fix(theme-mermaid): serialize initialize and render calls to prevent empty SVGs (#12757)

packages/docusaurus-theme-mermaid/src/__tests__/renderMermaid.test.ts

@@ -0,0 +1,167 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {describe, expect, it, vi, beforeEach} from 'vitest';
+import {renderMermaid, resetRenderQueueForTest} from '../client/index';
+import * as loadMermaidModule from '../client/loadMermaid';
+import type {MermaidConfig, RenderResult} from 'mermaid';
+
+describe('renderMermaid', () => {
+  beforeEach(() => {
+    resetRenderQueueForTest();
+    vi.restoreAllMocks();
+  });
+
+  it('serialize concurrent renders so initialize and render never overlap', async () => {
+    const executionTrace: string[] = [];
+    let activeRenders = 0;
+    let maxConcurrentRenders = 0;
+
+    const mockMermaid = {
+      initialize: vi.fn((config: MermaidConfig) => {
+        executionTrace.push(`initialize:${config.theme}`);
+      }),
+      render: vi.fn(async (id: string, text: string): Promise<RenderResult> => {
+        activeRenders++;
+        if (activeRenders > maxConcurrentRenders) {
+          maxConcurrentRenders = activeRenders;
+        }
+        executionTrace.push(`render:start:${id}`);
+
+        // Simulate async layout/rendering time
+        await new Promise((resolve) => setTimeout(resolve, 20));
+
+        executionTrace.push(`render:end:${id}`);
+        activeRenders--;
+        return {
+          svg: `<svg id="${id}">${text}</svg>`,
+          diagramType: 'flowchart',
+        };
+      }),
+    };
+
+    vi.spyOn(loadMermaidModule, 'loadMermaid').mockResolvedValue(
+      mockMermaid as unknown as typeof loadMermaidModule.loadMermaid extends () => Promise<
+        infer M
+      >
+        ? M
+        : never,
+    );
+
+    // Call 3 renders concurrently without awaiting
+    const p1 = renderMermaid({
+      id: 'd1',
+      text: 'graph TD; A-->B',
+      config: {theme: 'default'},
+    });
+    const p2 = renderMermaid({
+      id: 'd2',
+      text: 'graph TD; C-->D',
+      config: {theme: 'forest'},
+    });
+    const p3 = renderMermaid({
+      id: 'd3',
+      text: 'graph TD; E-->F',
+      config: {theme: 'dark'},
+    });
+
+    const [r1, r2, r3] = await Promise.all([p1, p2, p3]);
+
+    expect(r1.svg).toBe('<svg id="d1">graph TD; A-->B</svg>');
+    expect(r2.svg).toBe('<svg id="d2">graph TD; C-->D</svg>');
+    expect(r3.svg).toBe('<svg id="d3">graph TD; E-->F</svg>');
+
+    // Must never have more than 1 render in flight simultaneously
+    expect(maxConcurrentRenders).toBe(1);
+
+    // Strict sequential execution:
+    // initialize -> render start -> render end -> initialize next...
+    expect(executionTrace).toEqual([
+      'initialize:default',
+      'render:start:d1',
+      'render:end:d1',
+      'initialize:forest',
+      'render:start:d2',
+      'render:end:d2',
+      'initialize:dark',
+      'render:start:d3',
+      'render:end:d3',
+    ]);
+  });
+
+  it('continue executing queued renders even if an earlier render throws', async () => {
+    const mockMermaid = {
+      initialize: vi.fn(),
+      render: vi.fn(async (id: string, text: string): Promise<RenderResult> => {
+        if (id === 'failing-diagram') {
+          throw new Error('Syntax error in diagram');
+        }
+        return {
+          svg: `<svg id="${id}">${text}</svg>`,
+          diagramType: 'flowchart',
+        };
+      }),
+    };
+
+    vi.spyOn(loadMermaidModule, 'loadMermaid').mockResolvedValue(
+      mockMermaid as unknown as typeof loadMermaidModule.loadMermaid extends () => Promise<
+        infer M
+      >
+        ? M
+        : never,
+    );
+
+    const p1 = renderMermaid({
+      id: 'failing-diagram',
+      text: 'syntax error',
+      config: {theme: 'default'},
+    });
+    const p2 = renderMermaid({
+      id: 'succeeding-diagram',
+      text: 'graph TD; A-->B',
+      config: {theme: 'default'},
+    });
+
+    await expect(p1).rejects.toThrow('Syntax error in diagram');
+    const r2 = await p2;
+    expect(r2.svg).toBe('<svg id="succeeding-diagram">graph TD; A-->B</svg>');
+  });
+
+  it('clean up error container from document on error if present', async () => {
+    const removeFn = vi.fn();
+    const querySelectorSpy = vi.fn().mockReturnValue({remove: removeFn});
+    vi.stubGlobal('document', {querySelector: querySelectorSpy});
+
+    const mockMermaid = {
+      initialize: vi.fn(),
+      render: vi.fn(async () => {
+        throw new Error('Render failed');
+      }),
+    };
+
+    vi.spyOn(loadMermaidModule, 'loadMermaid').mockResolvedValue(
+      mockMermaid as unknown as typeof loadMermaidModule.loadMermaid extends () => Promise<
+        infer M
+      >
+        ? M
+        : never,
+    );
+
+    await expect(
+      renderMermaid({
+        id: 'error-id',
+        text: 'invalid',
+        config: {theme: 'default'},
+      }),
+    ).rejects.toThrow('Render failed');
+
+    expect(querySelectorSpy).toHaveBeenCalledWith('#derror-id');
+    expect(removeFn).toHaveBeenCalled();
+
+    vi.unstubAllGlobals();
+  });
+});

packages/docusaurus-theme-mermaid/src/client/index.ts

@@ -32,7 +32,13 @@ export function useMermaidConfig(): MermaidConfig {
   );
 }
 
-async function renderMermaid({
+let renderQueue: Promise<unknown> = Promise.resolve();
+
+export function resetRenderQueueForTest(): void {
+  renderQueue = Promise.resolve();
+}
+
+export async function renderMermaid({
   id,
   text,
   config,
@@ -41,36 +47,47 @@ async function renderMermaid({
   text: string;
   config: MermaidConfig;
 }): Promise<RenderResult> {
-  const mermaid = await loadMermaid();
-
-  /*
-  Mermaid API is really weird :s
-  It is a big mutable singleton with multiple config levels
-  Note: most recent API type definitions are missing
-
-  There are 2 kind of configs:
-
-  - siteConfig: some kind of global/protected shared config
-    you can only set with "initialize"
-
-  - config/currentConfig
-    the config the renderer will use
-    it is reset to siteConfig before each render
-    but it can be altered by the mermaid txt content itself through directives
-
-  To use a new mermaid config (on colorMode change for example) we should
-  update siteConfig, and it can only be done with initialize()
-   */
-  mermaid.initialize(config);
-
-  try {
-    return await mermaid.render(id, text);
-  } catch (e) {
-    // Because Mermaid add a weird SVG/Message to the DOM on error
-    // https://github.com/mermaid-js/mermaid/issues/3205#issuecomment-1719620183
-    document.querySelector(`#d${id}`)?.remove();
-    throw e;
-  }
+  const run = async (): Promise<RenderResult> => {
+    const mermaid = await loadMermaid();
+
+    /*
+    Mermaid API is really weird :s
+    It is a big mutable singleton with multiple config levels
+    Note: most recent API type definitions are missing
+
+    There are 2 kind of configs:
+
+    - siteConfig: some kind of global/protected shared config
+      you can only set with "initialize"
+
+    - config/currentConfig
+      the config the renderer will use
+      it is reset to siteConfig before each render
+      but it can be altered by the mermaid txt content itself through directives
+
+    To use a new mermaid config (on colorMode change for example) we should
+    update siteConfig, and it can only be done with initialize()
+     */
+    mermaid.initialize(config);
+
+    try {
+      return await mermaid.render(id, text);
+    } catch (e) {
+      // Because Mermaid add a weird SVG/Message to the DOM on error
+      // https://github.com/mermaid-js/mermaid/issues/3205#issuecomment-1719620183
+      if (typeof document !== 'undefined') {
+        document.querySelector(`#d${id}`)?.remove();
+      }
+      throw e;
+    }
+  };
+
+  // Multiple diagrams on a page mount concurrently. Mermaid is a mutable
+  // singleton, so concurrent initialize() + render() calls can corrupt
+  // internal state and produce empty SVGs. We serialize renders in a queue.
+  const result = renderQueue.then(run, run);
+  renderQueue = result.catch(() => {});
+  return result;
 }
 
 export function useMermaidRenderResult({
@@ -92,20 +109,32 @@ export function useMermaidRenderResult({
   const config = providedConfig ?? defaultMermaidConfig;
 
   useEffect(() => {
+    let isCancelled = false;
+
     // Mermaid removes existing DOM elements with the requested ID. Reusing an
     // ID can delete the displayed SVG, which React won't restore if the new
     // SVG markup is identical. Give every render its own ID instead.
     renderMermaid({id: `${id}-${renderCount.current++}`, text, config})
       // TODO maybe try to use Suspense here and throw the promise?
       // See also https://github.com/pmndrs/suspend-react
-      .then(setResult)
+      .then((res) => {
+        if (!isCancelled) {
+          setResult(res);
+        }
+      })
       .catch((e) => {
-        // Funky way to trigger parent React error boundary
-        // See https://x.com/sebastienlorber/status/1628340871899893768
-        setResult(() => {
-          throw e;
-        });
+        if (!isCancelled) {
+          // Funky way to trigger parent React error boundary
+          // See https://x.com/sebastienlorber/status/1628340871899893768
+          setResult(() => {
+            throw e;
+          });
+        }
       });
+
+    return () => {
+      isCancelled = true;
+    };
   }, [id, text, config]);
 
   return result;