feat: allow using fetchEvent for wasi:http/incoming-handler@0.2.0 optionally (#122)

Author: karthik2804

README.md

@@ -133,6 +133,10 @@ Note that pure components **will not report errors and will instead trap**, so t
 Note that features explicitly imported by the target world cannot be disabled - if you target a component to a world
 that imports `wasi:clocks`, then `disableFeatures: ['clocks']` will not be supported.
 
+## Using StarlingMonkey's `fetch-event`
+
+The StarlingMonkey engine provides the ability to use `fetchEvent` to handle calls to `wasi:http/incoming-handler@0.2.0#handle`. When targeting worlds that export `wasi:http/incoming-handler@0.2.0` the fetch event will automatically be attached. Alternatively, to override the fetch event with a custom handler, export an explict `incomingHandler` or `'wasi:http/incoming-handler@0.2.0'` object. Using the `fetchEvent` requires enabling the `http` feature. 
+
 ## API
 
 ```ts

crates/spidermonkey-embedding-splicer/src/bindgen.rs

@@ -1,4 +1,4 @@
-use crate::{uwrite, uwriteln};
+use crate::{uwrite, uwriteln, Features};
 use anyhow::{bail, Result};
 use heck::*;
 use js_component_bindgen::function_bindgen::{
@@ -131,6 +131,7 @@ pub fn componentize_bindgen(
     name: &str,
     guest_imports: &Vec<String>,
     guest_exports: &Vec<String>,
+    features: Vec<Features>,
 ) -> Result<Componentization> {
     let mut bindgen = JsBindgen {
         src: Source::default(),
@@ -156,7 +157,7 @@ pub fn componentize_bindgen(
 
     bindgen.imports_bindgen(&guest_imports);
 
-    bindgen.exports_bindgen(&guest_exports)?;
+    bindgen.exports_bindgen(&guest_exports, features)?;
     bindgen.esm_bindgen.populate_export_aliases();
 
     // consolidate import specifiers and generate wrappers
@@ -370,7 +371,11 @@ impl JsBindgen<'_> {
         return intrinsic.name().to_string();
     }
 
-    fn exports_bindgen(&mut self, guest_exports: &Vec<String>) -> Result<()> {
+    fn exports_bindgen(
+        &mut self,
+        guest_exports: &Vec<String>,
+        features: Vec<Features>,
+    ) -> Result<()> {
         for (key, export) in &self.resolve.worlds[self.world].exports {
             let name = self.resolve.name_world_key(key);
 
@@ -382,9 +387,25 @@ impl JsBindgen<'_> {
                 WorldKey::Interface(iface) => {
                     if !guest_exports.contains(&name) {
                         let iface = &self.resolve.interfaces[*iface];
-                        if let Some(name) = iface.name.as_ref() {
-                            let camel_case_name = name.to_lower_camel_case();
+                        if let Some(iface_name) = iface.name.as_ref() {
+                            let camel_case_name = iface_name.to_lower_camel_case();
                             if !guest_exports.contains(&camel_case_name) {
+                                // For wasi:http/incoming-handler, we treat it
+                                // as a special case as the engine already
+                                // provides the export using fetchEvent and that
+                                // can be used when an explicit export is not
+                                // defined by the guest content.
+                                if iface_name == "incoming-handler"
+                                    || name == "wasi:http/incoming-handler@0.2.0"
+                                {
+                                    if !features.contains(&Features::Http) {
+                                        bail!(
+                                            "JS export definition for '{}' not found. Cannot use fetchEvent because the http feature is not enabled.",
+                                            camel_case_name
+                                        )
+                                    }
+                                    continue;
+                                }
                                 bail!("Expected a JS export definition for '{}'", camel_case_name);
                             }
                             // TODO: move populate_export_aliases to a preprocessing

crates/spidermonkey-embedding-splicer/src/lib.rs

@@ -115,6 +115,7 @@ impl Guest for SpidermonkeyEmbeddingSplicerComponent {
         world_name: Option<String>,
         mut guest_imports: Vec<String>,
         guest_exports: Vec<String>,
+        features: Vec<Features>,
         debug: bool,
     ) -> Result<SpliceResult, String> {
         let source_name = source_name.unwrap_or("source.js".to_string());
@@ -194,6 +195,7 @@ impl Guest for SpidermonkeyEmbeddingSplicerComponent {
             &source_name,
             &guest_imports,
             &guest_exports,
+            features,
         )
         .map_err(|err| err.to_string())?;
 

crates/spidermonkey-embedding-splicer/wit/spidermonkey-embedding-splicer.wit

@@ -33,5 +33,5 @@ world spidermonkey-embedding-splicer {
 
   export stub-wasi: func(engine: list<u8>, features: list<features>, wit-world: option<string>, wit-path: option<string>, world-name: option<string>) -> result<list<u8>, string>;
 
-  export splice-bindings: func(source-name: option<string>, spidermonkey-engine: list<u8>, wit-world: option<string>, wit-path: option<string>, world-name: option<string>, guest-imports: list<string>, guest-exports: list<string>, debug: bool) -> result<splice-result, string>;
+  export splice-bindings: func(source-name: option<string>, spidermonkey-engine: list<u8>, wit-world: option<string>, wit-path: option<string>, world-name: option<string>, guest-imports: list<string>, guest-exports: list<string>, features: list<features>,  debug: bool) -> result<splice-result, string>;
 }

package-lock.json

@@ -66,16 +66,6 @@ export async function componentize(jsSource, witWorld, opts) {
     guestExports.push(k.n);
   });
 
-  let { wasm, jsBindings, importWrappers, exports, imports } = spliceBindings(
-    sourceName,
-    await readFile(engine),
-    witWorld,
-    maybeWindowsPath(witPath),
-    worldName,
-    guestImports,
-    guestExports,
-    false
-  );
 
   // we never disable a feature that is already in the target world usage
   const features = [];
@@ -92,6 +82,19 @@ export async function componentize(jsSource, witWorld, opts) {
     features.push('http');
   }
 
+  let { wasm, jsBindings, importWrappers, exports, imports } = spliceBindings(
+    sourceName,
+    await readFile(engine),
+    witWorld,
+    maybeWindowsPath(witPath),
+    worldName,
+    guestImports,
+    guestExports,
+    features,
+    false
+  );
+
+
   if (DEBUG_BINDINGS) {
     console.log('--- JS Source ---');
     console.log(jsSource);