(crate) unsafe fn c_callback_to_rust_fn(
+    callback: wasmtime_func_callback_t,
+    data: *mut c_void,
+    finalizer: Option<extern "C" fn(*mut std::ffi::c_void)>,
+) -> impl Fn(Caller<'_, crate::StoreData>, &[Val], &mut [Val]) -> Result<(), Trap> {
+    let foreign = crate::ForeignData { data, finalizer };
+    move |caller, params, results| {
         let params = params
             .iter()
             .cloned()
@@ -234,8 +246,7 @@ pub unsafe extern "C" fn wasmtime_func_new(
             results[i] = unsafe { result.to_val() };
         }
         Ok(())
-    });
-    *func = f;
+    }
 }
 
 #[no_mangle]

crates/c-api/src/linker.rs

@@ -1,7 +1,9 @@
+use crate::func::c_callback_to_rust_fn;
 use crate::{
-    bad_utf8, handle_result, wasm_engine_t, wasm_trap_t, wasmtime_error_t, wasmtime_extern_t,
-    wasmtime_module_t, CStoreContextMut,
+    bad_utf8, handle_result, wasm_engine_t, wasm_functype_t, wasm_trap_t, wasmtime_error_t,
+    wasmtime_extern_t, wasmtime_func_callback_t, wasmtime_module_t, CStoreContextMut,
 };
+use std::ffi::c_void;
 use std::mem::MaybeUninit;
 use std::str;
 use wasmtime::{Func, Instance, Linker};
@@ -54,6 +56,25 @@ pub unsafe extern "C" fn wasmtime_linker_define(
     handle_result(linker.define(module, name, item), |_linker| ())
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_define_func(
+    linker: &mut wasmtime_linker_t,
+    module: *const u8,
+    module_len: usize,
+    name: *const u8,
+    name_len: usize,
+    ty: &wasm_functype_t,
+    callback: wasmtime_func_callback_t,
+    data: *mut c_void,
+    finalizer: Option<extern "C" fn(*mut std::ffi::c_void)>,
+) -> Option<Box<wasmtime_error_t>> {
+    let ty = ty.ty().ty.clone();
+    let module = to_str!(module, module_len);
+    let name = to_str!(name, name_len);
+    let cb = c_callback_to_rust_fn(callback, data, finalizer);
+    handle_result(linker.linker.func_new(module, name, ty, cb), |_linker| ())
+}
+
 #[cfg(feature = "wasi")]
 #[no_mangle]
 pub extern "C" fn wasmtime_linker_define_wasi(