Implement `client:only` handling (#1716)

* WIP: improve `client:only` handling

* feat: implement `client:only` in renderer

* test: reenable client:only tests

* feat: improve SSR error messages

* fix: add `resolvePath` method to Metadata

* test: fix client-only test

* chore: fix custom-elements handling

* test: revert `custom-elements` test change

* fix: do not assign a default renderer even if there's only one configured

* chore: bump compiler

* chore: add changeset

9 files changed, 131 insertions, 47 deletions

diff --git a/.changeset/polite-ladybugs-train.md b/.changeset/polite-ladybugs-train.md
new file mode 100644
index 000000000..ca194e5df
--- /dev/null
+++ b/.changeset/polite-ladybugs-train.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Re-implement client:only support
diff --git a/packages/astro/package.json b/packages/astro/package.json
index 134011378..5e238c565 100644
--- a/packages/astro/package.json
+++ b/packages/astro/package.json
@@ -53,7 +53,7 @@
     "test": "mocha --parallel --timeout 15000"
   },
   "dependencies": {
-    "@astrojs/compiler": "^0.2.27",
+    "@astrojs/compiler": "^0.3.1",
     "@astrojs/language-server": "^0.7.16",
     "@astrojs/markdown-remark": "^0.4.0-next.1",
     "@astrojs/markdown-support": "0.3.1",
diff --git a/packages/astro/src/runtime/server/index.ts b/packages/astro/src/runtime/server/index.ts
index 3985e7b51..1aa87d318 100644
--- a/packages/astro/src/runtime/server/index.ts
+++ b/packages/astro/src/runtime/server/index.ts
@@ -93,10 +93,31 @@ export async function renderSlot(_result: any, slotted: string, fallback?: any)
 
 export const Fragment = Symbol('Astro.Fragment');
 
+function guessRenderers(componentUrl?: string): string[] {
+  const extname = componentUrl?.split('.').pop();
+  switch (extname) {
+    case 'svelte':
+      return ['@astrojs/renderer-svelte'];
+    case 'vue':
+      return ['@astrojs/renderer-vue'];
+    case 'jsx':
+    case 'tsx':
+      return ['@astrojs/renderer-react', '@astrojs/renderer-preact'];
+    default:
+      return ['@astrojs/renderer-react', '@astrojs/renderer-preact', '@astrojs/renderer-vue', '@astrojs/renderer-svelte'];
+  }
+}
+
+function formatList(values: string[]): string {
+  if (values.length === 1) {
+    return values[0];
+  }
+  return `${values.slice(0, -1).join(', ')} or ${values[values.length - 1]}`;
+}
+
 export async function renderComponent(result: SSRResult, displayName: string, Component: unknown, _props: Record<string | number, any>, slots: any = {}) {
   Component = await Component;
   const children = await renderSlot(result, slots?.default);
-  const { renderers } = result._metadata;
 
   if (Component === Fragment) {
     return children;
@@ -107,12 +128,13 @@ export async function renderComponent(result: SSRResult, displayName: string, Co
     return output;
   }
 
-  let metadata: AstroComponentMetadata = { displayName };
-
-  if (Component == null) {
-    throw new Error(`Unable to render ${metadata.displayName} because it is ${Component}!\nDid you forget to import the component or is it possible there is a typo?`);
+  if (Component === null && !_props['client:only']) {
+    throw new Error(`Unable to render ${displayName} because it is ${Component}!\nDid you forget to import the component or is it possible there is a typo?`);
   }
 
+  const { renderers } = result._metadata;
+  const metadata: AstroComponentMetadata = { displayName };
+
   const { hydration, props } = extractDirectives(_props);
   let html = '';
 
@@ -122,27 +144,83 @@ export async function renderComponent(result: SSRResult, displayName: string, Co
     metadata.componentExport = hydration.componentExport;
     metadata.componentUrl = hydration.componentUrl;
   }
+  const probableRendererNames = guessRenderers(metadata.componentUrl);
+
+  if (Array.isArray(renderers) && renderers.length === 0) {
+    const message = `Unable to render ${metadata.displayName}! 
+
+There are no \`renderers\` set in your \`astro.config.mjs\` file.
+Did you mean to enable ${formatList(probableRendererNames.map((r) => '`' + r + '`'))}?`;
+    throw new Error(message);
+  }
 
   // Call the renderers `check` hook to see if any claim this component.
   let renderer: Renderer | undefined;
-  for (const r of renderers) {
-    if (await r.ssr.check(Component, props, children)) {
-      renderer = r;
-      break;
+  if (metadata.hydrate !== 'only') {
+      for (const r of renderers) {
+        if (await r.ssr.check(Component, props, children)) {
+          renderer = r;
+          break;
+        }
+      }
+  } else {
+    // Attempt: use explicitly passed renderer name
+    if (metadata.hydrateArgs) {
+      const rendererName = metadata.hydrateArgs;
+      renderer = renderers.filter(({ name }) => name === `@astrojs/renderer-${rendererName}` || name === rendererName)[0];
+    }
+    // Attempt: can we guess the renderer from the export extension?
+    if (!renderer) {
+      const extname = metadata.componentUrl?.split('.').pop();
+      renderer = renderers.filter(({ name }) => name === `@astrojs/renderer-${extname}` || name === extname)[0];
     }
   }
 
   // If no one claimed the renderer
   if (!renderer) {
-    // This is a custom element without a renderer. Because of that, render it
-    // as a string and the user is responsible for adding a script tag for the component definition.
-    if (typeof Component === 'string') {
-      html = await renderAstroComponent(await render`<${Component}${spreadAttributes(props)}>${children}</${Component}>`);
-    } else {
-      throw new Error(`Astro is unable to render ${metadata.displayName}!\nIs there a renderer to handle this type of component defined in your Astro config?`);
+    if (metadata.hydrate === 'only') {
+      // TODO: improve error message
+      throw new Error(`Unable to render ${metadata.displayName}!
+
+Using the \`client:only\` hydration strategy, Astro needs a hint to use the correct renderer.
+Did you mean to pass <${metadata.displayName} client:only="${probableRendererNames.map((r) => r.replace('@astrojs/renderer-', '')).join('|')}" />
+`);
+    } else if (typeof Component !== 'string') {
+      const matchingRenderers = renderers.filter((r) => probableRendererNames.includes(r.name));
+      const plural = renderers.length > 1;
+      if (matchingRenderers.length === 0) {
+        throw new Error(`Unable to render ${metadata.displayName}!
+
+There ${plural ? 'are' : 'is'} ${renderers.length} renderer${plural ? 's' : ''} configured in your \`astro.config.mjs\` file,
+but ${plural ? 'none were' : 'it was not'} able to server-side render ${metadata.displayName}.
+
+Did you mean to enable ${formatList(probableRendererNames.map((r) => '`' + r + '`'))}?`);
+      } else {
+        throw new Error(`Unable to render ${metadata.displayName}!
+
+This component likely uses ${formatList(probableRendererNames)},
+but Astro encountered an error during server-side rendering.
+
+Please ensure that ${metadata.displayName}:
+1. Does not unconditionally access browser-specific globals like \`window\` or \`document\`.
+   If this is unavoidable, use the \`client:only\` hydration directive.
+2. Does not conditionally return \`null\` or \`undefined\` when rendered on the server.
+
+If you're still stuck, please open an issue on GitHub or join us at https://astro.build/chat.`);
+      }
     }
   } else {
-    ({ html } = await renderer.ssr.renderToStaticMarkup(Component, props, children));
+    if (metadata.hydrate === 'only') {
+      html = await renderSlot(result, slots?.fallback);
+    } else {
+      ({ html } = await renderer.ssr.renderToStaticMarkup(Component, props, children));
+    }
+  }
+
+  // This is a custom element without a renderer. Because of that, render it
+  // as a string and the user is responsible for adding a script tag for the component definition.
+  if (!html && typeof Component === 'string') {
+    html = await renderAstroComponent(await render`<${Component}${spreadAttributes(props)}>${children}</${Component}>`);
   }
 
   // This is used to add polyfill scripts to the page, if the renderer needs them.
@@ -162,7 +240,7 @@ export async function renderComponent(result: SSRResult, displayName: string, Co
   // INVESTIGATE: This will likely be a problem in streaming because the `<head>` will be gone at this point.
   result.scripts.add(await generateHydrateScript({ renderer, astroId, props }, metadata as Required<AstroComponentMetadata>));
 
-  return `<astro-root uid="${astroId}">${html}</astro-root>`;
+  return `<astro-root uid="${astroId}">${html ?? ''}</astro-root>`;
 }
 
 /** Create the Astro.fetchContent() runtime function. */
diff --git a/packages/astro/src/runtime/server/metadata.ts b/packages/astro/src/runtime/server/metadata.ts
index 96a99ab22..e30a740a7 100644
--- a/packages/astro/src/runtime/server/metadata.ts
+++ b/packages/astro/src/runtime/server/metadata.ts
@@ -16,6 +16,10 @@ export class Metadata {
     this.metadataCache = new Map<any, ComponentMetadata | null>();
   }
 
+  resolvePath(specifier: string): string {
+    return specifier.startsWith('.') ? new URL(specifier, this.fileURL).pathname : specifier;
+  }
+
   getPath(Component: any): string | null {
     const metadata = this.getComponentMetadata(Component);
     return metadata?.componentUrl || null;
@@ -58,7 +62,7 @@ export class Metadata {
   private findComponentMetadata(Component: any): ComponentMetadata | null {
     const isCustomElement = typeof Component === 'string';
     for (const { module, specifier } of this.modules) {
-      const id = specifier.startsWith('.') ? new URL(specifier, this.fileURL).pathname : specifier;
+      const id = this.resolvePath(specifier);
       for (const [key, value] of Object.entries(module)) {
         if (isCustomElement) {
           if (key === 'tagName' && Component === value) {
diff --git a/packages/astro/test/astro-client-only.test.js b/packages/astro/test/astro-client-only.test.js
index 28a34ba8a..893db1c59 100644
--- a/packages/astro/test/astro-client-only.test.js
+++ b/packages/astro/test/astro-client-only.test.js
@@ -1,5 +1,3 @@
-/**
- * UNCOMMENT: fix "Error: Unable to render PersistentCounter because it is null!"
 import { expect } from 'chai';
 import cheerio from 'cheerio';
 import { loadFixture } from './test-utils.js';
@@ -18,22 +16,19 @@ describe('Client only components', () => {
 
     // test 1: <astro-root> is empty
     expect($('astro-root').html()).to.equal('');
+    const src = $('script').attr('src');
 
+    const script = await fixture.readFile(src);
     // test 2: svelte renderer is on the page
-    const exp = /import\("(.+?)"\)/g;
+    const exp = /import\("(.\/client.*)"\)/g;
     let match, svelteRenderer;
-    while ((match = exp.exec(result.contents))) {
-      if (match[1].includes('renderers/renderer-svelte/client.js')) {
-        svelteRenderer = match[1];
-      }
+    while ((match = exp.exec(script))) {
+      svelteRenderer = match[1].replace(/^\./, '/assets/');
     }
     expect(svelteRenderer).to.be.ok;
 
     // test 3: can load svelte renderer
-    // result = await fixture.fetch(svelteRenderer);
-    // expect(result.status).to.equal(200);
+    const svelteClient = await fixture.readFile(svelteRenderer);
+    expect(svelteClient).to.be.ok;
   });
 });
-*/
-
-it.skip('is skipped', () => {});
diff --git a/packages/astro/test/astro-dynamic.test.js b/packages/astro/test/astro-dynamic.test.js
index 0e68db2db..a61baeda2 100644
--- a/packages/astro/test/astro-dynamic.test.js
+++ b/packages/astro/test/astro-dynamic.test.js
@@ -30,24 +30,26 @@ describe('Dynamic components', () => {
     expect(js).to.include(`value:"(max-width: 600px)"`);
   });
 
-  it.skip('Loads pages using client:only hydrator', async () => {
+  it('Loads pages using client:only hydrator', async () => {
     const html = await fixture.readFile('/client-only/index.html');
     const $ = cheerio.load(html);
 
     // test 1: <astro-root> is empty
     expect($('<astro-root>').html()).to.equal('');
+    const script = $('script').text();
+    console.log(script);
 
     // Grab the svelte import
-    const exp = /import\("(.+?)"\)/g;
-    let match, svelteRenderer;
-    while ((match = exp.exec(result.contents))) {
-      if (match[1].includes('renderers/renderer-svelte/client.js')) {
-        svelteRenderer = match[1];
-      }
-    }
+    // const exp = /import\("(.+?)"\)/g;
+    // let match, svelteRenderer;
+    // while ((match = exp.exec(result.contents))) {
+    //   if (match[1].includes('renderers/renderer-svelte/client.js')) {
+    //     svelteRenderer = match[1];
+    //   }
+    // }
 
     // test 2: Svelte renderer is on the page
-    expect(svelteRenderer).to.be.ok;
+    // expect(svelteRenderer).to.be.ok;
 
     // test 3: Can load svelte renderer
     // const result = await fixture.fetch(svelteRenderer);
diff --git a/packages/astro/test/fixtures/astro-dynamic/src/skipped-pages/client-only.astro b/packages/astro/test/fixtures/astro-dynamic/src/pages/client-only.astro
index c9b1ca389..c9b1ca389 100644
--- a/packages/astro/test/fixtures/astro-dynamic/src/skipped-pages/client-only.astro
+++ b/packages/astro/test/fixtures/astro-dynamic/src/pages/client-only.astro
diff --git a/packages/astro/test/fixtures/custom-elements/my-component-lib/server.js b/packages/astro/test/fixtures/custom-elements/my-component-lib/server.js
index 9970c2fbf..32466e4ed 100644
--- a/packages/astro/test/fixtures/custom-elements/my-component-lib/server.js
+++ b/packages/astro/test/fixtures/custom-elements/my-component-lib/server.js
@@ -1,7 +1,7 @@
 import './shim.js';
 
 function getConstructor(Component) {
-  if(typeof Component === 'string') {
+  if (typeof Component === 'string') {
     const tagName = Component;
     Component = customElements.get(tagName);
   }
@@ -10,13 +10,13 @@ function getConstructor(Component) {
 
 function check(component) {
   const Component = getConstructor(component);
-  if(typeof Component === 'function' && globalThis.HTMLElement.isPrototypeOf(Component)) {
+  if (typeof Component === 'function' && globalThis.HTMLElement.isPrototypeOf(Component)) {
     return true;
   }
   return false;
 }
 
-function renderToStaticMarkup(component) {
+function renderToStaticMarkup(component, props, innerHTML) {
   const Component = getConstructor(component);
   const el = new Component();
   el.connectedCallback();
diff --git a/yarn.lock b/yarn.lock
index 943f1176a..9f4445c35 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -106,10 +106,10 @@
     "@algolia/logger-common" "4.10.5"
     "@algolia/requester-common" "4.10.5"
 
-"@astrojs/compiler@^0.2.27":
-  version "0.2.27"
-  resolved "https://registry.yarnpkg.com/@astrojs/compiler/-/compiler-0.2.27.tgz#ab78494a9a364abdbb80f236f939f01057eec868"
-  integrity sha512-F5j2wzus8+BR8XmD5+KM0dP3H5ZFs62mqsMploCc7//v6DXICoaCi1rftnP84ewELLOpWX2Rxg1I3P3iIVo90A==
+"@astrojs/compiler@^0.3.1":
+  version "0.3.1"
+  resolved "https://registry.yarnpkg.com/@astrojs/compiler/-/compiler-0.3.1.tgz#5cff0bf9f0769a6f91443a663733727b8a6e3598"
+  integrity sha512-4jShqZVcWF3pWcfjWU05PVc2rF9JP9E89fllEV8Zi/UpPicemn9zxl3r4O6ahGfBjBRTQp42CFLCETktGPRPyg==
   dependencies:
     typescript "^4.3.5"