fix(mdx-loader): fix cross-compiler cache randomly loading mdx with client/server envs (#10553)

packages/docusaurus-mdx-loader/src/loader.ts

@@ -17,6 +17,7 @@ import {
   compileToJSX,
   createAssetsExportCode,
   extractContentTitleData,
+  promiseWithResolvers,
 } from './utils';
 import type {WebpackCompilerName} from '@docusaurus/utils';
 import type {Options} from './options';
@@ -138,31 +139,77 @@ async function loadMDXWithCaching({
   options: Options;
   compilerName: WebpackCompilerName;
 }): Promise<string> {
+  const {crossCompilerCache} = options;
+  if (!crossCompilerCache) {
+    return loadMDX({
+      fileContent,
+      filePath,
+      options,
+      compilerName,
+    });
+  }
+
   // Note we "resource" as cache key, not "filePath" nor "fileContent"
   // This is because:
   // - the same file can be compiled in different variants (blog.mdx?truncated)
   // - the same content can be processed differently (versioned docs links)
   const cacheKey = resource;
 
-  const cachedPromise = options.crossCompilerCache?.get(cacheKey);
-  if (cachedPromise) {
-    // We can clean up the cache and free memory here
+  // We can clean up the cache and free memory after cache entry consumption
   // We know there are only 2 compilations for the same file
   // Note: once we introduce RSCs we'll probably have 3 compilations
   // Note: we can't use string keys in WeakMap
   // But we could eventually use WeakRef for the values
-    options.crossCompilerCache?.delete(cacheKey);
-    return cachedPromise;
-  }
+  const deleteCacheEntry = () => crossCompilerCache.delete(cacheKey);
+
+  const cacheEntry = crossCompilerCache?.get(cacheKey);
+
+  // When deduplicating client/server compilations, we always use the client
+  // compilation and not the server compilation
+  // This is important because the server compilation usually skips some steps
+  // Notably: the server compilation does not emit file-loader assets
+  // Using the server compilation otherwise leads to broken images
+  // See https://github.com/facebook/docusaurus/issues/10544#issuecomment-2390943794
+  // See https://github.com/facebook/docusaurus/pull/10553
+  // TODO a problem with this: server bundle will use client inline loaders
+  //  This means server bundle will use ?emit=true for assets
+  //  We should try to get rid of inline loaders to cleanup this caching logic
+  if (compilerName === 'client') {
     const promise = loadMDX({
       fileContent,
       filePath,
       options,
       compilerName,
     });
-  options.crossCompilerCache?.set(cacheKey, promise);
+    if (cacheEntry) {
+      promise.then(cacheEntry.resolve, cacheEntry.reject);
+      deleteCacheEntry();
+    } else {
+      const noop = () => {
+        throw new Error('this should never be called');
+      };
+      crossCompilerCache.set(cacheKey, {
+        promise,
+        resolve: noop,
+        reject: noop,
+      });
+    }
     return promise;
   }
+  // Server compilation always uses the result of the client compilation above
+  else if (compilerName === 'server') {
+    if (cacheEntry) {
+      deleteCacheEntry();
+      return cacheEntry.promise;
+    } else {
+      const {promise, resolve, reject} = promiseWithResolvers<string>();
+      crossCompilerCache.set(cacheKey, {promise, resolve, reject});
+      return promise;
+    }
+  } else {
+    throw new Error(`Unexpected compilerName=${compilerName}`);
+  }
+}
 
 export async function mdxLoader(
   this: LoaderContext<Options>,

packages/docusaurus-mdx-loader/src/options.ts

@@ -8,6 +8,7 @@
 import type {MDXOptions, SimpleProcessors} from './processor';
 import type {MarkdownConfig} from '@docusaurus/types';
 import type {ResolveMarkdownLink} from './remark/resolveMarkdownLinks';
+import type {PromiseWithResolvers} from './utils';
 
 export type Options = Partial<MDXOptions> & {
   markdownConfig: MarkdownConfig;
@@ -25,5 +26,7 @@ export type Options = Partial<MDXOptions> & {
 
   // Will usually be created by "createMDXLoaderItem"
   processors?: SimpleProcessors;
-  crossCompilerCache?: Map<string, Promise<string>>; // MDX => Promise<JSX> cache
+  crossCompilerCache?: Map<string, CrossCompilerCacheEntry>; // MDX => Promise<JSX> cache
 };
+
+type CrossCompilerCacheEntry = PromiseWithResolvers<string>;

packages/docusaurus-mdx-loader/src/remark/unusedDirectives/index.ts

@@ -155,6 +155,7 @@ const plugin: Plugin = function plugin(this: Processor): Transformer {
     // We only enable these warnings for the client compiler
     // This avoids emitting duplicate warnings in prod mode
     // Note: the client compiler is used in both dev/prod modes
+    // Also: the client compiler is what gets used when using crossCompilerCache
     if (file.data.compilerName === 'client') {
       logUnusedDirectivesWarning({
         directives: unusedDirectives,

packages/docusaurus-mdx-loader/src/utils.ts

@@ -132,3 +132,20 @@ export async function compileToJSX({
     );
   }
 }
+
+// TODO Docusaurus v4, remove temporary polyfill when upgrading to Node 22+
+export interface PromiseWithResolvers<T> {
+  promise: Promise<T>;
+  resolve: (value: T | PromiseLike<T>) => void;
+  reject: (reason?: any) => void;
+}
+// TODO Docusaurus v4, remove temporary polyfill when upgrading to Node 22+
+export function promiseWithResolvers<T>(): PromiseWithResolvers<T> {
+  // @ts-expect-error: it's fine
+  const out: PromiseWithResolvers<T> = {};
+  out.promise = new Promise((resolve, reject) => {
+    out.resolve = resolve;
+    out.reject = reject;
+  });
+  return out;
+}