fix(vite): inject client entry on islands-free pages for HMR

The dev server emits `fresh:reload` over the Vite WebSocket whenever an
SSR-only module changes, but the listener for that event lives in the
client entry. The Vite plugin only injected the client entry when a page
had at least one island, so projects with islands-free routes never had
the `fresh:reload` listener attached and edits did not refresh the
browser.

Wire `hmrClientEntry` through the build snapshot so that in dev the SSR
runtime always emits the boot script, which loads the client entry and
attaches the HMR listener regardless of whether the route uses islands.

Closes #3806

Author: fibibot

packages/fresh/src/build_cache.ts

@@ -17,6 +17,12 @@ export interface FileSnapshot {
 export interface BuildSnapshot<State> {
   version: string;
   clientEntry: string;
+  /**
+   * Pathname for an HMR-only client entry. When defined, the SSR runtime
+   * always emits the boot script so HMR listeners attach to pages that
+   * have no islands. Undefined outside of dev.
+   */
+  hmrClientEntry?: string;
   fsRoutes: FsRouteFile<State>[];
   staticFiles: Map<string, FileSnapshot>;
   islands: ServerIslandRegistry;
@@ -51,13 +57,15 @@ export class ProdBuildCache<State> implements BuildCache<State> {
   #snapshot: BuildSnapshot<State>;
   islandRegistry: ServerIslandRegistry;
   clientEntry: string;
+  hmrClientEntry: string | undefined;
   features = { errorOverlay: false };
 
   constructor(public root: string, snapshot: BuildSnapshot<State>) {
     setBuildId(snapshot.version);
     this.#snapshot = snapshot;
     this.islandRegistry = snapshot.islands;
     this.clientEntry = snapshot.clientEntry;
+    this.hmrClientEntry = snapshot.hmrClientEntry;
   }
 
   getEntryAssets(): string[] {

packages/fresh/src/dev/dev_build_cache.ts

@@ -458,6 +458,7 @@ export async function generateSnapshotServer(
     outDir: string;
     buildId: string;
     clientEntry: string;
+    hmrClientEntry?: string;
     islands: IslandModChunk[];
     // deno-lint-ignore no-explicit-any
     fsRoutesFiles: FsRouteFileNoMod<any>[];
@@ -524,12 +525,17 @@ export async function generateSnapshotServer(
   const entryAssets = options.entryAssets.map((url) => JSON.stringify(url))
     .join(",\n");
 
+  const hmrClientEntryDecl = options.hmrClientEntry !== undefined
+    ? `export const hmrClientEntry = ${JSON.stringify(options.hmrClientEntry)}`
+    : "";
+
   return `${EDIT_WARNING}
 import { IslandPreparer } from "fresh/internal";
 ${islandImports}
 ${fsRouteImports}
 
 export const clientEntry = ${JSON.stringify(options.clientEntry)}
+${hmrClientEntryDecl}
 export const version = ${JSON.stringify(options.buildId)}
 
 export const islands = new Map();

packages/plugin-vite/src/plugins/server_snapshot.ts

@@ -165,10 +165,18 @@ export function serverSnapshot(options: ResolvedFreshViteConfig): Plugin[] {
           const staticFiles: PendingStaticFile[] = [];
           let islandMods: IslandModChunk[] = [];
           let clientEntry = "/@id/fresh:client-entry";
+          let hmrClientEntry: string | undefined;
           let buildId = "";
           const entryAssets: string[] = [];
 
           if (isDev && server !== undefined) {
+            // The client entry hosts the HMR listener. Set hmrClientEntry so
+            // the SSR runtime always emits a boot script in dev, even when
+            // a page has zero islands. Without this, edits to islands-free
+            // routes never trigger a browser reload because the
+            // `fresh:reload` WebSocket listener is never attached.
+            hmrClientEntry = clientEntry;
+
             for (const id of islands.keys()) {
               const mod = server.environments.client.moduleGraph.getModuleById(
                 id,
@@ -380,6 +388,7 @@ export function serverSnapshot(options: ResolvedFreshViteConfig): Plugin[] {
             staticFiles,
             buildId,
             clientEntry,
+            hmrClientEntry,
             entryAssets,
             fsRoutesFiles: result.routes,
             islands: islandMods,

packages/plugin-vite/tests/dev_server_test.ts

@@ -69,6 +69,23 @@ integrationTest("vite dev - starts without islands/ dir", async () => {
   });
 });
 
+// Issue: https://github.com/denoland/fresh/issues/3806
+// Pages without islands must still load the client entry in dev so the
+// HMR `fresh:reload` listener attaches and route edits trigger a refresh.
+integrationTest(
+  "vite dev - injects client entry on islands-free pages for HMR",
+  async () => {
+    const fixture = path.join(FIXTURE_DIR, "no_islands");
+    await withDevServer(fixture, async (address) => {
+      const res = await fetch(`${address}/`);
+      const text = await res.text();
+      expect(text).toContain("ok");
+      expect(text).toContain("/@id/fresh:client-entry");
+      expect(text).toMatch(/import\s*\{\s*boot\s*\}/);
+    });
+  },
+);
+
 integrationTest("vite dev - starts without routes/ dir", async () => {
   const fixture = path.join(FIXTURE_DIR, "no_routes");
   await withDevServer(fixture, async (address) => {