Core: Fix EEXIST race condition in static file copying during build

[flt3150sk]

Closes #18686

What I did

Added force: true to all fs.cp() calls in the static build pipeline.

In buildStaticStandalone, the preview build (Vite/Webpack) and static file copies run concurrently via Promise.all:

await Promise.all([
  previewBuilder.build({ startTime, options: fullOptions }),
  ...effects  // includes copyAllStaticFilesRelativeToMain() and cp()
]);

When both operations write to the same outputDir, they can attempt to mkdir the same subdirectory simultaneously. Without force: true, fs.cp internally calls mkdir(dest) (not mkdir(dest, { recursive: true })), so the second call fails with EEXIST.

This is timing-dependent and primarily affects CI environments where I/O is slower, causing the copy and build to overlap for longer periods.

Changes

• code/core/src/core-server/utils/copy-all-static-files.ts: Add force: true to both copyAllStaticFiles and copyAllStaticFilesRelativeToMain
• code/core/src/core-server/build-static.ts: Add force: true to the core server public dir copy

How to test

1. Configure staticDirs pointing to a directory with nested subdirectories
2. Run storybook build repeatedly in a CI-like environment (constrained I/O)
3. Before this fix, builds intermittently fail with EEXIST: file already exists, mkdir
4. After this fix, builds complete reliably

Summary by CodeRabbit

• Bug Fixes
  • Improved static file generation to ensure files are properly overwritten when regenerating output. Previously, existing files in the output directory could block updates, potentially causing build inconsistencies or stale content. This fix applies to both standalone and main build configurations, ensuring clean and complete static output generation in all scenarios.

[coderabbitai]

📝 Walkthrough

Walkthrough

The static build now copies Storybook browser assets to the output directory with fs/promises.cp using { recursive: true, force: true }, enabling overwrites of existing destination targets.

Changes

Static Build File Copy

Layer / File(s)	Summary
Build static copy updated code/core/src/core-server/build-static.ts	Changed the copy call for storybook/assets/browser to pass force: true to fs/promises.cp (keeps recursive copy and existing async coordination).

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

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[github-actions]

	Fails
🚫	PR description is missing the mandatory "#### Manual testing" section. Please add it so that reviewers know how to manually test your changes.

Generated by 🚫 dangerJS against d07098c

[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

code/core/src/core-server/build-static.ts (1)

115-119: ⚠️ Potential issue | 🟠 Major | ⚡ Quick win

Restore disableTelemetry guard on telemetry sends.

Line 115 and Line 211 currently send telemetry even when users disable it via core.disableTelemetry. Please add the opt-out guard back in both blocks.

Proposed fix

-  if (invokedBy) {
+  if (invokedBy && !core?.disableTelemetry) {
     // NOTE: we don't await this event to avoid slowing things down.
     // This could result in telemetry events being lost.
     telemetry('test-run', { runner: invokedBy, watch: false }, { configDir: options.configDir });
   }
@@
-  if (!options.test) {
+  if (!options.test && !core?.disableTelemetry) {
     try {
       const generator = await storyIndexGeneratorPromise;
       const storyIndex = await generator?.getIndex();
       const payload: any = {
         precedingUpgrade: await getPrecedingUpgrade(),
       };

Also applies to: 211-229

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@code/core/src/core-server/build-static.ts` around lines 115 - 119, The
telemetry sends are missing the opt-out check; wrap both places that call
telemetry(...) (the block that runs when invokedBy is set and the other block
later that also calls telemetry) with a guard checking core.disableTelemetry
(e.g. only call telemetry if core.disableTelemetry is falsy), so use the
existing disableTelemetry flag (core.disableTelemetry) to skip the
telemetry('test-run', ...) invocation and the other telemetry invocation in the
file.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Outside diff comments:
In `@code/core/src/core-server/build-static.ts`:
- Around line 115-119: The telemetry sends are missing the opt-out check; wrap
both places that call telemetry(...) (the block that runs when invokedBy is set
and the other block later that also calls telemetry) with a guard checking
core.disableTelemetry (e.g. only call telemetry if core.disableTelemetry is
falsy), so use the existing disableTelemetry flag (core.disableTelemetry) to
skip the telemetry('test-run', ...) invocation and the other telemetry
invocation in the file.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: d47a5492-5e88-4256-82b6-3272a481325f

📥 Commits

Reviewing files that changed from the base of the PR and between e7fc8d0 and d07098c.

📒 Files selected for processing (1)

• code/core/src/core-server/build-static.ts

[storybook-app-bot]

Package Benchmarks

^{Commit: d07098c, ran on 22 May 2026 at 10:55:16 UTC}

The following packages have significant changes to their size or dependencies:

@storybook/builder-webpack5

	Before	After	Difference
Dependency count	184	184	0
Self size	79 KB	79 KB	🎉 -48 B 🎉
Dependency size	33.35 MB	33.36 MB	🚨 +16 KB 🚨
Bundle Size Analyzer	Link	Link

@storybook/angular

	Before	After	Difference
Dependency count	185	185	0
Self size	160 KB	160 KB	0 B
Dependency size	30.73 MB	30.75 MB	🚨 +16 KB 🚨
Bundle Size Analyzer	Link	Link

@storybook/ember

	Before	After	Difference
Dependency count	188	188	0
Self size	15 KB	15 KB	0 B
Dependency size	30.07 MB	30.08 MB	🚨 +16 KB 🚨
Bundle Size Analyzer	Link	Link

@storybook/nextjs

	Before	After	Difference
Dependency count	534	534	0
Self size	662 KB	662 KB	0 B
Dependency size	61.51 MB	61.52 MB	🚨 +16 KB 🚨
Bundle Size Analyzer	Link	Link

@storybook/react-webpack5

	Before	After	Difference
Dependency count	271	271	0
Self size	23 KB	23 KB	0 B
Dependency size	46.05 MB	46.06 MB	🚨 +16 KB 🚨
Bundle Size Analyzer	Link	Link

@storybook/server-webpack5

	Before	After	Difference
Dependency count	196	196	0
Self size	16 KB	16 KB	0 B
Dependency size	34.61 MB	34.63 MB	🚨 +16 KB 🚨
Bundle Size Analyzer	Link	Link

@storybook/preset-react-webpack

	Before	After	Difference
Dependency count	164	164	0
Self size	18 KB	18 KB	0 B
Dependency size	32.35 MB	32.37 MB	🚨 +16 KB 🚨
Bundle Size Analyzer	Link	Link