Merge #803

803: Add method to call function at index on Ctx r=MarkMcCaskey a=MarkMcCaskey

For #638 and #670

```Rust
fn call_guest_fn(ctx: &mut Ctx, guest_fn: u32) -> u32 {
    println!("{}", guest_fn);

    let guest_fn_typed = unsafe { std::mem::transmute(guest_fn) };

    let result = ctx.call_with_table_index(guest_fn_typed, &[]).unwrap();
    println!("  -> {:?}", result);

    0
}
```
is what this looks like from the Host side

See `examples/callback.rs` for an example that doesn't require `transmute`


# Review

- [x] Create a short description of the the change in the CHANGELOG.md file


Co-authored-by: Mark McCaskey <mark@wasmer.io>
Co-authored-by: Mark McCaskey <markmccaskey@users.noreply.github.com>

lib/runtime-core/src/instance.rs

@@ -527,29 +527,12 @@ fn call_func_with_index(
     args: &[Value],
     rets: &mut Vec<Value>,
 ) -> CallResult<()> {
-    rets.clear();
-
     let sig_index = *info
         .func_assoc
         .get(func_index)
         .expect("broken invariant, incorrect func index");
 
     let signature = &info.signatures[sig_index];
-    let num_results = signature.returns().len();
-    let num_results = num_results
-        + signature
-            .returns()
-            .iter()
-            .filter(|&&ty| ty == Type::V128)
-            .count();
-    rets.reserve(num_results);
-
-    if !signature.check_param_value_types(args) {
-        Err(ResolveError::Signature {
-            expected: signature.clone(),
-            found: args.iter().map(|val| val.ty()).collect(),
-        })?
-    }
 
     let func_ptr = match func_index.local_or_import(info) {
         LocalOrImport::Local(local_func_index) => {
@@ -567,6 +550,39 @@ fn call_func_with_index(
         }
     };
 
+    let wasm = runnable
+        .get_trampoline(info, sig_index)
+        .expect("wasm trampoline");
+
+    call_func_with_index_inner(ctx_ptr, func_ptr, signature, wasm, args, rets)
+}
+
+pub(crate) fn call_func_with_index_inner(
+    ctx_ptr: *mut vm::Ctx,
+    func_ptr: NonNull<vm::Func>,
+    signature: &FuncSig,
+    wasm: Wasm,
+    args: &[Value],
+    rets: &mut Vec<Value>,
+) -> CallResult<()> {
+    rets.clear();
+
+    let num_results = signature.returns().len();
+    let num_results = num_results
+        + signature
+            .returns()
+            .iter()
+            .filter(|&&ty| ty == Type::V128)
+            .count();
+    rets.reserve(num_results);
+
+    if !signature.check_param_value_types(args) {
+        Err(ResolveError::Signature {
+            expected: signature.clone(),
+            found: args.iter().map(|val| val.ty()).collect(),
+        })?
+    }
+
     let mut raw_args: SmallVec<[u64; 8]> = SmallVec::new();
     for v in args {
         match v {
@@ -598,9 +614,7 @@ fn call_func_with_index(
         trampoline,
         invoke,
         invoke_env,
-    } = runnable
-        .get_trampoline(info, sig_index)
-        .expect("wasm trampoline");
+    } = wasm;
 
     let run_wasm = |result_space: *mut u64| unsafe {
         let mut trap_info = WasmTrapInfo::Unknown;

lib/runtime-core/src/table/anyfunc.rs

@@ -44,7 +44,7 @@ impl<'a> From<DynFunc<'a>> for Anyfunc<'a> {
 }
 
 pub struct AnyfuncTable {
-    backing: Vec<vm::Anyfunc>,
+    pub(crate) backing: Vec<vm::Anyfunc>,
     max: Option<u32>,
 }
 

lib/runtime-core/src/table/mod.rs

@@ -10,7 +10,7 @@ use std::{cell::RefCell, fmt, ptr, rc::Rc};
 mod anyfunc;
 
 pub use self::anyfunc::Anyfunc;
-use self::anyfunc::AnyfuncTable;
+pub(crate) use self::anyfunc::AnyfuncTable;
 use crate::error::GrowError;
 
 pub enum Element<'a> {

lib/runtime-core/src/vm.rs

@@ -1,15 +1,19 @@
 pub use crate::backing::{ImportBacking, LocalBacking, INTERNALS_SIZE};
 use crate::{
+    error::CallResult,
+    instance::call_func_with_index_inner,
     memory::{Memory, MemoryType},
     module::{ModuleInfo, ModuleInner},
+    sig_registry::SigRegistry,
     structures::TypedIndex,
-    types::{LocalOrImport, MemoryIndex},
+    types::{LocalOrImport, MemoryIndex, TableIndex, Value},
     vmcalls,
 };
 use std::{
     cell::UnsafeCell,
     ffi::c_void,
-    mem, ptr,
+    mem,
+    ptr::{self, NonNull},
     sync::atomic::{AtomicUsize, Ordering},
     sync::Once,
 };
@@ -393,6 +397,41 @@ impl Ctx {
             (*self.internal.internals)[field.index()] = value;
         }
     }
+
+    /// Calls a host or Wasm function at the given table index
+    pub fn call_with_table_index(
+        &mut self,
+        index: TableIndex,
+        args: &[Value],
+    ) -> CallResult<Vec<Value>> {
+        let anyfunc_table =
+            unsafe { &*((**self.internal.tables).table as *mut crate::table::AnyfuncTable) };
+        let Anyfunc { func, ctx, sig_id } = anyfunc_table.backing[index.index()];
+
+        let signature = SigRegistry.lookup_signature(unsafe { std::mem::transmute(sig_id.0) });
+        let mut rets = vec![];
+
+        let wasm = {
+            let module = unsafe { &*self.module };
+            let runnable = &module.runnable_module;
+
+            let sig_index = SigRegistry.lookup_sig_index(signature.clone());
+            runnable
+                .get_trampoline(&module.info, sig_index)
+                .expect("wasm trampoline")
+        };
+
+        call_func_with_index_inner(
+            ctx,
+            NonNull::new(func as *mut _).unwrap(),
+            &signature,
+            wasm,
+            args,
+            &mut rets,
+        )?;
+
+        Ok(rets)
+    }
 }
 
 #[doc(hidden)]