fix(core): allow esm extensions not included in snapshot (#18980)

Fixes #18979.

This changes the predicate for allowing `ext:` specifier resolution from
`snapshot_loaded_and_not_snapshotting` to `ext_resolution_allowed` which
is only set to true during the extension module loading phase. Module
loaders as used in core
are now declared as `ExtModuleLoader` rather than `dyn ModuleLoader`.

9 files changed, 219 insertions, 363 deletions

diff --git a/bench_util/js_runtime.rs b/bench_util/js_runtime.rs
index 57085ef96..a12e6ca62 100644
--- a/bench_util/js_runtime.rs
+++ b/bench_util/js_runtime.rs
@@ -10,9 +10,7 @@ use crate::profiling::is_profiling;
 pub fn create_js_runtime(setup: impl FnOnce() -> Vec<Extension>) -> JsRuntime {
   JsRuntime::new(RuntimeOptions {
     extensions: setup(),
-    module_loader: Some(
-      std::rc::Rc::new(deno_core::ExtModuleLoader::default()),
-    ),
+    module_loader: None,
     ..Default::default()
   })
 }
diff --git a/cli/tests/testdata/run/extension_dynamic_import.ts.out b/cli/tests/testdata/run/extension_dynamic_import.ts.out
index 18b05ea47..081318960 100644
--- a/cli/tests/testdata/run/extension_dynamic_import.ts.out
+++ b/cli/tests/testdata/run/extension_dynamic_import.ts.out
@@ -1,4 +1,4 @@
-error: Uncaught TypeError: Cannot load extension module from external code
+error: Uncaught (in promise) TypeError: Cannot load extension module from external code
 await import("ext:runtime/01_errors.js");
 ^
     at [WILDCARD]/extension_dynamic_import.ts:1:1
diff --git a/core/bindings.rs b/core/bindings.rs
index 2d9c91461..1437bc657 100644
--- a/core/bindings.rs
+++ b/core/bindings.rs
@@ -10,7 +10,6 @@ use crate::error::is_instance_of_error;
 use crate::error::JsStackFrame;
 use crate::modules::get_asserted_module_type_from_assertions;
 use crate::modules::parse_import_assertions;
-use crate::modules::resolve_helper;
 use crate::modules::validate_import_assertions;
 use crate::modules::ImportAssertionsKind;
 use crate::modules::ModuleMap;
@@ -259,46 +258,43 @@ pub fn host_import_module_dynamically_callback<'s>(
     .unwrap()
     .to_rust_string_lossy(scope);
 
-  let is_ext_module = specifier_str.starts_with("ext:");
   let resolver = v8::PromiseResolver::new(scope).unwrap();
   let promise = resolver.get_promise(scope);
 
-  if !is_ext_module {
-    let assertions = parse_import_assertions(
-      scope,
-      import_assertions,
-      ImportAssertionsKind::DynamicImport,
-    );
+  let assertions = parse_import_assertions(
+    scope,
+    import_assertions,
+    ImportAssertionsKind::DynamicImport,
+  );
 
-    {
-      let tc_scope = &mut v8::TryCatch::new(scope);
-      validate_import_assertions(tc_scope, &assertions);
-      if tc_scope.has_caught() {
-        let e = tc_scope.exception().unwrap();
-        resolver.reject(tc_scope, e);
-      }
+  {
+    let tc_scope = &mut v8::TryCatch::new(scope);
+    validate_import_assertions(tc_scope, &assertions);
+    if tc_scope.has_caught() {
+      let e = tc_scope.exception().unwrap();
+      resolver.reject(tc_scope, e);
     }
-    let asserted_module_type =
-      get_asserted_module_type_from_assertions(&assertions);
+  }
+  let asserted_module_type =
+    get_asserted_module_type_from_assertions(&assertions);
 
-    let resolver_handle = v8::Global::new(scope, resolver);
-    {
-      let state_rc = JsRuntime::state(scope);
-      let module_map_rc = JsRuntime::module_map(scope);
+  let resolver_handle = v8::Global::new(scope, resolver);
+  {
+    let state_rc = JsRuntime::state(scope);
+    let module_map_rc = JsRuntime::module_map(scope);
 
-      debug!(
-        "dyn_import specifier {} referrer {} ",
-        specifier_str, referrer_name_str
-      );
-      ModuleMap::load_dynamic_import(
-        module_map_rc,
-        &specifier_str,
-        &referrer_name_str,
-        asserted_module_type,
-        resolver_handle,
-      );
-      state_rc.borrow_mut().notify_new_dynamic_import();
-    }
+    debug!(
+      "dyn_import specifier {} referrer {} ",
+      specifier_str, referrer_name_str
+    );
+    ModuleMap::load_dynamic_import(
+      module_map_rc,
+      &specifier_str,
+      &referrer_name_str,
+      asserted_module_type,
+      resolver_handle,
+    );
+    state_rc.borrow_mut().notify_new_dynamic_import();
   }
   // Map errors from module resolution (not JS errors from module execution) to
   // ones rethrown from this scope, so they include the call stack of the
@@ -311,16 +307,6 @@ pub fn host_import_module_dynamically_callback<'s>(
 
   let promise = promise.catch(scope, map_err).unwrap();
 
-  if is_ext_module {
-    let message = v8::String::new_external_onebyte_static(
-      scope,
-      b"Cannot load extension module from external code",
-    )
-    .unwrap();
-    let exception = v8::Exception::type_error(scope, message);
-    resolver.reject(scope, exception);
-  }
-
   Some(promise)
 }
 
@@ -375,13 +361,7 @@ fn import_meta_resolve(
     url_prop.to_rust_string_lossy(scope)
   };
   let module_map_rc = JsRuntime::module_map(scope);
-  let (loader, snapshot_loaded_and_not_snapshotting) = {
-    let module_map = module_map_rc.borrow();
-    (
-      module_map.loader.clone(),
-      module_map.snapshot_loaded_and_not_snapshotting,
-    )
-  };
+  let loader = module_map_rc.borrow().loader.clone();
   let specifier_str = specifier.to_rust_string_lossy(scope);
 
   if specifier_str.starts_with("npm:") {
@@ -389,13 +369,8 @@ fn import_meta_resolve(
     return;
   }
 
-  match resolve_helper(
-    snapshot_loaded_and_not_snapshotting,
-    loader,
-    &specifier_str,
-    &referrer,
-    ResolutionKind::DynamicImport,
-  ) {
+  match loader.resolve(&specifier_str, &referrer, ResolutionKind::DynamicImport)
+  {
     Ok(resolved) => {
       let resolved_val = serde_v8::to_v8(scope, resolved.as_str()).unwrap();
       rv.set(resolved_val);
diff --git a/core/extensions.rs b/core/extensions.rs
index a8b52eb3b..ba151da3d 100644
--- a/core/extensions.rs
+++ b/core/extensions.rs
@@ -471,6 +471,16 @@ impl Extension {
   pub fn disable(self) -> Self {
     self.enabled(false)
   }
+
+  pub(crate) fn find_esm(
+    &self,
+    specifier: &str,
+  ) -> Option<&ExtensionFileSource> {
+    self
+      .get_esm_sources()?
+      .iter()
+      .find(|s| s.specifier == specifier)
+  }
 }
 
 // Provides a convenient builder pattern to declare Extensions
diff --git a/core/modules.rs b/core/modules.rs
index c63c4dd30..bc795de5c 100644
--- a/core/modules.rs
+++ b/core/modules.rs
@@ -9,6 +9,7 @@ use crate::module_specifier::ModuleSpecifier;
 use crate::resolve_import;
 use crate::resolve_url;
 use crate::snapshot_util::SnapshottedData;
+use crate::Extension;
 use crate::JsRuntime;
 use crate::OpState;
 use anyhow::Error;
@@ -379,25 +380,6 @@ impl ModuleLoader for NoopModuleLoader {
   }
 }
 
-/// Helper function, that calls into `loader.resolve()`, but denies resolution
-/// of `ext` scheme if we are running with a snapshot loaded and not
-/// creating a snapshot
-pub(crate) fn resolve_helper(
-  snapshot_loaded_and_not_snapshotting: bool,
-  loader: Rc<dyn ModuleLoader>,
-  specifier: &str,
-  referrer: &str,
-  kind: ResolutionKind,
-) -> Result<ModuleSpecifier, Error> {
-  if snapshot_loaded_and_not_snapshotting && specifier.starts_with("ext:") {
-    return Err(generic_error(
-      "Cannot load extension module from external code",
-    ));
-  }
-
-  loader.resolve(specifier, referrer, kind)
-}
-
 /// Function that can be passed to the `ExtModuleLoader` that allows to
 /// transpile sources before passing to V8.
 pub type ExtModuleLoaderCb =
@@ -405,7 +387,8 @@ pub type ExtModuleLoaderCb =
 
 pub struct ExtModuleLoader {
   module_loader: Rc<dyn ModuleLoader>,
-  esm_sources: Vec<ExtensionFileSource>,
+  extensions: Rc<RefCell<Vec<Extension>>>,
+  ext_resolution_allowed: RefCell<bool>,
   used_esm_sources: RefCell<HashMap<String, bool>>,
   maybe_load_callback: Option<ExtModuleLoaderCb>,
 }
@@ -414,7 +397,8 @@ impl Default for ExtModuleLoader {
   fn default() -> Self {
     Self {
       module_loader: Rc::new(NoopModuleLoader),
-      esm_sources: vec![],
+      extensions: Default::default(),
+      ext_resolution_allowed: Default::default(),
       used_esm_sources: RefCell::new(HashMap::default()),
       maybe_load_callback: None,
     }
@@ -424,70 +408,46 @@ impl Default for ExtModuleLoader {
 impl ExtModuleLoader {
   pub fn new(
     module_loader: Option<Rc<dyn ModuleLoader>>,
-    esm_sources: Vec<ExtensionFileSource>,
+    extensions: Rc<RefCell<Vec<Extension>>>,
     maybe_load_callback: Option<ExtModuleLoaderCb>,
   ) -> Self {
-    let used_esm_sources: HashMap<String, bool> = esm_sources
+    let used_esm_sources: HashMap<String, bool> = extensions
+      .borrow()
       .iter()
+      .flat_map(|e| e.get_esm_sources())
+      .flatten()
       .map(|file_source| (file_source.specifier.to_string(), false))
       .collect();
 
     ExtModuleLoader {
       module_loader: module_loader.unwrap_or_else(|| Rc::new(NoopModuleLoader)),
-      esm_sources,
+      extensions,
+      ext_resolution_allowed: Default::default(),
       used_esm_sources: RefCell::new(used_esm_sources),
       maybe_load_callback,
     }
   }
-}
-
-impl Drop for ExtModuleLoader {
-  fn drop(&mut self) {
-    let used_esm_sources = self.used_esm_sources.get_mut();
-    let unused_modules: Vec<_> = used_esm_sources
-      .iter()
-      .filter(|(_s, v)| !*v)
-      .map(|(s, _)| s)
-      .collect();
-
-    if !unused_modules.is_empty() {
-      let mut msg =
-        "Following modules were passed to ExtModuleLoader but never used:\n"
-          .to_string();
-      for m in unused_modules {
-        msg.push_str("  - ");
-        msg.push_str(m);
-        msg.push('\n');
-      }
-      panic!("{}", msg);
-    }
-  }
-}
 
-impl ModuleLoader for ExtModuleLoader {
-  fn resolve(
+  pub fn resolve(
     &self,
     specifier: &str,
     referrer: &str,
     kind: ResolutionKind,
   ) -> Result<ModuleSpecifier, Error> {
-    if let Ok(url_specifier) = ModuleSpecifier::parse(specifier) {
-      if url_specifier.scheme() == "ext" {
-        let referrer_specifier = ModuleSpecifier::parse(referrer).ok();
-        if referrer == "." || referrer_specifier.unwrap().scheme() == "ext" {
-          return Ok(url_specifier);
-        } else {
-          return Err(generic_error(
-            "Cannot load extension module from external code",
-          ));
-        };
+    if specifier.starts_with("ext:") {
+      if !referrer.starts_with("ext:") && referrer != "."
+        || !*self.ext_resolution_allowed.borrow()
+      {
+        return Err(generic_error(
+          "Cannot load extension module from external code",
+        ));
       }
+      return Ok(ModuleSpecifier::parse(specifier)?);
     }
-
     self.module_loader.resolve(specifier, referrer, kind)
   }
 
-  fn load(
+  pub fn load(
     &self,
     module_specifier: &ModuleSpecifier,
     maybe_referrer: Option<&ModuleSpecifier>,
@@ -502,10 +462,10 @@ impl ModuleLoader for ExtModuleLoader {
     }
 
     let specifier = module_specifier.to_string();
-    let maybe_file_source = self
-      .esm_sources
+    let extensions = self.extensions.borrow();
+    let maybe_file_source = extensions
       .iter()
-      .find(|file_source| file_source.specifier == module_specifier.as_str());
+      .find_map(|e| e.find_esm(module_specifier.as_str()));
 
     if let Some(file_source) = maybe_file_source {
       {
@@ -538,7 +498,7 @@ impl ModuleLoader for ExtModuleLoader {
     .boxed_local()
   }
 
-  fn prepare_load(
+  pub fn prepare_load(
     &self,
     op_state: Rc<RefCell<OpState>>,
     module_specifier: &ModuleSpecifier,
@@ -556,6 +516,37 @@ impl ModuleLoader for ExtModuleLoader {
       is_dyn_import,
     )
   }
+
+  pub fn allow_ext_resolution(&self) {
+    *self.ext_resolution_allowed.borrow_mut() = true;
+  }
+
+  pub fn disallow_ext_resolution(&self) {
+    *self.ext_resolution_allowed.borrow_mut() = false;
+  }
+}
+
+impl Drop for ExtModuleLoader {
+  fn drop(&mut self) {
+    let used_esm_sources = self.used_esm_sources.get_mut();
+    let unused_modules: Vec<_> = used_esm_sources
+      .iter()
+      .filter(|(_s, v)| !*v)
+      .map(|(s, _)| s)
+      .collect();
+
+    if !unused_modules.is_empty() {
+      let mut msg =
+        "Following modules were passed to ExtModuleLoader but never used:\n"
+          .to_string();
+      for m in unused_modules {
+        msg.push_str("  - ");
+        msg.push_str(m);
+        msg.push('\n');
+      }
+      panic!("{}", msg);
+    }
+  }
 }
 
 /// Basic file system module loader.
@@ -643,8 +634,7 @@ pub(crate) struct RecursiveModuleLoad {
   // These three fields are copied from `module_map_rc`, but they are cloned
   // ahead of time to avoid already-borrowed errors.
   op_state: Rc<RefCell<OpState>>,
-  loader: Rc<dyn ModuleLoader>,
-  snapshot_loaded_and_not_snapshotting: bool,
+  loader: Rc<ExtModuleLoader>,
 }
 
 impl RecursiveModuleLoad {
@@ -700,9 +690,6 @@ impl RecursiveModuleLoad {
       init,
       state: LoadState::Init,
       module_map_rc: module_map_rc.clone(),
-      snapshot_loaded_and_not_snapshotting: module_map_rc
-        .borrow()
-        .snapshot_loaded_and_not_snapshotting,
       op_state,
       loader,
       pending: FuturesUnordered::new(),
@@ -731,29 +718,17 @@ impl RecursiveModuleLoad {
 
   fn resolve_root(&self) -> Result<ModuleSpecifier, Error> {
     match self.init {
-      LoadInit::Main(ref specifier) => resolve_helper(
-        self.snapshot_loaded_and_not_snapshotting,
-        self.loader.clone(),
-        specifier,
-        ".",
-        ResolutionKind::MainModule,
-      ),
-      LoadInit::Side(ref specifier) => resolve_helper(
-        self.snapshot_loaded_and_not_snapshotting,
-        self.loader.clone(),
-        specifier,
-        ".",
-        ResolutionKind::Import,
-      ),
-      LoadInit::DynamicImport(ref specifier, ref referrer, _) => {
-        resolve_helper(
-          self.snapshot_loaded_and_not_snapshotting,
-          self.loader.clone(),
-          specifier,
-          referrer,
-          ResolutionKind::DynamicImport,
-        )
+      LoadInit::Main(ref specifier) => {
+        self
+          .loader
+          .resolve(specifier, ".", ResolutionKind::MainModule)
+      }
+      LoadInit::Side(ref specifier) => {
+        self.loader.resolve(specifier, ".", ResolutionKind::Import)
       }
+      LoadInit::DynamicImport(ref specifier, ref referrer, _) => self
+        .loader
+        .resolve(specifier, referrer, ResolutionKind::DynamicImport),
     }
   }
 
@@ -762,29 +737,21 @@ impl RecursiveModuleLoad {
 
     let (module_specifier, maybe_referrer) = match self.init {
       LoadInit::Main(ref specifier) => {
-        let spec = resolve_helper(
-          self.snapshot_loaded_and_not_snapshotting,
-          self.loader.clone(),
-          specifier,
-          ".",
-          ResolutionKind::MainModule,
-        )?;
+        let spec =
+          self
+            .loader
+            .resolve(specifier, ".", ResolutionKind::MainModule)?;
         (spec, None)
       }
       LoadInit::Side(ref specifier) => {
-        let spec = resolve_helper(
-          self.snapshot_loaded_and_not_snapshotting,
-          self.loader.clone(),
-          specifier,
-          ".",
-          ResolutionKind::Import,
-        )?;
+        let spec =
+          self
+            .loader
+            .resolve(specifier, ".", ResolutionKind::Import)?;
         (spec, None)
       }
       LoadInit::DynamicImport(ref specifier, ref referrer, _) => {
-        let spec = resolve_helper(
-          self.snapshot_loaded_and_not_snapshotting,
-          self.loader.clone(),
+        let spec = self.loader.resolve(
           specifier,
           referrer,
           ResolutionKind::DynamicImport,
@@ -1093,7 +1060,7 @@ pub(crate) struct ModuleMap {
   pub(crate) next_load_id: ModuleLoadId,
 
   // Handling of futures for loading module sources
-  pub loader: Rc<dyn ModuleLoader>,
+  pub loader: Rc<ExtModuleLoader>,
   op_state: Rc<RefCell<OpState>>,
   pub(crate) dynamic_import_map:
     HashMap<ModuleLoadId, v8::Global<v8::PromiseResolver>>,
@@ -1105,8 +1072,6 @@ pub(crate) struct ModuleMap {
   // This store is used temporarly, to forward parsed JSON
   // value from `new_json_module` to `json_module_evaluation_steps`
   json_value_store: HashMap<v8::Global<v8::Module>, v8::Global<v8::Value>>,
-
-  pub(crate) snapshot_loaded_and_not_snapshotting: bool,
 }
 
 impl ModuleMap {
@@ -1381,9 +1346,8 @@ impl ModuleMap {
   }
 
   pub(crate) fn new(
-    loader: Rc<dyn ModuleLoader>,
+    loader: Rc<ExtModuleLoader>,
     op_state: Rc<RefCell<OpState>>,
-    snapshot_loaded_and_not_snapshotting: bool,
   ) -> ModuleMap {
     Self {
       handles: vec![],
@@ -1397,7 +1361,6 @@ impl ModuleMap {
       preparing_dynamic_imports: FuturesUnordered::new(),
       pending_dynamic_imports: FuturesUnordered::new(),
       json_value_store: HashMap::new(),
-      snapshot_loaded_and_not_snapshotting,
     }
   }
 
@@ -1526,9 +1489,7 @@ impl ModuleMap {
         return Err(ModuleError::Exception(exception));
       }
 
-      let module_specifier = match resolve_helper(
-        self.snapshot_loaded_and_not_snapshotting,
-        self.loader.clone(),
+      let module_specifier = match self.loader.resolve(
         &import_specifier,
         name.as_ref(),
         if is_dynamic_import {
@@ -1717,20 +1678,9 @@ impl ModuleMap {
       .dynamic_import_map
       .insert(load.id, resolver_handle);
 
-    let (loader, snapshot_loaded_and_not_snapshotting) = {
-      let module_map = module_map_rc.borrow();
-      (
-        module_map.loader.clone(),
-        module_map.snapshot_loaded_and_not_snapshotting,
-      )
-    };
-    let resolve_result = resolve_helper(
-      snapshot_loaded_and_not_snapshotting,
-      loader,
-      specifier,
-      referrer,
-      ResolutionKind::DynamicImport,
-    );
+    let loader = module_map_rc.borrow().loader.clone();
+    let resolve_result =
+      loader.resolve(specifier, referrer, ResolutionKind::DynamicImport);
     let fut = match resolve_result {
       Ok(module_specifier) => {
         if module_map_rc
@@ -1764,14 +1714,10 @@ impl ModuleMap {
     referrer: &str,
     import_assertions: HashMap<String, String>,
   ) -> Option<v8::Local<'s, v8::Module>> {
-    let resolved_specifier = resolve_helper(
-      self.snapshot_loaded_and_not_snapshotting,
-      self.loader.clone(),
-      specifier,
-      referrer,
-      ResolutionKind::Import,
-    )
-    .expect("Module should have been already resolved");
+    let resolved_specifier = self
+      .loader
+      .resolve(specifier, referrer, ResolutionKind::Import)
+      .expect("Module should have been already resolved");
 
     let module_type =
       get_asserted_module_type_from_assertions(&import_assertions);
@@ -3042,48 +2988,34 @@ if (import.meta.url != 'file:///main_with_code.js') throw Error();
   }
 
   #[test]
-  fn ext_module_loader() {
+  fn ext_resolution() {
+    let loader = ExtModuleLoader::default();
+    loader.allow_ext_resolution();
+    loader
+      .resolve("ext:core.js", "ext:referrer.js", ResolutionKind::Import)
+      .unwrap();
+    loader
+      .resolve("ext:core.js", ".", ResolutionKind::Import)
+      .unwrap();
+  }
+
+  #[test]
+  fn ext_resolution_failure() {
     let loader = ExtModuleLoader::default();
-    assert!(loader
-      .resolve("ext:foo", "ext:bar", ResolutionKind::Import)
-      .is_ok());
+    loader.allow_ext_resolution();
     assert_eq!(
       loader
-        .resolve("ext:foo", "file://bar", ResolutionKind::Import)
+        .resolve("ext:core.js", "file://bar", ResolutionKind::Import,)
         .err()
         .map(|e| e.to_string()),
       Some("Cannot load extension module from external code".to_string())
     );
+    loader.disallow_ext_resolution();
     assert_eq!(
       loader
-        .resolve("file://foo", "file://bar", ResolutionKind::Import)
+        .resolve("ext:core.js", "ext:referrer.js", ResolutionKind::Import,)
         .err()
         .map(|e| e.to_string()),
-      Some(
-        "Module loading is not supported; attempted to resolve: \"file://foo\" from \"file://bar\""
-          .to_string()
-      )
-    );
-    assert_eq!(
-      loader
-        .resolve("file://foo", "ext:bar", ResolutionKind::Import)
-        .err()
-        .map(|e| e.to_string()),
-      Some(
-        "Module loading is not supported; attempted to resolve: \"file://foo\" from \"ext:bar\""
-        .to_string()
-      )
-    );
-    assert_eq!(
-      resolve_helper(
-        true,
-        Rc::new(loader),
-        "ext:core.js",
-        "file://bar",
-        ResolutionKind::Import,
-      )
-      .err()
-      .map(|e| e.to_string()),
       Some("Cannot load extension module from external code".to_string())
     );
   }
diff --git a/core/runtime.rs b/core/runtime.rs
index 9676ce571..1cbefb6fe 100644
--- a/core/runtime.rs
+++ b/core/runtime.rs
@@ -132,7 +132,7 @@ pub struct JsRuntime {
   v8_isolate: Option<v8::OwnedIsolate>,
   snapshot_options: snapshot_util::SnapshotOptions,
   allocations: IsolateAllocations,
-  extensions: Vec<Extension>,
+  extensions: Rc<RefCell<Vec<Extension>>>,
   event_loop_middlewares: Vec<Box<OpEventLoopFn>>,
   // Marks if this is considered the top-level runtime. Used only be inspector.
   is_main: bool,
@@ -416,7 +416,7 @@ impl JsRuntime {
     let global_context;
     let mut maybe_snapshotted_data = None;
 
-    let (mut isolate, snapshot_options) = if snapshot_options.will_snapshot() {
+    let mut isolate = if snapshot_options.will_snapshot() {
       let snapshot_creator =
         snapshot_util::create_snapshot_creator(refs, options.startup_snapshot);
       let mut isolate = JsRuntime::setup_isolate(snapshot_creator);
@@ -433,7 +433,7 @@ impl JsRuntime {
 
         global_context = v8::Global::new(scope, context);
       }
-      (isolate, snapshot_options)
+      isolate
     } else {
       #[cfg(not(target_env = "msvc"))]
       let vtable: &'static v8::RustAllocatorVtable<
@@ -492,7 +492,7 @@ impl JsRuntime {
         global_context = v8::Global::new(scope, context);
       }
 
-      (isolate, snapshot_options)
+      isolate
     };
 
     // SAFETY: this is first use of `isolate_ptr` so we are sure we're
@@ -521,61 +521,33 @@ impl JsRuntime {
       None
     };
 
-    let loader = if snapshot_options != snapshot_util::SnapshotOptions::Load {
-      let esm_sources = options
+    let loader = options
+      .module_loader
+      .unwrap_or_else(|| Rc::new(NoopModuleLoader));
+    #[cfg(feature = "include_js_files_for_snapshotting")]
+    if snapshot_options.will_snapshot() {
+      for source in options
         .extensions
         .iter()
-        .flat_map(|ext| match ext.get_esm_sources() {
-          Some(s) => s.to_owned(),
-          None => vec![],
-        })
-        .collect::<Vec<ExtensionFileSource>>();
-
-      #[cfg(feature = "include_js_files_for_snapshotting")]
-      if snapshot_options != snapshot_util::SnapshotOptions::None {
-        for source in &esm_sources {
-          use crate::ExtensionFileSourceCode;
-          if let ExtensionFileSourceCode::LoadedFromFsDuringSnapshot(path) =
-            &source.code
-          {
-            println!("cargo:rerun-if-changed={}", path.display())
-          }
-        }
-      }
-
-      #[cfg(feature = "include_js_files_for_snapshotting")]
+        .flat_map(|e| vec![e.get_esm_sources(), e.get_js_sources()])
+        .flatten()
+        .flatten()
       {
-        let js_sources = options
-          .extensions
-          .iter()
-          .flat_map(|ext| match ext.get_js_sources() {
-            Some(s) => s.to_owned(),
-            None => vec![],
-          })
-          .collect::<Vec<ExtensionFileSource>>();
-
-        if snapshot_options != snapshot_util::SnapshotOptions::None {
-          for source in &js_sources {
-            use crate::ExtensionFileSourceCode;
-            if let ExtensionFileSourceCode::LoadedFromFsDuringSnapshot(path) =
-              &source.code
-            {
-              println!("cargo:rerun-if-changed={}", path.display())
-            }
-          }
+        use crate::ExtensionFileSourceCode;
+        if let ExtensionFileSourceCode::LoadedFromFsDuringSnapshot(path) =
+          &source.code
+        {
+          println!("cargo:rerun-if-changed={}", path.display())
         }
       }
-
-      Rc::new(crate::modules::ExtModuleLoader::new(
-        options.module_loader,
-        esm_sources,
-        options.snapshot_module_load_cb,
-      ))
-    } else {
-      options
-        .module_loader
-        .unwrap_or_else(|| Rc::new(NoopModuleLoader))
-    };
+    }
+    let num_extensions = options.extensions.len();
+    let extensions = Rc::new(RefCell::new(options.extensions));
+    let ext_loader = Rc::new(crate::modules::ExtModuleLoader::new(
+      Some(loader.clone()),
+      extensions.clone(),
+      options.snapshot_module_load_cb,
+    ));
 
     {
       let mut state = state_rc.borrow_mut();
@@ -589,12 +561,8 @@ impl JsRuntime {
       Self::STATE_DATA_OFFSET,
       Rc::into_raw(state_rc.clone()) as *mut c_void,
     );
-
-    let module_map_rc = Rc::new(RefCell::new(ModuleMap::new(
-      loader,
-      op_state,
-      snapshot_options == snapshot_util::SnapshotOptions::Load,
-    )));
+    let module_map_rc =
+      Rc::new(RefCell::new(ModuleMap::new(ext_loader, op_state)));
     if let Some(snapshotted_data) = maybe_snapshotted_data {
       let scope =
         &mut v8::HandleScope::with_context(&mut isolate, global_context);
@@ -610,10 +578,10 @@ impl JsRuntime {
       v8_isolate: Some(isolate),
       snapshot_options,
       allocations: IsolateAllocations::default(),
-      event_loop_middlewares: Vec::with_capacity(options.extensions.len()),
-      extensions: options.extensions,
+      event_loop_middlewares: Vec::with_capacity(num_extensions),
+      extensions,
       state: state_rc,
-      module_map: Some(module_map_rc),
+      module_map: Some(module_map_rc.clone()),
       is_main: options.is_main,
     };
 
@@ -621,7 +589,9 @@ impl JsRuntime {
     // available during the initialization process.
     js_runtime.init_extension_ops().unwrap();
     let realm = js_runtime.global_realm();
+    module_map_rc.borrow().loader.allow_ext_resolution();
     js_runtime.init_extension_js(&realm).unwrap();
+    module_map_rc.borrow().loader.disallow_ext_resolution();
 
     js_runtime
   }
@@ -722,7 +692,21 @@ impl JsRuntime {
       JsRealm::new(v8::Global::new(scope, context))
     };
 
+    self
+      .module_map
+      .as_ref()
+      .unwrap()
+      .borrow()
+      .loader
+      .allow_ext_resolution();
     self.init_extension_js(&realm)?;
+    self
+      .module_map
+      .as_ref()
+      .unwrap()
+      .borrow()
+      .loader
+      .disallow_ext_resolution();
     Ok(realm)
   }
 
@@ -790,7 +774,7 @@ impl JsRuntime {
 
     // Take extensions to avoid double-borrow
     let extensions = std::mem::take(&mut self.extensions);
-    for ext in &extensions {
+    for ext in extensions.borrow().iter() {
       {
         if let Some(esm_files) = ext.get_esm_sources() {
           if let Some(entry_point) = ext.get_esm_entry_point() {
@@ -863,23 +847,15 @@ impl JsRuntime {
   /// Initializes ops of provided Extensions
   fn init_extension_ops(&mut self) -> Result<(), Error> {
     let op_state = self.op_state();
-    // Take extensions to avoid double-borrow
-    {
-      let mut extensions: Vec<Extension> = std::mem::take(&mut self.extensions);
-
-      // Setup state
-      for e in extensions.iter_mut() {
-        // ops are already registered during in bindings::initialize_context();
-        e.init_state(&mut op_state.borrow_mut());
-
-        // Setup event-loop middleware
-        if let Some(middleware) = e.init_event_loop_middleware() {
-          self.event_loop_middlewares.push(middleware);
-        }
+    // Setup state
+    for e in self.extensions.borrow_mut().iter_mut() {
+      // ops are already registered during in bindings::initialize_context();
+      e.init_state(&mut op_state.borrow_mut());
+
+      // Setup event-loop middleware
+      if let Some(middleware) = e.init_event_loop_middleware() {
+        self.event_loop_middlewares.push(middleware);
       }
-
-      // Restore extensions
-      self.extensions = extensions;
     }
     Ok(())
   }
diff --git a/runtime/examples/hello_runtime.js b/runtime/examples/hello_runtime.js
index 066fa21d6..5b079d8d8 100644
--- a/runtime/examples/hello_runtime.js
+++ b/runtime/examples/hello_runtime.js
@@ -1,3 +1,4 @@
 // Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
 console.log("Hello world!");
 console.log(Deno);
+Extension.hello();
diff --git a/runtime/examples/hello_runtime.rs b/runtime/examples/hello_runtime.rs
index 2e930a03f..157a200f4 100644
--- a/runtime/examples/hello_runtime.rs
+++ b/runtime/examples/hello_runtime.rs
@@ -1,72 +1,31 @@
 // Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
 
-use deno_core::anyhow::Context;
 use deno_core::error::AnyError;
 use deno_core::FsModuleLoader;
-use deno_runtime::deno_broadcast_channel::InMemoryBroadcastChannel;
-use deno_runtime::deno_web::BlobStore;
 use deno_runtime::permissions::PermissionsContainer;
 use deno_runtime::worker::MainWorker;
 use deno_runtime::worker::WorkerOptions;
-use deno_runtime::BootstrapOptions;
 use std::path::Path;
 use std::rc::Rc;
-use std::sync::Arc;
 
-fn get_error_class_name(e: &AnyError) -> &'static str {
-  deno_runtime::errors::get_error_class_name(e).unwrap_or("Error")
-}
+deno_core::extension!(hello_runtime, esm = ["hello_runtime_bootstrap.js"]);
 
 #[tokio::main]
 async fn main() -> Result<(), AnyError> {
-  let module_loader = Rc::new(FsModuleLoader);
-  let create_web_worker_cb = Arc::new(|_| {
-    todo!("Web workers are not supported in the example");
-  });
-  let web_worker_event_cb = Arc::new(|_| {
-    todo!("Web workers are not supported in the example");
-  });
-
-  let options = WorkerOptions {
-    bootstrap: BootstrapOptions::default(),
-    extensions: vec![],
-    startup_snapshot: None,
-    unsafely_ignore_certificate_errors: None,
-    root_cert_store_provider: None,
-    seed: None,
-    source_map_getter: None,
-    format_js_error_fn: None,
-    web_worker_preload_module_cb: web_worker_event_cb.clone(),
-    web_worker_pre_execute_module_cb: web_worker_event_cb,
-    create_web_worker_cb,
-    maybe_inspector_server: None,
-    should_break_on_first_statement: false,
-    should_wait_for_inspector_session: false,
-    module_loader,
-    node_fs: None,
-    npm_resolver: None,
-    get_error_class_fn: Some(&get_error_class_name),
-    cache_storage_dir: None,
-    origin_storage_dir: None,
-    blob_store: BlobStore::default(),
-    broadcast_channel: InMemoryBroadcastChannel::default(),
-    shared_array_buffer_store: None,
-    compiled_wasm_module_store: None,
-    stdio: Default::default(),
-  };
-
   let js_path =
     Path::new(env!("CARGO_MANIFEST_DIR")).join("examples/hello_runtime.js");
   let main_module = deno_core::resolve_path(
     &js_path.to_string_lossy(),
-    &std::env::current_dir().context("Unable to get CWD")?,
+    &std::env::current_dir()?,
   )?;
-  let permissions = PermissionsContainer::allow_all();
-
   let mut worker = MainWorker::bootstrap_from_options(
     main_module.clone(),
-    permissions,
-    options,
+    PermissionsContainer::allow_all(),
+    WorkerOptions {
+      module_loader: Rc::new(FsModuleLoader),
+      extensions: vec![hello_runtime::init_ops_and_esm()],
+      ..Default::default()
+    },
   );
   worker.execute_main_module(&main_module).await?;
   worker.run_event_loop(false).await?;
diff --git a/runtime/examples/hello_runtime_bootstrap.js b/runtime/examples/hello_runtime_bootstrap.js
new file mode 100644
index 000000000..759dde939
--- /dev/null
+++ b/runtime/examples/hello_runtime_bootstrap.js
@@ -0,0 +1,5 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+function hello() {
+  console.log("Hello from extension!");
+}
+globalThis.Extension = { hello };