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WalkthroughThe change removes MIME metadata from shared Blob stores and uses each Blob’s content type for MIME updates and lookups. It updates store and Blob construction paths, adjusts Blob memory-size estimates, and adds coverage for MIME type isolation in asset, web, and worker scenarios. ChangesBlob MIME type isolation
Priority: ⬆️ High Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
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| Description check |  |
The description clearly explains the problem, fix, impact, scope, and verification results. It does not use the exact template headings, but it provides the required information in equivalent sections… |
| Title check | |
The title is concise, specific, and accurately summarizes the main change: removing Blob MIME type data from the shared byte store. |
| Linked Issues check | |
Issue #35284 requires Blob.type to remain fixed and isolated from unrelated Response and Request bodies. The supplied PR evidence shows removal of Store.mime_type, its readers and writers, and MimeTyp… |
| Out of Scope Changes check | |
The supplied changes remain connected to issue #35284. Removing obsolete Store initialization and MimeType::NONE follows removal of Store.mime_type. The Blob and Store heap-block accounting change pre… |
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The description clearly explains the problem, fix, impact, scope, and verification results. It does not use the exact template headings, but it provides the required information in equivalent sections.
Full details: Linked Issues check
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Explanation
Issue
#35284requires Blob.type to remain fixed and isolated from unrelated Response and Request bodies. The supplied PR evidence shows removal of Store.mime_type, its readers and writers, and MimeType::NONE. The evidence also shows that each affected path keeps the type on the individual Blob. Regression tests cover Blob and slice isolation, Response and Request bodies, blob URL imports, build outputs, and cross-thread access. The whole-PR diff was unavailable, but the supplied change summary and current PR description establish the relevant implementation and tests.
Full details: Out of Scope Changes check
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Explanation
The supplied changes remain connected to issue
#35284. Removing obsolete Store initialization and MimeType::NONE follows removal of Store.mime_type. The Blob and Store heap-block accounting change preserves JSC memory reporting after the layout change. The added tests validate type isolation, related MIME behavior, and the worker failure mode. No unrelated change is demonstrated.
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Updated 7:04 AM PT - Oct 20th, 2026
✅ @robobun, your commit 99b9712d7386a1a6418bdab377e6b006ef7b89a6 passed in 🧪 To try this PR locally: bunx bun-pr 46114That installs a local version of the PR into your bun-46114 --bun |
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Status: ready for review. How I reproduced it. On a release build of main and on 1.4.3, with the script from #35284: The freed-memory read is the same write seen from another thread. Four Workers resolve one object URL and loop Tests. Each new test fails on a release build of main:
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The second push addresses two of my earlier threads (the Bun.build() artifact now marks its type as set, and the race fixture ends on its own loop with no per-test timeout). Beyond this run's inline findings, I also checked for leftover readers or writers of the removed Store-level type: no MimeType::NONE, Category::None, or Store.mime_type references remain in Rust, C++, Zig, or the generated MIME list inputs; the surviving mime_type fields are the per-File/S3 ones, which the diff leaves intact.
Extended reasoning...
The PR removes the shared Store.mime_type field and its four writers across Blob.rs, Body.rs, output_file_jsc.rs, standalone_graph_jsc.rs and Store initializers, plus a PathLike::estimated_size repair; it touches cross-thread Blob store memory but no auth, crypto or injection surface. A grep of src for the deleted constant, variant and field confirmed the class was removed completely, including generated-binding inputs. Inline findings from this run (the Fd-variant estimate gap and two test nits) and the still-open slice-of-body thread mean a human should still weigh the accepted behaviour change.
Findings marked 🟡 are optional suggestions and need no follow-up push.
Still open from earlier reviews (2):
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src/runtime/webcore/Blob.rs:1890—Users who slice a Blob obtained from Response.blob() or Request.blob() now get type "" where main returned the body's c… - Also unresolved: 1 minor or pre-existing.
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…ap blocks A slice with no type keeps the type of its parent only when that type is interned. A header type came from MimeType::init as a heap copy, so a slice of response.blob() lost a type such as image/png. The header path now takes the interned name when the MIME table has the same bytes. calculate_estimated_byte_size counts the Blob and the Store as the heap blocks they are (steps of 16 bytes). A const assert keeps the sum above the 256 bytes that JSC needs before it counts a report.
The fixture loaded node:worker_threads in both threads. That load takes most of a second in a debug build. The global Worker API needs no module, so the fixture runs in less than half the time.
…s on main A slice of a body Blob, and a Blob made from the stream of a Bun.build() output, read their type through the store. Two earlier commits gave them a type of their own: an interned header name, and content_type_was_set on an output. That also put the type into multipart parts, Response and Request headers, fetch() bodies and static routes, which never had it on main. This commit removes both. No header changes. The `.type` of those two Blobs is now empty, which is what every other reader of them already got. The size estimate no longer counts the path of a file store. The estimate in heap blocks keeps every Blob that has a store above the 256 bytes that JSC needs before it counts a report, so the path count is not necessary.
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Actionable comments posted: 1
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Inline comments:
Review comments at @test/js/web/workers/worker_blob.test.ts:
- Around line 129-183: Update the worker in the Blob race test to validate only
that blob.type remains the empty string: remove the types payload and expected
set, initialize unexpected from blob.type === "", and increment it whenever
blob.type !== "" during the loop. Preserve the existing synchronization and
final assertions.
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📒 Files selected for processing: 5
src/runtime/api/BunObject.rssrc/runtime/api/output_file_jsc.rssrc/runtime/webcore/Blob.rstest/js/web/fetch/blob.test.tstest/js/web/workers/worker_blob.test.ts
💤 Files with no reviewable changes: 2
- src/runtime/api/BunObject.rs
- src/runtime/api/output_file_jsc.rs
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The Worker counted a read as unexpected only when the value was not one of the types that the main thread wrote. The Blob of the Worker has no type, so each read that is not empty is wrong.
…6270) ### What does this PR do? Hardening follow-up to #46021. It fixes regressions from #46021 and #44618 that were never released, includes more of the open pull requests that fix a crash, and fixes a few crashes that had no pull request. #### Regressions fixed (none was released) - `spyOn(Bun, "$")`, and the same for `"sql"`, `"postgres"` and `"SQL"`, threw "does not support accessor properties yet". - Async `fs.read`, `fs.write` and `zlib.brotliCompress` on a view over a `WebAssembly.Memory` could lose data or copy far more than they needed. - `node:vm` with `vm.constants.DONT_CONTEXTIFY`: `this` in host accessors, prototype handling, `util.types.isProxy`, `Bun.deepEquals`, `assert.deepEqual` and `console.log` of a context. - `zlib` `stream.flush()` threw for arguments it used to accept, and streams had a new enumerable property. - `Bun.Transpiler` `exports.replace` with the name `"default"`. - `process.env.X = 3000` could store the number once the function was JIT-compiled. - `node:sqlite` and `bun:sqlite` could lose rows at exit. - Dev server: stylesheets shared between pages after a build error. - Shell `cp` of a file onto itself exited 1. - `error.stack`: the frame of `require()` reads `at anonymous (unknown)` again. - `import * as process from "process"` followed by `process.on(...)` threw at load. The EventEmitter methods that the module exports now act on `process`. - `delete res.statusCode; res.end()` in a `node:http` handler threw. It sends 200, as in Node. (from #46128) - `node:tls`, `node:https` and `node:http2`: connection teardown and write callbacks behave as in 1.4.3 and Node again. #### Crash fixes Bundler, parser and CSS: #42286, #46092, #46097, #46177, #46239, #36334, #43024, #40769. `bun test`, `--hot` and `--watch`: #30776, #38206, #38974. Shell: #43183, #37743, #46168, #46207, #34900. `node:http`, `node:http2`, `Bun.serve`, streams: #43449, #45380, #44457, #42197, #42485, #38082, #38124, #46074, #45999, #46045. `node:vm`, native addons, workers: #46129, #46198, #45615, #33412, #38031. Dev server: #42790, #42850, #39488. Other: #35991, #38030, #38586, #40526, #41830, #46029, #46078, #46114, and the bug of #42215. #### Behavior changes - `crypto.randomFill(view, cb)` reports `ERR_CRYPTO_OPERATION_FAILED` if it could not fill the view. It used to report success. - A function registered twice on `process` runs twice, as in Node. (#41830) - Under `--hot`, a reload after an uncaught exception cancels the timers that the failed run created. The new run's timers fire, and so do timers and `fs.watchFile` watchers that earlier runs kept on `globalThis`. The reload also resets the exit code to 0. (#38206) - The parent of a Worker no longer gets an `'error'` event for a failed `Bun.serve` request in that Worker. The error is printed, as on the main thread. - `hash.write()` after `digest()` drops the chunk, as in Node. (#35991) - `http2` over TLS with `server.emit("connection", socket)`: `session.socket` is a `TLSSocket` and a `'connection'` listener fires, as in Node. (#38124) - A `slice()` with no type of a body `Blob` has `type === ""`, as in Node. (#46114) - `util.types.isProxy(globalThis)` is false, as in Node. - A write on a `node:net` or `node:tls` socket that is still pending when the connection closes calls back with `ECANCELED`, as in Node. - `HTMLRewriter.on()` throws for a selector that is nested too deeply. - `Bun.Image`: `maxPixels` also bounds PNG metadata, `metadata()` of a GIF reports the size that gets decoded, and a path with a NUL byte throws. (#40526) - `bun build --no-bundle` with the `base64` or `dataurl` loader prints an error. (#36334) - Bundled output for a default-exported function or class expression changes to `var file_default = function name() {}`, as in esbuild. `.name` is unchanged. (#46097) - A Proxy in the prototype chain of a `node:vm` global is allowed. jsdom 30 with `runScripts` needs this. - `bun:sqlite` commits a write that `iterate()` left open when the main thread exits. An explicit `BEGIN` is still rolled back. - When a Worker stops while a native addon's async work is still running, the addon's finalizers run after that work is done, as in Node. - Shell: `mv -n` and `cp -n` don't overwrite, and `cp` reads `R`, `v` and `n` anywhere in a flag group. #### Performance - Ordinary `node:vm` contexts run at the same speed. With `DONT_CONTEXTIFY`, stores to globals and reads of inherited names in a hot loop cost 10 to 20 % more than on `main`. - A `process.env` write costs 20 to 35 % less than after #46021. - An async read into a `WebAssembly.Memory` view looks through the live memories: about 1.5 µs more with one memory. #### Not included - Taken in part, so these should stay open for the rest: #36334, #38974, #39488, #40526, #40769, #42485, #45615, #45999, #46029, #46092, #46114, #46168, #46207, #46239. - Needs a WebKit change: #34655 #39204 #40170 #40253 #41587 #41900 #42699 #42897 #42903 #43084 #43813 #44518 #44949 #45770 #45804 #45827. - #44723 (covered by #46097), #41675, #42753, #38762, #41475, #46016. ### How did you verify your code works? - Every item was reproduced first on release builds of `main`, of its parent and of 1.4.3, and compared with Node v26 where Node has the API. - Every fix has a test that fails on `main` and passes on a debug + ASAN build of this branch, except where that isn't practical (huge inputs, code that only runs on Windows or macOS). - The regressions were checked on grids of calls against the build before #46021 and against 1.4.3. - Bun's own test suites and Node's `test-vm-*`, `test-http2-*` and `test-zlib-*` files for the areas touched.
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Fixes #35284
Problem
typealso lived on the byte store, so reading one body retyped an unrelated Blob. In the issue's scriptoriginal.typebecomes"application/x-attacker-controlled"..typethrough an object URL (set_blob_content_type,Body.rs:62). A 4-Worker probe segfaults main in 4 of 4 runs.Fix
Store.mime_type(webcore_types.rs:538), its four writers, its two readers andMimeType::NONE. Each writer already set the type on its own Blob.blob.test.ts, a two-thread fixture inworker_blob.test.ts, one row incompile-asset-bunfs.test.ts. All fail on main.Background
Storeof bytes. Slices, copies and every thread that resolves oneblob:URL share it..typegetter and the loader of ablob:import read the store's type. Headers and multipart parts read the Blob's own type..blob()and puts a type into multipart parts that had none.Downsides
.typebecomes""for a Blob that read it through the store: a typelessslice()of a body Blob, a Blob made from the stream of aBun.build()output.new Response(thatBlob)already sent no type.blob:import of a typed slice loads by that type, not as TSX.Notes
The freed-memory read. Release builds, linux-x64. Workers read
.typethrough an object URL while the main thread loopsnew Response(blob, { headers: { "content-type": <a 4 KB type that is not in the MIME table> } }).blob()..typereads that return a value never written, 3 runsUnder ASAN the same fixture reports
heap-use-after-free, a READ inBlobExt::get_type, freed byset_blob_content_typeunderResponse::get_blob_with_this_value. The new test asserts the values, so it also fails on a release build of main. Nobody reported this. A review of the diff found it.The four writers.
Body.rs:62is the only one that writes a store that is already shared.output_file_jsc.rs:29,standalone_graph_jsc.rs:53andfrom_url_search_paramswrite a store they have just allocated. All four set the Blob's own type as well. The field is from #2416 (2023), when.blob()had nowhere else to put the type. #40436 made the same move for the File name.What moves, and what does not. Every reader of one typeless slice of
await new Response("hello", { headers: { "content-type": "image/png" } }).blob():slice.typeimage/png""Bun.readableStreamToBlob(slice.stream())then.typeimage/png""form.append("f", slice, "a.bin")application/octet-streamapplication/octet-streamnew Response(slice)Content-Typenew Request(url, { body: slice })Content-Typefetch(blobUrlOf(slice))Content-TypestructuredClone(slice).type""""A header of
text/htmlkeeps its type in every row on both builds. For 8 header types (text/html,text/plain,application/jsonand five more)MimeType::initreturns a static constant, andslice()keeps a static type. Node gives""for every typeless slice.The same holds for a
Bun.build()output:output.type,new Response(output),new Request(url, { body: output }), aBun.servehandler,fetch(body: output)and a static route are the same on both builds. Only(await Bun.readableStreamToBlob(output.stream())).typemoves, fromtext/javascript;charset=utf-8to"".new Response(thatBlob)sent no Content-Type on main.Two ways to keep those
.typevalues, both measured and not taken.slice()keeps a MIME-table name): the multipart part, the three Content-Type rows andstructuredClone(slice).typeabove all change toimage/png, a value such astext/csv; charset=utf-8still moves to"", andresponse.blob()goes from 1165 to 1601 instructions.content_type_was_seton a build output (so its stream carries the type):fetch(body: output)and a static route start to sendtext/javascript;charset=utf-8, andnew Response(output.stream()).blob()changes fromtext/plain;charset=utf-8. Set ETag header from BuildArtifact in new Response #36596 proposes that line with its own tests.Self-review, declined sub-items. No doc comment on
content_type_was_set: embedded files set it for a type inferred from the extension, so one rule would be wrong.get_mime_type_or_content_typekeeps its name, so the diff stays a removal.Overlap. #42016 rewrites
blob().typefrom the Content-Type header, removes the same store write and claims #35284 as well. It conflicts with main today. This PR is the field removal alone. #46173 is stacked on it: it stopsnew Blob([blob])from taking the type of its part. #41554 changes what a Blob reports to the GC. Four open PRs write the field (#40221, #40411, #31487, #30812) and two useMimeType::NONE(#40416, #43896). Each needs one line on rebase.Measurements. gdb single-step of one native call, 3 hits, release builds of the merge base (3880151) and this branch. The three hits agree.
new Blob(["hello world"])new Blob([new Uint8Array(11)])new Blob(["hello world"], { type: "text/plain" })new File(["x"], "n")new Blob([typedBlob])new Blob([Bun.file(p)])new File([Bun.file(p)], "n").type, untyped Blob.type, typed Blob.type,Bun.file(p)response.blob(), headerimage/pngresponse.blob(), headertext/htmlsize_of::<Store>()184 to 152 bytes,size_of::<Blob>()104 on both (release layout, checked on linux-x64, aarch64-apple-darwin and x86_64-pc-windows-msvc). Release.text81,664,741 to 81,662,693 bytes. Wall clock ofnew Blob(["hello world"]), 10 interleaved pinned runs: median 133.5 ns on main and 134.3 ns here. The runs range from 120 to 168 ns.GC accounting. JSC drops a report of 256 bytes or less (
size > minExtraMemory). With the smaller store, 104 + 152 is exactly 256. In heap blocks the sum is 112 + 160. Peak RSS of a loop that makes 2M Blobs, 3 runs each:Bun.file(path)Bun.file(fd)blob.slice(0, 0)Suites. On the debug ASAN build:
blob.test.ts,worker_blob.test.ts,compile-asset-bunfs.test.ts(167 pass), andbody,body-clone,body-mixin-errors,FormData,FormData-multipart-serialization,client-fetch,stream-fast-path,streams,archive,structured-clone-blob-file,structured-clone,bun-serve-static,bun-serve-file,blob-write,bun-build-api(2564 pass). Four heavy tests inbun-build-api.test.tsandbody.test.tshit their 5 s or 180 s timeout on that debug build. Both files pass 851 of 851 on release builds of main and of this branch.