fix(serve-static): compare If-Modified-Since at whole-second precision by francisjohnjohnston-web · Pull Request #1394 · h3js/h3

francisjohnjohnston-web · 2026-05-24T16:09:36Z

What

serveStatic now compares the If-Modified-Since request value against the file mtime truncated to whole-second precision, matching the granularity of the Last-Modified response header it emits.

Why

Last-Modified is serialized via Date.toUTCString(), which has whole-second granularity. The conditional-request check, however, compared the client's If-Modified-Since against the full-precision mtime. When a file's mtime carries sub-second milliseconds, a conditional request echoing back the server's own Last-Modified value never satisfied ifModifiedSince >= mtime, so the server returned a full 200 instead of 304 — silently defeating conditional-request caching for any asset whose mtime is not exactly on a second boundary (the common case on many filesystems).

Safety

Behaviour-preserving for whole-second mtimes. The only change is that sub-second-mtime assets now correctly return 304 on a matching conditional request. This mirrors the existing whole-second handling already used in handleCacheHeaders (cache.ts), so the two paths are now consistent.

Testing

Added "Handles cache (if-modified-since) for sub-second mtime" to test/static.test.ts (both web and node targets). It fails on the current code (expected 200 to deeply equal 304) and passes after the fix. Full test suite green (1154 passed / 14 skipped), typecheck clean, oxlint clean on changed files.

Impact

Restores correct 304 Not Modified responses for conditional requests on static assets with sub-second mtimes — fixing avoidable full-body re-transfers.

Summary by CodeRabbit

Bug Fixes
- Fixed timestamp precision handling in HTTP conditional requests to ensure proper cache validation. Timestamps are now normalized to second-level granularity, preventing mismatches when clients check if cached resources remain valid.

The `last-modified` header is emitted via `toUTCString()`, which truncates to whole seconds (HTTP-date granularity). When `meta.mtime` carries sub-second milliseconds, the `if-modified-since` comparison used the full-precision mtime, so a client echoing back our own `last-modified` value never satisfied `ifModifiedSince >= mtime` and received a full 200 response instead of 304 on every conditional request. Truncate the mtime to whole seconds before comparing, matching `handleCacheHeaders`.

coderabbitai · 2026-05-24T16:09:46Z

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 84244b4 and 9932a77.

📒 Files selected for processing (2)

src/utils/static.ts
test/static.test.ts

📝 Walkthrough

Walkthrough

The serveStatic function now truncates mtime to whole-second precision by zeroing milliseconds before evaluating conditional GET requests and setting response headers, preventing client-side matching failures due to sub-second datetime mismatches. A test case validates the behavior when mtime contains milliseconds.

Changes

HTTP Date Precision Fix

Layer / File(s)	Summary
HTTP date precision for conditional GET `src/utils/static.ts`, `test/static.test.ts`	The `serveStatic` function truncates `mtimeDate` milliseconds to ensure `if-modified-since` comparison and `last-modified` emission use whole-second HTTP-date granularity; test validates that a `304` response is returned when the server-truncated `last-modified` matches the request header despite sub-second `mtime` values.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

h3js/h3#1262: Normalizes mtime to whole-second precision in handleCacheHeaders to fix the same If-Modified-Since conditional GET mismatch with sub-second precision.

Suggested reviewers

pi0

Poem

🐰 A rabbit's watch ticks true and fast,
But HTTP clocks with seconds mast,
We trim the fractions, dust each millisecond away,
So browsers and servers agree on the day! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and clearly describes the main change: fixing If-Modified-Since comparison to use whole-second precision in the serve-static functionality.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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francisjohnjohnston-web requested a review from pi0 as a code owner May 24, 2026 16:09

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fix(serve-static): compare If-Modified-Since at whole-second precision#1394

fix(serve-static): compare If-Modified-Since at whole-second precision#1394
francisjohnjohnston-web wants to merge 1 commit into
h3js:mainfrom
francisjohnjohnston-web:fix/static-if-modified-since-subsecond

francisjohnjohnston-web commented May 24, 2026 •

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coderabbitai Bot commented May 24, 2026 •

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francisjohnjohnston-web commented May 24, 2026 •

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