PR #36001 - oven-sh/bun

What

new Blob([blobA, blobB, ...]) now shares each Blob-typed part's backing store instead of memcpy'ing every part into one fresh contiguous buffer at construction time.

Why

The constructor walked every part through a StringJoiner and joined them into a single Vec<u8> (Blob.rs from_js_without_defer_gc → joiner.done() → out.extend_from_slice(node.slice())). Composing N × 100 MB blobs cost N × 100 MB of RSS even though each part's bytes are already immutable and refcounted behind a Store.

The existing storage model forced this: Blob = { store: Option<StoreRef>, offset, size } is one window into one Store, and Store::Data = Bytes | File | S3 has no multi-part variant, so composition had to materialise contiguous bytes.

How

Add Store::Data::Rope: a Vec<{ StoreRef, offset, len }> of windows into other in-memory stores.

The constructor pushes Blob-typed parts as rope segments (cloning the StoreRef, zero copy) and keeps snapshotting string / typed-array / ArrayBuffer / DataView parts into owned runs, so a caller mutating a source typed array after construction still cannot alias the blob.
Consecutive non-Blob parts are coalesced into one Bytes segment per run; a rope-backed source splices its segments into the outer rope so depth stays at one.
StoreRef::flatten_if_rope collapses a rope into a single Bytes in place on the first consumer that needs a contiguous view (shared_view / shared_view_raw / Bun.write / S3 upload). ByteBlobLoader reads through the flattened store so .stream() / .arrayBuffer() / .text() all see the same bytes.
Every Data match site gains a Rope arm that either reuses the Bytes behaviour or flattens first. The is_memory_backed() predicate is factored out so existsSync, write-target validation, structured clone, wasm streaming, and console inspection all treat Rope like Bytes.

Follow-up (not in this PR): teach ByteBlobLoader / Bun.write / the HTTP body writer to walk rope segments directly so .stream() and Bun.serve never flatten at all.

Measurements

RSS delta around construction, isolated child process:

Case	before	after
`new Blob(8 × [100 MB blob])`	+1000 MB	+2 MB
`new Blob([hdr, 100 MB blob, ftr])`	+300 MB	+2 MB
`new Blob([blob])` (single-part fast path)	+0.7 MB	unchanged

Node passes all five cases of the same verify harness; Bun went from 3/5 to 5/5.

Verification

test/js/web/fetch/blob-rope-store.test.ts (6 tests) covers the two RSS deltas plus byte-exact arrayBuffer/stream/slice/text round-trips, the typed-array-snapshot aliasing guarantee, nested-rope splicing, and structuredClone of a rope-backed File. The two RSS tests fail on current main (+576 MB / +128 MB) and pass with this change. Existing blob.test.ts, blob-array-fast-path.test.ts, blob-cow.test.ts, blob-oom.test.ts, the deno blob suite, FormData.test.ts, body.test.ts, structured-clone-blob-file.test.ts, worker_blob.test.ts, and archive.test.ts all pass unchanged.

[review] gate passed · iteration 5 · 10 files touched

fails on main (without fix)

ASAN without fix: 2 FAILED
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/js/web/fetch/blob-rope-store.test.ts
bun test v1.4.0 (5af3a3818)

test/js/web/fetch/blob-rope-store.test.ts:
48 |     const after = rssMB();
49 |     process.stdout.write(JSON.stringify({ delta: after - before, size: framed.size }));
50 |   `);
51 |   expect(result.size).toBe(64 * 1024 * 1024 + 32);
52 |   // Pre-rope: one 64 MB memcpy into the joined buffer.
53 |   expect(result.delta).toBeLessThan(isASAN || isDebug ? 32 : 16);
                            ^
error: expect(received).toBeLessThan(expected)

Expected: < 32
Received: 128

      at <anonymous> (/workspace/bun/test/js/web/fetch/blob-rope-store.test.ts:53:24)
(fail) new Blob([header, big, footer]) shares the Blob part and snapshots the scalar parts [1204.88ms]
32 |     process.stdout.write(JSON.stringify({ delta: after - before, size: composite.size }));
33 |   `);
34 |   expect(result.size).toBe(8 * 64 * 1024 * 1024);
35 |   // Pre-rope: copies 8 * 64 MB = 512 MB into a fresh buffer (+512 MB RSS).
36 |   // Rope: 8 StoreRef clones + one small Vec<RopeSegment>.
37 |   ex
... (truncated)

release without fix: all passed
bun test v1.4.0-canary.1 (5c1a54c1e)

test/js/web/fetch/blob-rope-store.test.ts:
(pass) new Blob([header, big, footer]) shares the Blob part and snapshots the scalar parts [61.54ms]
(pass) new Blob([big, big, ...]) shares the source Blob's store instead of copying [63.27ms]
(pass) rope-backed Blob is byte-exact across arrayBuffer/stream/slice/text [1.20ms]
(pass) typed-array parts are snapshotted at construction time [0.10ms]
(pass) a Blob part that is itself rope-backed splices into the outer rope [0.09ms]
(pass) structuredClone of a rope-backed File preserves bytes and name [0.25ms]
(pass) FormData.append filename sticks for a rope-backed Blob [0.13ms]
(pass) Bun.Archive accepts a rope-backed Blob [1.42ms]
(pass) Bun.write rejects a rope-backed Blob destination like any other in-memory Blob [0.13ms]
(pass) a multi-part File whose non-Blob parts are empty does not alias the source Blob's stored_name [0.13ms]
(pass) URL.createObjectURL flattens a rope before it can be reached from another JS thread [3.29ms]

 11 pass
 0 fail
 31 expect() calls
Ran 11 tests across 1 file. [222.00ms]
__F:0:S:0

passes on PR (with fix)

ASAN with fix: all passed
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/js/web/fetch/blob-rope-store.test.ts
bun test v1.4.0 (5af3a3818)

test/js/web/fetch/blob-rope-store.test.ts:
(pass) new Blob([big, big, ...]) shares the source Blob's store instead of copying [1217.83ms]
(pass) new Blob([header, big, footer]) shares the Blob part and snapshots the scalar parts [1181.72ms]
(pass) rope-backed Blob is byte-exact across arrayBuffer/stream/slice/text [41.21ms]
(pass) typed-array parts are snapshotted at construction time [5.70ms]
(pass) a Blob part that is itself rope-backed splices into the outer rope [6.71ms]
(pass) structuredClone of a rope-backed File preserves bytes and name [13.54ms]
(pass) FormData.append filename sticks for a rope-backed Blob [7.48ms]
(pass) Bun.Archive accepts a rope-backed Blob [188.67ms]
(pass) Bun.write rejects a rope-backed Blob destination like any other in-memory Blob [6.47ms]
(pass) a multi-part File whose non-Blob parts are empty does not alias the source Blob's stored_name [9.60ms]
(pass) URL.createObjectURL flattens a rope before it can be reached from another JS thr
... (truncated)

release with fix: all passed
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 751ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[1/6] gen generated_host_exports.rs
generated_host_exports.rs: 94 exports (host=3, lazy=10, generic=81, rust=0); 240 extern-C blocks audited
[1/6] cargo bun_bin → libbun_rust.a (--target x86_64-unknown-linux-gnu)

  nightly-2026-07-20-x86_64-unknown-linux-gnu unchanged - rustc 1.99.0-nightly (9f36de775 2026-07-19)

[1m[92m   Compiling[0m bun_core v0.0.0 (/workspace/bun/src/bun_core)
[1m[92m   Compiling[0m bun_errno v0.0.0 (/workspace/bun/src/errno)
[1m[92m   Compiling[0m bun_ptr v0.0.0 (/workspace/bun/src/ptr)
[1m[92m   Compiling[0m bun_boringssl_sys v0.0.0 (/workspace/bun/src/boringssl_sys)
[1m[92m   Compiling[0m bun_safety v0.0.0 (/workspace/bun/src/safety)
[1m[92m   Compiling[0m bun_zlib_sys v0.0.0 (/workspace/bun/src/zlib_sys)
[1m[92m   Compiling[0m bun_cares_sys v0.0.0 (/workspace/bun/src/cares_sys)
[1m[92m   Compiling[0m bun_zstd v0.0.0 (/workspace/bun/src/zstd)
[1m[92m   Compiling[0m bun_picohttp v0.0.0 (/workspace/bun/src/picohttp)
[1m[92m   Compiling[0m bun_output v
... (truncated)

diff hotspot

src/jsc/webcore_types.rs                  | 146 ++++++++++++++++++++++++-
 src/runtime/api/Archive.rs                |   2 +
 src/runtime/webcore/Blob.rs               | 126 +++++++++++++++-------
 src/runtime/webcore/ByteBlobLoader.rs     |   1 +
 src/runtime/webcore/FileReader.rs         |   4 +-
 src/runtime/webcore/ObjectURLRegistry.rs  |   3 +
 src/runtime/webcore/ReadableStream.rs     |   2 +-
 src/runtime/webcore/blob/Store.rs         |  11 +-
 src/runtime/webcore/wasm_streaming.rs     |   7 +-
 test/js/web/fetch/blob-rope-store.test.ts | 174 ++++++++++++++++++++++++++++++
 10 files changed, 431 insertions(+), 45 deletions(-)

gate history · 6 passed · 1 rejected · iteration 5

evidence per changed file

file                                       reads  edits  tests
src/jsc/webcore_types.rs                       9     26      0
src/runtime/api/Archive.rs                     2      2      0
src/runtime/webcore/Blob.rs                    8     28      0
src/runtime/webcore/ByteBlobLoader.rs          2      1      0
src/runtime/webcore/FileReader.rs              1      1      0
src/runtime/webcore/ObjectURLRegistry.rs       1      1      0
src/runtime/webcore/ReadableStream.rs          1      1      0
src/runtime/webcore/blob/Store.rs              3      4      0
src/runtime/webcore/wasm_streaming.rs          3      3      0
test/js/web/fetch/blob-rope-store.test.ts      3      9      0

#36001 webcore(Blob): share Blob-typed parts via a rope store instead of eager memcpy

What

Why

How

Measurements

Verification

webcore(Blob): share Blob-typed parts via a rope store instead of eager memcpy