fix: serialize concurrent mermaid renders in markdown preview by anik8das · Pull Request #3478 · wavetermdev/waveterm

anik8das · 2026-08-18T22:24:10Z

Background is in that thread. My analysis and a position bisect are in this comment: a file with six blocks where block 6 was byte identical to block 1, in which blocks 1 and 2 rendered every time, block 6 never did, and the set changed between reloads. Identical content at a different position giving a different result is what ruled out syntax and pointed at a race.

Problem

Each <Mermaid> component calls mermaid.run() from its own useEffect, so a markdown file with several diagrams starts every render in parallel. Two things go wrong.

1. Colliding SVG ids. mermaid.run() mutates module-global state and derives its element id from Date.now():

this.next = deterministic ? () => this.count++ : () => Date.now();

Calls that overlap, or land in the same millisecond, get the same mermaid-<id>. Mermaid scopes each diagram's <style> block by that id, so colliding ids let one diagram's styles apply to another.

2. Repeated initialize(). initializeMermaid() sets mermaidInitialized = true after await import("mermaid"). Every component that mounts before that import resolves gets past the guard, so they all call mermaid.initialize(), resetting global config while other renders are in flight.

Reproduce it

A. See the symptom

Save this as repro.md and preview it in Wave. On main the diagrams overlap or fail to appear, and which ones break changes between reloads. With this PR all four render in place.

# repro

```mermaid
flowchart TD
  A[Start] --> B{Decision}
  B -->|Yes| C[Action 1]
  B -->|No| D[Action 2]
```

```mermaid
sequenceDiagram
  participant Alice
  participant Bob
  Alice->>Bob: Hello
  Bob-->>Alice: Hi back
```

```mermaid
flowchart LR
  X[Input] --> Y[Process] --> Z[Output]
```

```mermaid
flowchart TD
  P[One] --> Q[Two]
  Q --> R[Three]
```

B. Measure the id collision

This needs no new dependencies. Save it at the repo root as mermaid-race-repro.html (after npm install) and open it in a browser. It renders 8 diagrams the way markdown.tsx does, then the way this PR does, and prints how many ended up sharing an SVG id.

mermaid-race-repro.html

<!doctype html><meta charset="utf-8">
<title>mermaid concurrent render repro</title>
<style>body{font:14px system-ui;margin:24px}pre{background:#f4f4f5;padding:12px;border-radius:6px}#diagrams{display:none}</style>
<h3>mermaid concurrent render repro</h3>
<pre id="out">running...</pre>
<div id="diagrams"></div>
<script src="./node_modules/mermaid/dist/mermaid.min.js"></script>
<script>
const N = 8, TRIALS = 6;
const log = [];
const make = i => {
  const el = document.createElement("div");
  el.textContent = `flowchart TD\n  A${i}["a${i}"] --> B${i}["b${i}"]`;
  document.getElementById("diagrams").appendChild(el);
  return el;
};
// the queue this PR adds
let queue = Promise.resolve();
const runQueued = (node, text) => {
  const r = queue.then(() => { node.removeAttribute("data-processed"); node.textContent = text; return mermaid.run({ nodes: [node] }); });
  queue = r.catch(() => {});
  return r;
};
(async () => {
  mermaid.initialize({ startOnLoad: false, theme: "dark", securityLevel: "strict" });
  for (const queued of [false, true]) {
    let worstDupes = 0, worstRendered = N;
    for (let t = 0; t < TRIALS; t++) {
      document.getElementById("diagrams").innerHTML = "";
      const els = Array.from({ length: N }, (_, i) => make(i));
      const texts = els.map(e => e.textContent);
      // every <Mermaid> useEffect fires at once, as in markdown.tsx
      await Promise.all(els.map((e, i) => queued ? runQueued(e, texts[i]) : mermaid.run({ nodes: [e] })));
      const ids = els.map(e => e.querySelector("svg")?.id).filter(Boolean);
      worstDupes = Math.max(worstDupes, ids.length - new Set(ids).size);
      worstRendered = Math.min(worstRendered, els.filter(e => e.querySelector("svg")).length);
    }
    log.push(`${queued ? "queued (this PR)  " : "concurrent (before)"}  worst duplicate ids: ${worstDupes}/${N}   rendered: ${worstRendered}/${N}`);
    document.getElementById("out").textContent = log.join("\n");
  }
})();
</script>

Output here, on this repo's mermaid 11.15.0:

concurrent (before)  worst duplicate ids: 6/8   rendered: 8/8
queued (this PR)     worst duplicate ids: 0/8   rendered: 8/8

What these do and do not show

B measures the id collision deterministically and runs in any browser. A shows the user-visible symptom, but I have only been able to observe it in Wave. In headless Chromium all eight diagrams still render even with fully concurrent calls, so the overlap and the missing diagrams appear to need the app's markdown preview specifically. That is why B measures the underlying id collision rather than the visual result: it is the part of the failure I can demonstrate anywhere.

Fix

Queue the run() calls so only one executes at a time, and memoize the init promise instead of setting a flag after the await. No change in behavior for single-diagram documents.

The node's text is written inside the queued task rather than before queueing, because the node contents are what mermaid.run() reads when the task executes. Writing them earlier would leave a window where a later effect on the same node could replace the text an earlier queued render was about to read.

Note on `deterministicIds`

The issue suggests deterministicIds: true as a possible fix. It makes things worse. The id generator is constructed inside each run() call, so with one node per call every diagram gets mermaid-0:

deterministicIds	ids from 3 sequential `run()` calls
`false`	`mermaid-1787091579824`, `mermaid-1787091579860`, `mermaid-1787091579867`
`true`	`mermaid-0`, `mermaid-0`, `mermaid-0`

Testing

prettier --check passes on the changed code. The one remaining warning is a pre-existing import ordering difference that is also present on main, so I left it alone rather than mix an unrelated reformat into this PR.
tsc --noEmit reports the same 17 pre-existing errors as the base commit, none in markdown.tsx.

Deliberately out of scope

useEffect has no cleanup, so a queued render for an unmounted component still runs and can call setError/setIsLoading afterwards. That is pre-existing and separate from the concurrency bug, so I left it out to keep this to one logical change. Happy to follow up in another PR if you want it.

Every Mermaid component calls mermaid.run() from its own useEffect, so a markdown file with several diagrams starts all of the renders in parallel. mermaid.run() mutates module-global state and derives its SVG element id from Date.now(), so calls that overlap or land in the same millisecond can produce colliding ids and render into one another. Queue the run() calls so only one executes at a time, and memoize the mermaid init promise instead of setting a flag after the await, which let every component get past the guard and call initialize() again while other renders were still in flight. Fixes wavetermdev#3191

CLAassistant · 2026-08-18T22:24:19Z

All committers have signed the CLA.

coderabbitai · 2026-08-18T22:24:31Z

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frontend/app/element/markdown.tsx

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Walkthrough

Mermaid rendering now passes normalized chart text into the serialized render queue. Each queued task prepares its target node when execution starts. This prevents queued renders from using overwritten chart contents. The queue still recovers after render failures. renderMermaid stores the initialized Mermaid instance.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to c3d80

The change serializes Mermaid rendering and prevents concurrent diagram ID collisions, but an older queued render may still update the preview after the diagram changes or the component unmounts, potentially showing stale error or loading state. The PR is mergeable with explicit owner awareness or follow-up to guard stale results.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue `#3191` by serializing Mermaid renders and preventing shared-state and SVG ID collisions between diagrams.
Out of Scope Changes check	✅ Passed	The changes remain focused on Mermaid render serialization and initialization; no unrelated code changes are identified.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title clearly identifies the main change: serializing concurrent Mermaid renders in the Markdown preview.
Description check	✅ Passed	The description directly explains the Mermaid rendering race, its effects, the queued-render fix, testing, and scope.

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Inline comments:
In `@frontend/app/element/markdown.tsx`:
- Around line 47-51: Update runMermaid and its calling effect so node
preparation, including textContent and data-processed changes, is queued
together with mermaidInstance.run, preventing later renders from racing with
earlier ones. Add effect cleanup and guard stale render failures so an older run
cannot set error or replace a newer diagram after cleanup or rerender.

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frontend/app/element/markdown.tsx

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The node's contents are what mermaid.run() reads when the queued task executes. Writing them before queueing left a window where a later effect on the same node could replace the text an earlier queued render was about to read, so a render could pick up the wrong chart.

anik8das · 2026-08-18T23:01:07Z

Addressed the review feedback in c3d805c: the node's text is now written inside the queued task instead of before it, so a render always reads its own chart. That was the substantive half of the finding, and it closes a window the queue had widened.

I left the useEffect cleanup out on purpose. A queued render belonging to an unmounted component can still call setError/setIsLoading afterwards, but that is pre-existing and separate from the concurrency bug, and CONTRIBUTING.md asks for one logical change per PR. Happy to do it as a follow-up if you want it.

I also added runnable reproduction steps to the description: a markdown file that shows the symptom in the preview, and a standalone HTML file that measures the SVG id collisions directly with no extra dependencies.

The earlier merge-gatekeeper failure was a timing artifact. It timed out waiting on license/cla before the CLA was signed, and it passes on the new commit.

coderabbitai Bot reviewed Aug 18, 2026

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fix: serialize concurrent mermaid renders in markdown preview - #3478

fix: serialize concurrent mermaid renders in markdown preview#3478
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Problem

Reproduce it

A. See the symptom

B. Measure the id collision

What these do and do not show

Fix

Note on deterministicIds

Testing

Deliberately out of scope

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anik8das commented Aug 18, 2026 •

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Note on `deterministicIds`

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