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…ore the headers
new Response(blob) derived the Content-Type header from the body only
when the header list was first materialized. Reading .body, .text(),
rewrapping with new Response(res.body, res), HTMLRewriter, or the
server's stream-to-blob fallback all moved the body out of its Blob
state first, so the header came back null and a Bun.file(path, { type })
override was replaced by the extension type on the wire.
Capture the body's Content-Type into Init at construction and fold it
into the header list when that is allocated. Init::clone carries it, the
server prefers it over the re-derived blob type, and Request (which has
no reason to defer) appends the header at construction.
…der list; widen tests The output body is a stream, and finalize_without_stream types a waiting .blob() from get_fetch_headers(), which does not see a pending type. Write it into the output's header list when the pipe is created so .blob().type does not depend on whether .headers was read. Tests: FormData and URLSearchParams bodies, the multipart boundary after arrayBuffer(), HTMLRewriter .blob().type, and fetch() of data:/blob:/file: URLs read body-first.
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WalkthroughThe change defers Blob-derived ChangesContent-Type preservation
Priority: ⬇️ Low — Defer this response-header fix because it targets body-derived Content-Type preservation across requests, responses, routes, and rewriting without elevated external urgency. Merge Risk: ⚪ Minimal · up to No concrete merge-blocking risk is established at the current head. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 11:16 AM PT - Sep 8th, 2026
❌ @robobun, your commit d74b38a has 1 failures in 🧪 To try this PR locally: bunx bun-pr 42015That installs a local version of the PR into your bun-42015 --bun |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/runtime/server/FileRoute.rs— Pre-existing: the static-route path still drops a body-derived Content-Type when the body was streamed first — the same ordering bug this PR fixes forrender_metadata.FileRoute::from_jsandStaticRoute::from_jsreadresponse.get_init_headers()(None) and the rebuilt blob fromto_blob_if_possible(), neither of which seespending_content_type; anew Response(res.body, res)or a Response whose.bodywas touched, registered underroutes:/static:, is served with the extension-derived type, exactly as on main. Fix: have both call sites consultresponse.pending_content_type()(orget_or_create_headers) before building the route headers. Sites: FileRoute.rs:168, StaticRoute.rs:227.Extended reasoning...
Trigger:
const r = new Response(Bun.file('a.txt', { type: 'image/png' })); Bun.serve({ routes: { '/x': new Response(r.body, r) } }).Init::clonegives the rewrapped Responsepending_content_type = "image/png"andheaders = None; its body isLocked. InFileRoute::from_js(server_body.rs:888 → FileRoute.rs:141)to_blob_if_possible()rebuilds the blob viaReadableStream::to_any_blob→Blob::init_with_store(store.clone(), …)(ReadableStream.rs:212), which setscontent_typefrom the store's extension mime and leavescontent_type_was_set = false.headers_from(response.get_init_headers(), &blob)at FileRoute.rs:168 receivesNoneandblob_content_type(&blob) = None(gated onhas_content_type_from_user()), so the route has no Content-Type header and the file is served astext/plain.pending_content_type = "image/png"is captured but never read on this path. StaticRoute.rs:163/227 has the same shape for non-file blobs. Base branch behaves identically (nopending_content_typeexisted), so this is a sibling the fix leaves unclosed rather than a regression;…Verification: pre-existing — the static-route sibling of the path this PR fixes still drops the body-derived Content-Type when the body was streamed first, and the base branch already fails the same way.
FileRoute::from_js(src/runtime/server/FileRoute.rs:141,168):rust body_value.to_blob_if_possible(); ... let headers = headers_from(response.get_init_headers(), &blob);headers_from… | pre-existing…
…te headers A Response registered under routes:/static: whose body was streamed first (new Response(res.body, res)) kept its Content-Type only in Init::pending_content_type, which the route builders did not read. Fold it into the header list before building the route's headers.
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Addressed the review notes in bb661df and 16a3b30:
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Actionable comments posted: 1
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Inline comments:
In `@test/js/web/fetch/response.test.ts`:
- Line 244: Replace the parameterized test patterns with describe.each() suites:
in test/js/web/fetch/response.test.ts at lines 244-244, replace the manual loop;
at lines 260-260, wrap the body-reader matrix in describe.each(); and at lines
316-316, replace test.each() with describe.each() plus a normal test. In
test/js/bun/http/bun-serve-file.test.ts at lines 900-900, replace it.each() with
describe.each() plus a normal test, preserving each case’s existing assertions
and setup.
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src/runtime/api/html_rewriter.rssrc/runtime/server/FileRoute.rssrc/runtime/server/RequestContext.rssrc/runtime/server/StaticRoute.rssrc/runtime/webcore/Request.rssrc/runtime/webcore/Response.rstest/js/bun/http/bun-serve-file.test.tstest/js/web/fetch/response.test.tstest/js/web/request/request.test.ts
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CI on d74b38a (build 113062): 180 of 181 jobs passed. The one red job is Ready for review. |
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Heads-up on an overlap: #42217 rewrites the same |
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Problem
new Response(Bun.file(p, { type: "image/png" })): reading.body(or.text(), ornew Response(res.body, res)) before.headersmakesheaders.get("content-type")returnnull, andBun.servethen sends the extension-derived type instead of the override. A typedBlob,FormDataandURLSearchParamsbody lose their header the same way, as doHTMLRewriteroutput andfetch()ofdata:/file:/blob:URLs.Response::get_or_create_headers(src/runtime/webcore/Response.rs) derived the header from whatever the body was at first.headersaccess. Once the body left itsBlobstate there was nothing to derive from. On the wire,RequestContext::render_metadatafell back to the Blob the server re-derives from a file stream, which carries the extension type, not the{ type }override.Fix
Initgainspending_content_type: theContent-Typetaken from the body when theResponseis constructed (JS constructor and nativeResponse::init).get_or_create_headersfolds it into the header list when that is allocated,Init::clonecarries it (clone(), aResponseused asResponseInit/RequestInit),HTMLRewriter.transformand theroutes:/static:builders write it into a header list, and the server prefers it over the re-derived Blob type.Requestappends the header at construction instead of deferring it. Nothing keys off aRequest's header list being unallocated, andfetch()builds one from it anyway. This also fixesfetch(req)sending noContent-Typeafterreq.bodywas read.new Response(body)stays lazily allocated, so the sliced-Bun.file()auto-206 rule inrender_metadata(which reads "no header list" as "no headers init") and thenew Response(file)fast path are unchanged.test/js/web/fetch/response.test.ts,test/js/web/request/request.test.ts,test/js/bun/http/bun-serve-file.test.ts(new cases fail on 1.4.3, pass here). Other suites in the notes. Self-reviewed: 3 concerns raised, 2 addressed, 1 deferred (below).Background
Content-Type; theResponseandRequestconstructors append it to the header list unless one is already there. So the header exists from construction, whatever is read first. Node behaves this way.Response'sFetchHeaders(a C++ object) only on first use, soreturn new Response(Bun.file(path))never builds one. The derivedContent-Typewas the one piece of header state that lived only in the body.Bun.serveturns an unread file stream back into a fileBlobbefore sending (to_blob_if_possible). That Blob is rebuilt from the store, whose type comes from the path extension.Notes
.headersfirstimage/png/image/png;.bodyfirstnull/image/png;new Response(r.body, r)null/image/png; typedBlobwith.bodyfirstnull/text/html;charset=utf-8. Wire:/direct,/bodycheck,/rewrapallimage/png(wereimage/png,text/html,text/html).Responseregistered underroutes:,.text()/.arrayBuffer()/.bytes()/.blob()/.textStream()first,clone()after.body,new Response("x", res)andnew Request(url, res)with an unreadres,HTMLRewriter.transform(res).headersand.blob().type,res.headersafterBun.write(dest, res), aFormDatarequest read with.arrayBuffer()first (the boundary inreq.headerswas lost), andfetch()responses fordata:,file:andblob:URLs read body-first.ReadableStreambody contributes noContent-Type; an explicitContent-Typeheader always wins; the 200/206 status of a slicedBun.file()response is exactly as before in every case.Bodymixin consumer and therefore has to change the auto-206 rule. Covers the typed-body part of HTMLRewriter: carry the input body's Content-Type onto the transformed Response #38570 (HTMLRewriteroutputContent-Type); itstext/plaindefault for string-body input is independent.Content-Typeheader (Responsewith text body should haveContent-Typeoftext/plain#8530), sonew Response("x")with.bodyread first is served asapplication/octet-stream; reading.headersonnew Response(Bun.file(p).slice(a, b))still turns the automatic 206 into a 200, becauserender_metadatakeys that on the header list's existence;(await res.blob()).typefor a body held as a stream ignores the header (Bun.serve: read Content-Type and Range through req.headers; give stream-body blob() its type #41922 routesblob()throughget_content_type(), which reads the pending type added here).HTMLRewriteroutput typed a waiting.blob()fromget_fetch_headers(), so its type depended on whether.headershad been read: addressed by writing the pending type into the output's header list inRewriterPipe::init, with a test. (2) Port theFormData/URLSearchParams/data:URL cases from webcore: fix Content-Type lost when Request/Response body is read before headers #32913 and theHTMLRewritercases from HTMLRewriter: carry the input body's Content-Type onto the transformed Response #38570: done inresponse.test.tsandrequest.test.ts. (3) Decouple the auto-206 rule from header-list existence and allocateResponseheaders eagerly likeRequest: deferred, it changes the sliced-file status contract and thenew Response(file)allocation profile, both out of scope for this bug.response.test.ts,request.test.ts,bun-serve-file.test.ts,body.test.ts,body-stream.test.ts,body-clone.test.ts,blob.test.ts,FormData.test.ts,headers.test.ts,html-rewriter.test.js,htmlrewriter-additional-bugs.test.ts,serve.test.ts,bun-serve-static.test.ts,serve-body-leak.test.ts,fetch-file-upload.test.ts,client-fetch.test.ts,inspect.test.js.response.test.ts"print size" snapshots the test file's own size, so its expected value changes with this diff.no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/http/bun-serve-file.test.ts