feat(core): facilitate op-disabling middleware (#11858)

* feat(core): facilitate op-disabling middleware

By providing `void_op_sync()` and `void_op_async() as well as `core/examples/disable_ops.rs`

core/examples/disable_ops.rs

@@ -0,0 +1,27 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+//!  This example shows you how to define ops in Rust and then call them from
+//!  JavaScript.
+
+use deno_core::Extension;
+use deno_core::JsRuntime;
+use deno_core::RuntimeOptions;
+
+fn main() {
+  let my_ext = Extension::builder()
+    .middleware(|name, opfn| match name {
+      "op_print" => deno_core::void_op_sync(),
+      _ => opfn,
+    })
+    .build();
+
+  // Initialize a runtime instance
+  let mut runtime = JsRuntime::new(RuntimeOptions {
+    extensions: vec![my_ext],
+    ..Default::default()
+  });
+
+  // Deno.core.print() will now be a NOP
+  runtime
+    .execute_script("<usage>", r#"Deno.core.print("I'm broken")"#)
+    .unwrap();
+}

core/lib.rs

@@ -79,6 +79,8 @@ pub use crate::ops_builtin::op_resources;
 pub use crate::ops_json::op_async;
 pub use crate::ops_json::op_async_unref;
 pub use crate::ops_json::op_sync;
+pub use crate::ops_json::void_op_async;
+pub use crate::ops_json::void_op_sync;
 pub use crate::resources::Resource;
 pub use crate::resources::ResourceId;
 pub use crate::resources::ResourceTable;

core/ops_json.rs

@@ -11,6 +11,33 @@ use std::cell::RefCell;
 use std::future::Future;
 use std::rc::Rc;
 
+/// A helper function that returns a sync NOP OpFn
+///
+/// It's mainly intended for embedders who want to disable ops, see ./examples/disable_ops.rs
+pub fn void_op_sync() -> Box<OpFn> {
+  // TODO(@AaronO): use this simpler implementation after changing serde_v8 to allow all values
+  // to deserialize to the unit type instead of failing with `ExpectedNull`
+  // op_sync(|_, _: (), _: ()| Ok(()))
+  Box::new(move |state, _| -> Op {
+    let op_result = serialize_op_result(Ok(()), state);
+    Op::Sync(op_result)
+  })
+}
+
+/// A helper function that returns an async NOP OpFn
+///
+/// It's mainly intended for embedders who want to disable ops, see ./examples/disable_ops.rs
+pub fn void_op_async() -> Box<OpFn> {
+  // TODO(@AaronO): use this simpler implementation after changing serde_v8 to allow all values
+  // to deserialize to the unit type instead of failing with `ExpectedNull`
+  // op_async(|_, _: (), _: ()| futures::future::ok(()))
+  Box::new(move |state, payload| -> Op {
+    let pid = payload.promise_id;
+    let op_result = serialize_op_result(Ok(()), state);
+    Op::Async(Box::pin(futures::future::ready((pid, op_result))))
+  })
+}
+
 /// Creates an op that passes data synchronously using JSON.
 ///
 /// The provided function `op_fn` has the following parameters: