Prepare RouterContentSource for basePath (vercel/turborepo#5218)

### Description

This is part 2 of [WEB-993](https://linear.app/vercel/issue/WEB-993)
basePath support. A few of our next-specific content sources will need
to be scoped under the `basePath` (like `_next/image` and
`__nextjs_original-stack-frame`). These are currently served with a
`RouterContentSource`, but it didn't have support for arbitrary
prefixes.

We _could_ have changed the subpath for these sources to include the
`basePath`, but that would require reading the `next_config.base_path()`
in the
[source](https://github.com/vercel/next.js/blob/2b1f0d9351610b04d01638efed19252ca81d0023/packages/next-swc/crates/next-dev/src/lib.rs#L413-L423)
method, and it would invalidate our entire call graph whenever the
`next.config.js` changed. Not a good choice.


### Testing Instructions

<!--
  Give a quick description of steps to test your changes.
-->

crates/turbopack-dev-server/src/source/router.rs

@@ -7,22 +7,105 @@ use crate::source::ContentSourcesVc;
 
 /// Binds different ContentSources to different subpaths. A fallback
 /// ContentSource will serve all other subpaths.
+// TODO(WEB-1151): Remove this and migrate all users to PrefixedRouterContentSource.
 #[turbo_tasks::value(shared)]
 pub struct RouterContentSource {
     pub routes: Vec<(String, ContentSourceVc)>,
     pub fallback: ContentSourceVc,
 }
 
-impl RouterContentSource {
-    fn get_source<'s, 'a>(&'s self, path: &'a str) -> (&'s ContentSourceVc, &'a str) {
-        for (route, source) in self.routes.iter() {
-            if path.starts_with(route) {
-                let path = &path[route.len()..];
-                return (source, path);
+/// Binds different ContentSources to different subpaths. The request path must
+/// begin with the prefix, which will be stripped (along with the subpath)
+/// before querying the ContentSource. A fallback ContentSource will serve all
+/// other subpaths, including if the request path does not include the prefix.
+#[turbo_tasks::value(shared)]
+pub struct PrefixedRouterContentSource {
+    prefix: StringVc,
+    routes: Vec<(String, ContentSourceVc)>,
+    fallback: ContentSourceVc,
+}
+
+#[turbo_tasks::value_impl]
+impl PrefixedRouterContentSourceVc {
+    #[turbo_tasks::function]
+    async fn new(
+        prefix: StringVc,
+        routes: Vec<(String, ContentSourceVc)>,
+        fallback: ContentSourceVc,
+    ) -> Result<Self> {
+        if cfg!(debug_assertions) {
+            let prefix_string = prefix.await?;
+            debug_assert!(prefix_string.is_empty() || prefix_string.ends_with('/'));
+            debug_assert!(prefix_string.starts_with('/'));
+        }
+        Ok(PrefixedRouterContentSource {
+            prefix,
+            routes,
+            fallback,
+        }
+        .cell())
+    }
+}
+
+/// If the `path` starts with `prefix`, then it will search each route to see if
+/// any subpath matches. If so, the remaining path (after removing the prefix
+/// and subpath) is used to query the matching ContentSource. If no match is
+/// found, then the fallback is queried with the original path.
+async fn get(
+    routes: &[(String, ContentSourceVc)],
+    fallback: &ContentSourceVc,
+    prefix: &str,
+    path: &str,
+    data: Value<ContentSourceData>,
+) -> Result<ContentSourceResultVc> {
+    let mut found = None;
+
+    if let Some(path) = path.strip_prefix(prefix) {
+        for (subpath, source) in routes {
+            if let Some(path) = path.strip_prefix(subpath) {
+                found = Some((source, path));
+                break;
             }
         }
-        (&self.fallback, path)
     }
+
+    let (source, path) = found.unwrap_or((fallback, path));
+    Ok(source.resolve().await?.get(path, data))
+}
+
+fn get_children(
+    routes: &[(String, ContentSourceVc)],
+    fallback: &ContentSourceVc,
+) -> ContentSourcesVc {
+    ContentSourcesVc::cell(
+        routes
+            .iter()
+            .map(|r| r.1)
+            .chain(std::iter::once(*fallback))
+            .collect(),
+    )
+}
+
+async fn get_introspection_children(
+    routes: &[(String, ContentSourceVc)],
+    fallback: &ContentSourceVc,
+) -> Result<IntrospectableChildrenVc> {
+    Ok(IntrospectableChildrenVc::cell(
+        routes
+            .iter()
+            .cloned()
+            .chain(std::iter::once((String::new(), *fallback)))
+            .map(|(path, source)| async move {
+                Ok(IntrospectableVc::resolve_from(source)
+                    .await?
+                    .map(|i| (StringVc::cell(path), i)))
+            })
+            .try_join()
+            .await?
+            .into_iter()
+            .flatten()
+            .collect(),
+    ))
 }
 
 #[turbo_tasks::value_impl]
@@ -33,51 +116,61 @@ impl ContentSource for RouterContentSource {
         path: &str,
         data: Value<ContentSourceData>,
     ) -> Result<ContentSourceResultVc> {
-        let (source, path) = self.get_source(path);
-        Ok(source.resolve().await?.get(path, data))
+        get(&self.routes, &self.fallback, "", path, data).await
     }
 
     #[turbo_tasks::function]
     fn get_children(&self) -> ContentSourcesVc {
-        let mut sources = Vec::with_capacity(self.routes.len() + 1);
+        get_children(&self.routes, &self.fallback)
+    }
+}
 
-        sources.extend(self.routes.iter().map(|r| r.1));
-        sources.push(self.fallback);
+#[turbo_tasks::value_impl]
+impl Introspectable for RouterContentSource {
+    #[turbo_tasks::function]
+    fn ty(&self) -> StringVc {
+        StringVc::cell("router content source".to_string())
+    }
 
-        ContentSourcesVc::cell(sources)
+    #[turbo_tasks::function]
+    async fn children(&self) -> Result<IntrospectableChildrenVc> {
+        get_introspection_children(&self.routes, &self.fallback).await
     }
 }
 
-#[turbo_tasks::function]
-fn introspectable_type() -> StringVc {
-    StringVc::cell("router content source".to_string())
+#[turbo_tasks::value_impl]
+impl ContentSource for PrefixedRouterContentSource {
+    #[turbo_tasks::function]
+    async fn get(
+        &self,
+        path: &str,
+        data: Value<ContentSourceData>,
+    ) -> Result<ContentSourceResultVc> {
+        let prefix = self.prefix.await?;
+        get(&self.routes, &self.fallback, &prefix, path, data).await
+    }
+
+    #[turbo_tasks::function]
+    fn get_children(&self) -> ContentSourcesVc {
+        get_children(&self.routes, &self.fallback)
+    }
 }
 
 #[turbo_tasks::value_impl]
-impl Introspectable for RouterContentSource {
+impl Introspectable for PrefixedRouterContentSource {
     #[turbo_tasks::function]
     fn ty(&self) -> StringVc {
-        introspectable_type()
+        StringVc::cell("prefixed router content source".to_string())
+    }
+
+    #[turbo_tasks::function]
+    async fn details(&self) -> Result<StringVc> {
+        let prefix = self.prefix.await?;
+        Ok(StringVc::cell(format!("prefix: '{}'", prefix)))
     }
 
     #[turbo_tasks::function]
     async fn children(&self) -> Result<IntrospectableChildrenVc> {
-        Ok(IntrospectableChildrenVc::cell(
-            self.routes
-                .iter()
-                .cloned()
-                .chain(std::iter::once((String::new(), self.fallback)))
-                .map(|(path, source)| (StringVc::cell(path), source))
-                .map(|(path, source)| async move {
-                    Ok(IntrospectableVc::resolve_from(source)
-                        .await?
-                        .map(|i| (path, i)))
-                })
-                .try_join()
-                .await?
-                .into_iter()
-                .flatten()
-                .collect(),
-        ))
+        get_introspection_children(&self.routes, &self.fallback).await
     }
 }

crates/turbopack-dev-server/src/source/static_assets.rs

@@ -22,6 +22,7 @@ pub struct StaticAssetsContentSource {
 
 #[turbo_tasks::value_impl]
 impl StaticAssetsContentSourceVc {
+    // TODO(WEB-1151): Remove this method and migrate users to `with_prefix`.
     #[turbo_tasks::function]
     pub fn new(prefix: String, dir: FileSystemPathVc) -> StaticAssetsContentSourceVc {
         StaticAssetsContentSourceVc::with_prefix(StringVc::cell(prefix), dir)
@@ -35,6 +36,7 @@ impl StaticAssetsContentSourceVc {
         if cfg!(debug_assertions) {
             let prefix_string = prefix.await?;
             debug_assert!(prefix_string.is_empty() || prefix_string.ends_with('/'));
+            debug_assert!(!prefix_string.starts_with('/'));
         }
         Ok(StaticAssetsContentSource { prefix, dir }.cell())
     }