Implement async module loading in CLI (#2084)

6 files changed, 278 insertions, 436 deletions

diff --git a/cli/errors.rs b/cli/errors.rs
index 3873f70ff..bd3e7ba73 100644
--- a/cli/errors.rs
+++ b/cli/errors.rs
@@ -217,3 +217,13 @@ impl fmt::Display for RustOrJsError {
     }
   }
 }
+
+// TODO(ry) This is ugly. They are essentially the same type.
+impl From<deno::JSErrorOr<DenoError>> for RustOrJsError {
+  fn from(e: deno::JSErrorOr<DenoError>) -> Self {
+    match e {
+      deno::JSErrorOr::JSError(err) => RustOrJsError::Js(err),
+      deno::JSErrorOr::Other(err) => RustOrJsError::Rust(err),
+    }
+  }
+}
diff --git a/cli/main.rs b/cli/main.rs
index dc2099595..b6cd49d0e 100644
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -20,7 +20,6 @@ mod global_timer;
 mod http_body;
 mod http_util;
 pub mod js_errors;
-pub mod modules;
 pub mod msg;
 pub mod msg_util;
 pub mod ops;
@@ -37,6 +36,7 @@ pub mod worker;
 
 use crate::errors::RustOrJsError;
 use crate::state::ThreadSafeState;
+use crate::worker::root_specifier_to_url;
 use crate::worker::Worker;
 use futures::lazy;
 use futures::Future;
@@ -74,6 +74,49 @@ where
   }
 }
 
+// TODO(ry) Move this to main.rs
+pub fn print_file_info(worker: &Worker, url: &str) {
+  let maybe_out =
+    worker::fetch_module_meta_data_and_maybe_compile(&worker.state, url, ".");
+  if let Err(err) = maybe_out {
+    println!("{}", err);
+    return;
+  }
+  let out = maybe_out.unwrap();
+
+  println!("{} {}", ansi::bold("local:".to_string()), &(out.filename));
+
+  println!(
+    "{} {}",
+    ansi::bold("type:".to_string()),
+    msg::enum_name_media_type(out.media_type)
+  );
+
+  if out.maybe_output_code_filename.is_some() {
+    println!(
+      "{} {}",
+      ansi::bold("compiled:".to_string()),
+      out.maybe_output_code_filename.as_ref().unwrap(),
+    );
+  }
+
+  if out.maybe_source_map_filename.is_some() {
+    println!(
+      "{} {}",
+      ansi::bold("map:".to_string()),
+      out.maybe_source_map_filename.as_ref().unwrap()
+    );
+  }
+
+  let deps = worker.modules.deps(&out.module_name);
+  println!("{}{}", ansi::bold("deps:\n".to_string()), deps.name);
+  if let Some(ref depsdeps) = deps.deps {
+    for d in depsdeps {
+      println!("{}", d);
+    }
+  }
+}
+
 fn main() {
   #[cfg(windows)]
   ansi_term::enable_ansi_support().ok(); // For Windows 10
@@ -102,17 +145,18 @@ fn main() {
   let should_display_info = flags.info;
 
   let state = ThreadSafeState::new(flags, rest_argv, ops::op_selector_std);
-  let mut main_worker = Worker::new(
+  let mut worker = Worker::new(
     "main".to_string(),
     startup_data::deno_isolate_init(),
     state.clone(),
   );
 
-  let main_future = lazy(move || {
-    // Setup runtime.
-    js_check(main_worker.execute("denoMain()"));
+  // TODO(ry) somehow combine the two branches below. They're very similar but
+  // it's difficult to get the types to workout.
 
-    if state.flags.eval {
+  if state.flags.eval {
+    let main_future = lazy(move || {
+      js_check(worker.execute("denoMain()"));
       // Wrap provided script in async function so asynchronous methods
       // work. This is required until top-level await is not supported.
       let js_source = format!(
@@ -125,25 +169,51 @@ fn main() {
       );
       // ATM imports in `deno eval` are not allowed
       // TODO Support ES modules once Worker supports evaluating anonymous modules.
-      js_check(main_worker.execute(&js_source));
-    } else {
-      // Execute main module.
-      if let Some(main_module) = state.main_module() {
-        debug!("main_module {}", main_module);
-        js_check(main_worker.execute_mod(&main_module, should_prefetch));
-        if should_display_info {
-          // Display file info and exit. Do not run file
-          main_worker.print_file_info(&main_module);
-          std::process::exit(0);
-        }
-      }
-    }
-
-    main_worker.then(|result| {
-      js_check(result);
-      Ok(())
-    })
-  });
-
-  tokio_util::run(main_future);
+      js_check(worker.execute(&js_source));
+      worker.then(|result| {
+        js_check(result);
+        Ok(())
+      })
+    });
+    tokio_util::run(main_future);
+  } else if let Some(main_module) = state.main_module() {
+    // Normal situation of executing a module.
+
+    let main_future = lazy(move || {
+      // Setup runtime.
+      js_check(worker.execute("denoMain()"));
+      debug!("main_module {}", main_module);
+
+      let main_url = root_specifier_to_url(&main_module).unwrap();
+
+      worker
+        .execute_mod_async(&main_url, should_prefetch)
+        .and_then(move |worker| {
+          if should_display_info {
+            // Display file info and exit. Do not run file
+            print_file_info(&worker, &main_module);
+            std::process::exit(0);
+          }
+          worker.then(|result| {
+            js_check(result);
+            Ok(())
+          })
+        }).map_err(|(err, _worker)| print_err_and_exit(err))
+    });
+    tokio_util::run(main_future);
+  } else {
+    // REPL situation.
+    let main_future = lazy(move || {
+      // Setup runtime.
+      js_check(worker.execute("denoMain()"));
+      worker
+        .then(|result| {
+          js_check(result);
+          Ok(())
+        }).map_err(|(err, _worker): (RustOrJsError, Worker)| {
+          print_err_and_exit(err)
+        })
+    });
+    tokio_util::run(main_future);
+  }
 }
diff --git a/cli/modules.rs b/cli/modules.rs
deleted file mode 100644
index f40f5ca08..000000000
--- a/cli/modules.rs
+++ /dev/null
@@ -1,263 +0,0 @@
-// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
-use crate::ansi;
-use crate::deno_dir::DenoDir;
-use crate::msg;
-use deno::deno_mod;
-use std::collections::HashMap;
-use std::collections::HashSet;
-use std::fmt;
-
-pub struct ModuleInfo {
-  name: String,
-  children: Vec<deno_mod>,
-}
-
-/// A symbolic module entity.
-pub enum SymbolicModule {
-  /// This module is an alias to another module.
-  /// This is useful such that multiple names could point to
-  /// the same underlying module (particularly due to redirects).
-  Alias(String),
-  /// This module associates with a V8 module by id.
-  Mod(deno_mod),
-}
-
-#[derive(Default)]
-/// Alias-able module name map
-pub struct ModuleNameMap {
-  inner: HashMap<String, SymbolicModule>,
-}
-
-impl ModuleNameMap {
-  pub fn new() -> Self {
-    ModuleNameMap {
-      inner: HashMap::new(),
-    }
-  }
-
-  /// Get the id of a module.
-  /// If this module is internally represented as an alias,
-  /// follow the alias chain to get the final module id.
-  pub fn get(&self, name: &str) -> Option<deno_mod> {
-    let mut mod_name = name;
-    loop {
-      let cond = self.inner.get(mod_name);
-      match cond {
-        Some(SymbolicModule::Alias(target)) => {
-          mod_name = target;
-        }
-        Some(SymbolicModule::Mod(mod_id)) => {
-          return Some(*mod_id);
-        }
-        _ => {
-          return None;
-        }
-      }
-    }
-  }
-
-  /// Insert a name assocated module id.
-  pub fn insert(&mut self, name: String, id: deno_mod) {
-    self.inner.insert(name, SymbolicModule::Mod(id));
-  }
-
-  /// Create an alias to another module.
-  pub fn alias(&mut self, name: String, target: String) {
-    self.inner.insert(name, SymbolicModule::Alias(target));
-  }
-}
-
-/// A collection of JS modules.
-#[derive(Default)]
-pub struct Modules {
-  pub info: HashMap<deno_mod, ModuleInfo>,
-  pub by_name: ModuleNameMap,
-}
-
-impl Modules {
-  pub fn new() -> Modules {
-    Self {
-      info: HashMap::new(),
-      by_name: ModuleNameMap::new(),
-    }
-  }
-
-  pub fn get_id(&self, name: &str) -> Option<deno_mod> {
-    self.by_name.get(name)
-  }
-
-  pub fn get_children(&self, id: deno_mod) -> Option<&Vec<deno_mod>> {
-    self.info.get(&id).map(|i| &i.children)
-  }
-
-  pub fn get_name(&self, id: deno_mod) -> Option<&String> {
-    self.info.get(&id).map(|i| &i.name)
-  }
-
-  pub fn is_registered(&self, name: &str) -> bool {
-    self.by_name.get(name).is_some()
-  }
-
-  pub fn register(&mut self, id: deno_mod, name: &str) {
-    let name = String::from(name);
-    debug!("register {}", name);
-    self.by_name.insert(name.clone(), id);
-    self.info.insert(
-      id,
-      ModuleInfo {
-        name,
-        children: Vec::new(),
-      },
-    );
-  }
-
-  pub fn alias(&mut self, name: &str, target: &str) {
-    self.by_name.alias(name.to_owned(), target.to_owned());
-  }
-
-  pub fn resolve_cb(
-    &mut self,
-    deno_dir: &DenoDir,
-    specifier: &str,
-    referrer: deno_mod,
-  ) -> deno_mod {
-    debug!("resolve_cb {}", specifier);
-
-    let maybe_info = self.info.get_mut(&referrer);
-    if maybe_info.is_none() {
-      debug!("cant find referrer {}", referrer);
-      return 0;
-    }
-    let info = maybe_info.unwrap();
-    let referrer_name = &info.name;
-    let r = deno_dir.resolve_module(specifier, referrer_name);
-    if let Err(err) = r {
-      debug!("potentially swallowed err: {}", err);
-      return 0;
-    }
-    let (name, _local_filename) = r.unwrap();
-
-    if let Some(child_id) = self.by_name.get(&name) {
-      info.children.push(child_id);
-      return child_id;
-    } else {
-      return 0;
-    }
-  }
-
-  pub fn print_file_info(&self, deno_dir: &DenoDir, filename: String) {
-    // TODO Note the --reload flag is ignored here.
-    let maybe_out = deno_dir.fetch_module_meta_data(&filename, ".", true);
-    if maybe_out.is_err() {
-      println!("{}", maybe_out.unwrap_err());
-      return;
-    }
-    let out = maybe_out.unwrap();
-
-    println!("{} {}", ansi::bold("local:".to_string()), &(out.filename));
-    println!(
-      "{} {}",
-      ansi::bold("type:".to_string()),
-      msg::enum_name_media_type(out.media_type)
-    );
-    if out.maybe_output_code_filename.is_some() {
-      println!(
-        "{} {}",
-        ansi::bold("compiled:".to_string()),
-        out.maybe_output_code_filename.as_ref().unwrap(),
-      );
-    }
-    if out.maybe_source_map_filename.is_some() {
-      println!(
-        "{} {}",
-        ansi::bold("map:".to_string()),
-        out.maybe_source_map_filename.as_ref().unwrap()
-      );
-    }
-
-    let deps = Deps::new(self, &out.module_name);
-    println!("{}{}", ansi::bold("deps:\n".to_string()), deps.name);
-    if let Some(ref depsdeps) = deps.deps {
-      for d in depsdeps {
-        println!("{}", d);
-      }
-    }
-  }
-}
-
-pub struct Deps {
-  pub name: String,
-  pub deps: Option<Vec<Deps>>,
-  prefix: String,
-  is_last: bool,
-}
-
-impl Deps {
-  pub fn new(modules: &Modules, module_name: &str) -> Deps {
-    let mut seen = HashSet::new();
-    let id = modules.get_id(module_name).unwrap();
-    Self::helper(&mut seen, "".to_string(), true, modules, id)
-  }
-
-  fn helper(
-    seen: &mut HashSet<deno_mod>,
-    prefix: String,
-    is_last: bool,
-    modules: &Modules,
-    id: deno_mod,
-  ) -> Deps {
-    let name = modules.get_name(id).unwrap().to_string();
-    if seen.contains(&id) {
-      Deps {
-        name,
-        prefix,
-        deps: None,
-        is_last,
-      }
-    } else {
-      seen.insert(id);
-      let child_ids = modules.get_children(id).unwrap();
-      let child_count = child_ids.iter().count();
-      let deps = child_ids
-        .iter()
-        .enumerate()
-        .map(|(index, dep_id)| {
-          let new_is_last = index == child_count - 1;
-          let mut new_prefix = prefix.clone();
-          new_prefix.push(if is_last { ' ' } else { '│' });
-          new_prefix.push(' ');
-          Self::helper(seen, new_prefix, new_is_last, modules, *dep_id)
-        }).collect();
-      Deps {
-        name,
-        prefix,
-        deps: Some(deps),
-        is_last,
-      }
-    }
-  }
-}
-
-impl fmt::Display for Deps {
-  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-    let mut has_children = false;
-    if let Some(ref deps) = self.deps {
-      has_children = !deps.is_empty();
-    }
-    write!(
-      f,
-      "{}{}─{} {}",
-      self.prefix,
-      if self.is_last { "└" } else { "├" },
-      if has_children { "┬" } else { "─" },
-      self.name
-    )?;
-
-    if let Some(ref deps) = self.deps {
-      for d in deps {
-        write!(f, "\n{}", d)?;
-      }
-    }
-    Ok(())
-  }
-}
diff --git a/cli/ops.rs b/cli/ops.rs
index 0d752e226..5e2a62439 100644
--- a/cli/ops.rs
+++ b/cli/ops.rs
@@ -19,6 +19,7 @@ use crate::state::ThreadSafeState;
 use crate::tokio_util;
 use crate::tokio_write;
 use crate::version;
+use crate::worker::root_specifier_to_url;
 use crate::worker::Worker;
 use deno::deno_buf;
 use deno::js_check;
@@ -1878,9 +1879,14 @@ fn op_create_worker(
       Worker::new(name, startup_data::deno_isolate_init(), child_state);
     js_check(worker.execute("denoMain()"));
     js_check(worker.execute("workerMain()"));
-    let result = worker.execute_mod(specifier, false);
+
+    let specifier_url =
+      root_specifier_to_url(specifier).map_err(DenoError::from)?;
+
+    // TODO(ry) Use execute_mod_async here.
+    let result = worker.execute_mod(&specifier_url, false);
     match result {
-      Ok(_) => {
+      Ok(worker) => {
         let mut workers_tl = parent_state.workers.lock().unwrap();
         workers_tl.insert(rid, worker.shared());
         let builder = &mut FlatBufferBuilder::new();
@@ -1898,8 +1904,10 @@ fn op_create_worker(
           },
         ))
       }
-      Err(errors::RustOrJsError::Js(_)) => Err(errors::worker_init_failed()),
-      Err(errors::RustOrJsError::Rust(err)) => Err(err),
+      Err((errors::RustOrJsError::Js(_), _worker)) => {
+        Err(errors::worker_init_failed())
+      }
+      Err((errors::RustOrJsError::Rust(err), _worker)) => Err(err),
     }
   }()))
 }
diff --git a/cli/state.rs b/cli/state.rs
index 3434566fa..24f2f5053 100644
--- a/cli/state.rs
+++ b/cli/state.rs
@@ -3,7 +3,6 @@ use crate::deno_dir;
 use crate::errors::DenoResult;
 use crate::flags;
 use crate::global_timer::GlobalTimer;
-use crate::modules::Modules;
 use crate::ops;
 use crate::permissions::DenoPermissions;
 use crate::resources;
@@ -54,7 +53,6 @@ pub struct State {
   pub permissions: DenoPermissions,
   pub flags: flags::DenoFlags,
   pub metrics: Metrics,
-  pub modules: Mutex<Modules>,
   pub worker_channels: Mutex<WorkerChannels>,
   pub global_timer: Mutex<GlobalTimer>,
   pub workers: Mutex<UserWorkerTable>,
@@ -106,7 +104,6 @@ impl ThreadSafeState {
       permissions: DenoPermissions::from_flags(&flags),
       flags,
       metrics: Metrics::default(),
-      modules: Mutex::new(Modules::new()),
       worker_channels: Mutex::new(internal_channels),
       global_timer: Mutex::new(GlobalTimer::new()),
       workers: Mutex::new(UserWorkerTable::new()),
diff --git a/cli/worker.rs b/cli/worker.rs
index 7178639c5..c43dbf6ee 100644
--- a/cli/worker.rs
+++ b/cli/worker.rs
@@ -9,19 +9,21 @@ use crate::msg;
 use crate::state::ThreadSafeState;
 use crate::tokio_util;
 use deno;
-use deno::deno_mod;
 use deno::JSError;
+use deno::Loader;
 use deno::StartupData;
 use futures::future::Either;
 use futures::Async;
 use futures::Future;
 use std::sync::atomic::Ordering;
+use url::Url;
 
 /// Wraps deno::Isolate to provide source maps, ops for the CLI, and
 /// high-level module loading
 pub struct Worker {
   inner: deno::Isolate<ThreadSafeState>,
-  state: ThreadSafeState,
+  pub modules: deno::Modules,
+  pub state: ThreadSafeState,
 }
 
 impl Worker {
@@ -33,6 +35,7 @@ impl Worker {
     let state_ = state.clone();
     Self {
       inner: deno::Isolate::new(startup_data, state_),
+      modules: deno::Modules::new(),
       state,
     }
   }
@@ -52,143 +55,133 @@ impl Worker {
     self.inner.execute(js_filename, js_source)
   }
 
-  // TODO(ry) make this return a future.
-  fn mod_load_deps(&self, id: deno_mod) -> Result<(), RustOrJsError> {
-    // basically iterate over the imports, start loading them.
-
-    let referrer_name = {
-      let g = self.state.modules.lock().unwrap();
-      g.get_name(id).unwrap().clone()
-    };
-
-    for specifier in self.inner.mod_get_imports(id) {
-      let (name, _local_filename) = self
-        .state
-        .dir
-        .resolve_module(&specifier, &referrer_name)
-        .map_err(DenoError::from)
-        .map_err(RustOrJsError::from)?;
-
-      debug!("mod_load_deps {}", name);
-
-      if !self.state.modules.lock().unwrap().is_registered(&name) {
-        let out = fetch_module_meta_data_and_maybe_compile(
-          &self.state,
-          &specifier,
-          &referrer_name,
-        )?;
-        let child_id = self.mod_new_and_register(
-          false,
-          &out.module_name.clone(),
-          &out.js_source(),
-        )?;
-
-        // The resolved module is an alias to another module (due to redirects).
-        // Save such alias to the module map.
-        if out.module_redirect_source_name.is_some() {
-          self.mod_alias(
-            &out.module_redirect_source_name.clone().unwrap(),
-            &out.module_name,
-          );
+  /// Consumes worker. Executes the provided JavaScript module.
+  pub fn execute_mod_async(
+    self,
+    js_url: &Url,
+    is_prefetch: bool,
+  ) -> impl Future<Item = Self, Error = (RustOrJsError, Self)> {
+    let recursive_load = deno::RecursiveLoad::new(js_url.as_str(), self);
+    recursive_load.and_then(
+      move |(id, mut self_)| -> Result<Self, (deno::JSErrorOr<DenoError>, Self)> {
+        if is_prefetch {
+          Ok(self_)
+        } else {
+          let result = self_.inner.mod_evaluate(id);
+          if let Err(err) = result {
+            Err((deno::JSErrorOr::JSError(err), self_))
+          } else {
+            Ok(self_)
+          }
         }
-
-        self.mod_load_deps(child_id)?;
-      }
-    }
-
-    Ok(())
+      },
+    )
+    .map_err(|(err, self_)| {
+      // Convert to RustOrJsError AND apply_source_map.
+      let err = match err {
+        deno::JSErrorOr::JSError(err) => RustOrJsError::Js(self_.apply_source_map(err)),
+        deno::JSErrorOr::Other(err) => RustOrJsError::Rust(err),
+      };
+      (err, self_)
+    })
   }
 
-  /// Executes the provided JavaScript module.
+  /// Consumes worker. Executes the provided JavaScript module.
   pub fn execute_mod(
-    &mut self,
-    js_filename: &str,
+    self,
+    js_url: &Url,
     is_prefetch: bool,
-  ) -> Result<(), RustOrJsError> {
-    // TODO move state::execute_mod impl here.
-    self
-      .execute_mod_inner(js_filename, is_prefetch)
-      .map_err(|err| match err {
-        RustOrJsError::Js(err) => RustOrJsError::Js(self.apply_source_map(err)),
-        x => x,
-      })
+  ) -> Result<Self, (RustOrJsError, Self)> {
+    tokio_util::block_on(self.execute_mod_async(js_url, is_prefetch))
   }
 
-  /// High-level way to execute modules.
-  /// This will issue HTTP requests and file system calls.
-  /// Blocks. TODO(ry) Don't block.
-  fn execute_mod_inner(
-    &mut self,
-    url: &str,
-    is_prefetch: bool,
-  ) -> Result<(), RustOrJsError> {
-    let out = fetch_module_meta_data_and_maybe_compile(&self.state, url, ".")
-      .map_err(RustOrJsError::from)?;
-
-    // Be careful.
-    // url might not match the actual out.module_name
-    // due to the mechanism of redirection.
-
-    let id = self
-      .mod_new_and_register(true, &out.module_name.clone(), &out.js_source())
-      .map_err(RustOrJsError::from)?;
-
-    // The resolved module is an alias to another module (due to redirects).
-    // Save such alias to the module map.
-    if out.module_redirect_source_name.is_some() {
-      self.mod_alias(
-        &out.module_redirect_source_name.clone().unwrap(),
-        &out.module_name,
-      );
-    }
+  /// Applies source map to the error.
+  fn apply_source_map(&self, err: JSError) -> JSError {
+    js_errors::apply_source_map(&err, &self.state.dir)
+  }
+}
 
-    self.mod_load_deps(id)?;
+// https://html.spec.whatwg.org/multipage/webappapis.html#resolve-a-module-specifier
+// TODO(ry) Add tests.
+// TODO(ry) Move this to core?
+pub fn resolve_module_spec(
+  specifier: &str,
+  base: &str,
+) -> Result<String, url::ParseError> {
+  // 1. Apply the URL parser to specifier. If the result is not failure, return
+  //    the result.
+  // let specifier = parse_local_or_remote(specifier)?.to_string();
+  if let Ok(specifier_url) = Url::parse(specifier) {
+    return Ok(specifier_url.to_string());
+  }
 
-    let state = self.state.clone();
+  // 2. If specifier does not start with the character U+002F SOLIDUS (/), the
+  //    two-character sequence U+002E FULL STOP, U+002F SOLIDUS (./), or the
+  //    three-character sequence U+002E FULL STOP, U+002E FULL STOP, U+002F
+  //    SOLIDUS (../), return failure.
+  if !specifier.starts_with("/")
+    && !specifier.starts_with("./")
+    && !specifier.starts_with("../")
+  {
+    // TODO(ry) This is (probably) not the correct error to return here.
+    return Err(url::ParseError::RelativeUrlWithCannotBeABaseBase);
+  }
 
-    let mut resolve = move |specifier: &str, referrer: deno_mod| -> deno_mod {
-      state.metrics.resolve_count.fetch_add(1, Ordering::Relaxed);
-      let mut modules = state.modules.lock().unwrap();
-      modules.resolve_cb(&state.dir, specifier, referrer)
-    };
+  // 3. Return the result of applying the URL parser to specifier with base URL
+  //    as the base URL.
+  let base_url = Url::parse(base)?;
+  let u = base_url.join(&specifier)?;
+  Ok(u.to_string())
+}
 
-    self
-      .inner
-      .mod_instantiate(id, &mut resolve)
-      .map_err(RustOrJsError::from)?;
-    if !is_prefetch {
-      self.inner.mod_evaluate(id).map_err(RustOrJsError::from)?;
-    }
-    Ok(())
+/// Takes a string representing a path or URL to a module, but of the type
+/// passed through the command-line interface for the main module. This is
+/// slightly different than specifiers used in import statements: "foo.js" for
+/// example is allowed here, whereas in import statements a leading "./" is
+/// required ("./foo.js"). This function is aware of the current working
+/// directory and returns an absolute URL.
+pub fn root_specifier_to_url(
+  root_specifier: &str,
+) -> Result<Url, url::ParseError> {
+  let maybe_url = Url::parse(root_specifier);
+  if let Ok(url) = maybe_url {
+    Ok(url)
+  } else {
+    let cwd = std::env::current_dir().unwrap();
+    let base = Url::from_directory_path(cwd).unwrap();
+    base.join(root_specifier)
   }
+}
 
-  /// Wraps Isolate::mod_new but registers with modules.
-  fn mod_new_and_register(
-    &self,
-    main: bool,
-    name: &str,
-    source: &str,
-  ) -> Result<deno_mod, JSError> {
-    let id = self.inner.mod_new(main, name, source)?;
-    self.state.modules.lock().unwrap().register(id, &name);
-    Ok(id)
-  }
+impl Loader for Worker {
+  type Dispatch = ThreadSafeState;
+  type Error = DenoError;
 
-  /// Create an alias for another module.
-  /// The alias could later be used to grab the module
-  /// which `target` points to.
-  fn mod_alias(&self, name: &str, target: &str) {
-    self.state.modules.lock().unwrap().alias(name, target);
+  fn resolve(specifier: &str, referrer: &str) -> Result<String, Self::Error> {
+    resolve_module_spec(specifier, referrer)
+      .map_err(|url_err| DenoError::from(url_err))
   }
 
-  pub fn print_file_info(&self, module: &str) {
-    let m = self.state.modules.lock().unwrap();
-    m.print_file_info(&self.state.dir, module.to_string());
+  /// Given an absolute url, load its source code.
+  fn load(&mut self, url: &str) -> Box<deno::SourceCodeFuture<Self::Error>> {
+    self
+      .state
+      .metrics
+      .resolve_count
+      .fetch_add(1, Ordering::SeqCst);
+    Box::new(
+      fetch_module_meta_data_and_maybe_compile_async(&self.state, url, ".")
+        .map_err(|err| {
+          eprintln!("{}", err);
+          err
+        }).map(|module_meta_data| module_meta_data.js_source()),
+    )
   }
 
-  /// Applies source map to the error.
-  fn apply_source_map(&self, err: JSError) -> JSError {
-    js_errors::apply_source_map(&err, &self.state.dir)
+  fn isolate_and_modules<'a: 'b + 'c, 'b, 'c>(
+    &'a mut self,
+  ) -> (&'b mut deno::Isolate<Self::Dispatch>, &'c mut deno::Modules) {
+    (&mut self.inner, &mut self.modules)
   }
 }
 
@@ -236,7 +229,7 @@ fn fetch_module_meta_data_and_maybe_compile_async(
     })
 }
 
-fn fetch_module_meta_data_and_maybe_compile(
+pub fn fetch_module_meta_data_and_maybe_compile(
   state: &ThreadSafeState,
   specifier: &str,
   referrer: &str,
@@ -260,46 +253,54 @@ mod tests {
   use std::sync::atomic::Ordering;
 
   #[test]
-  fn execute_mod() {
+  fn execute_mod_esm_imports_a() {
     let filename = std::env::current_dir()
       .unwrap()
       .join("tests/esm_imports_a.js");
-    let filename = filename.to_str().unwrap().to_string();
+    let js_url = Url::from_file_path(filename).unwrap();
 
-    let argv = vec![String::from("./deno"), filename.clone()];
+    let argv = vec![String::from("./deno"), js_url.to_string()];
     let (flags, rest_argv) = flags::set_flags(argv).unwrap();
 
     let state = ThreadSafeState::new(flags, rest_argv, op_selector_std);
     let state_ = state.clone();
     tokio_util::run(lazy(move || {
-      let mut worker =
-        Worker::new("TEST".to_string(), StartupData::None, state);
-      if let Err(err) = worker.execute_mod(&filename, false) {
-        eprintln!("execute_mod err {:?}", err);
-      }
+      let worker = Worker::new("TEST".to_string(), StartupData::None, state);
+      let result = worker.execute_mod(&js_url, false);
+      let worker = match result {
+        Err((err, worker)) => {
+          eprintln!("execute_mod err {:?}", err);
+          worker
+        }
+        Ok(worker) => worker,
+      };
       tokio_util::panic_on_error(worker)
     }));
 
     let metrics = &state_.metrics;
-    assert_eq!(metrics.resolve_count.load(Ordering::SeqCst), 1);
+    assert_eq!(metrics.resolve_count.load(Ordering::SeqCst), 2);
   }
 
   #[test]
   fn execute_mod_circular() {
     let filename = std::env::current_dir().unwrap().join("tests/circular1.js");
-    let filename = filename.to_str().unwrap().to_string();
+    let js_url = Url::from_file_path(filename).unwrap();
 
-    let argv = vec![String::from("./deno"), filename.clone()];
+    let argv = vec![String::from("./deno"), js_url.to_string()];
     let (flags, rest_argv) = flags::set_flags(argv).unwrap();
 
     let state = ThreadSafeState::new(flags, rest_argv, op_selector_std);
     let state_ = state.clone();
     tokio_util::run(lazy(move || {
-      let mut worker =
-        Worker::new("TEST".to_string(), StartupData::None, state);
-      if let Err(err) = worker.execute_mod(&filename, false) {
-        eprintln!("execute_mod err {:?}", err);
-      }
+      let worker = Worker::new("TEST".to_string(), StartupData::None, state);
+      let result = worker.execute_mod(&js_url, false);
+      let worker = match result {
+        Err((err, worker)) => {
+          eprintln!("execute_mod err {:?}", err);
+          worker
+        }
+        Ok(worker) => worker,
+      };
       tokio_util::panic_on_error(worker)
     }));
 
@@ -372,7 +373,7 @@ mod tests {
     tokio_util::init(|| {
       let mut worker = create_test_worker();
       js_check(
-        worker.execute("onmessage = () => { delete window['onmessage']; }"),
+        worker.execute("onmessage = () => { delete window.onmessage; }"),
       );
 
       let resource = worker.state.resource.clone();
@@ -400,4 +401,23 @@ mod tests {
       assert_eq!(resources::get_type(rid), None);
     })
   }
+
+  #[test]
+  fn execute_mod_resolve_error() {
+    // "foo" is not a vailid module specifier so this should return an error.
+    let worker = create_test_worker();
+    let js_url = root_specifier_to_url("does-not-exist").unwrap();
+    let result = worker.execute_mod_async(&js_url, false).wait();
+    assert!(result.is_err());
+  }
+
+  #[test]
+  fn execute_mod_002_hello() {
+    // This assumes cwd is project root (an assumption made throughout the
+    // tests).
+    let worker = create_test_worker();
+    let js_url = root_specifier_to_url("./tests/002_hello.ts").unwrap();
+    let result = worker.execute_mod_async(&js_url, false).wait();
+    assert!(result.is_ok());
+  }
 }