What?

Fixes inconsistent instrumentation.ts loading behavior between development and production environments when a src/ directory exists.

Why?

Problem:

• Development: Loads instrumentation.ts from root directory even when src/ directory exists
• Production: Correctly loads instrumentation.ts from src/ directory when it exists
• Expected: Should exclusively load from src/ when src/ directory exists, ignoring any root-level file

This inconsistency causes confusion and potential issues when projects have both instrumentation.ts in root and src/instrumentation.ts, as different files are loaded in different environments.

How?

Modified getPossibleInstrumentationHookFilenames() in packages/next/src/build/utils.ts:

1. Check for src/ directory existence using fs.existsSync()
2. If src/ exists: Only return paths within src/ directory
3. If no src/: Only return root directory paths
4. Result: Development behavior now matches production behavior

Before:

// Always returned both root AND src paths
files.push(
  path.join(folder, `${INSTRUMENTATION_HOOK_FILENAME}.${extension}`),
  path.join(folder, `src`, `${INSTRUMENTATION_HOOK_FILENAME}.${extension}`)
)

After:

// Only returns src paths if src/ exists, otherwise only root paths
if (hasSrcDir) {
  files.push(path.join(folder, `src`, `${INSTRUMENTATION_HOOK_FILENAME}.${extension}`))
} else {
  files.push(path.join(folder, `${INSTRUMENTATION_HOOK_FILENAME}.${extension}`))
}

Testing

Created test script that validates:

• ✅ When no src/ directory exists → only returns root paths
• ✅ When src/ directory exists → only returns src paths

Human Review Checklist

Critical items to verify:

• Logic correctly prioritizes src/ when directory exists
• No regressions in development server instrumentation loading
• File system check approach (fs.existsSync()) is appropriate for this context
• Consider if existing tests need updates for this behavior change
• Verify alignment with documented instrumentation.ts behavior

Potential risks:

• Synchronous fs.existsSync() call in hot path (though just directory check)
• Projects incorrectly depending on old buggy behavior might be affected
• Runtime require('fs') usage (follows existing codebase pattern)

Link to Devin run: https://app.devin.ai/sessions/dd6bf37499764766864e431921c83280
Requested by: @devjiwonchoi

This change ensures consistent instrumentation.ts loading behavior across all Next.js environments.