`Blob.stream()` concatenates all chunks into a single chunk instead of separate Uint8Arrays

[kevgeoleo]

What version of Bun is running?

1.3.11-canary.1+9e93bfa1b

What platform is your computer?

Linux 5.10.0-12-amd64 x86_64 x86_64

What steps can reproduce the bug?

Hi,

I would like to report a behavior in Bun that can be reproduced using the below snippet

async function main () {
  const blob = new Blob([
    new Uint8Array([
       58, 162, 101, 114, 111, 111, 116, 115, 129, 216,  42,
       88,  37,   0,   1,  85,  18,  32,  95, 102, 197,  19,
      218,  30, 103,  78, 133, 139,  57, 105,  28,  11,   0,
      246, 124, 167,  41, 140, 239, 220, 248, 168, 136,  35,
      196,  72, 236, 184, 232,  89, 103, 118, 101, 114, 115,
      105, 111, 110,   1
    ]),
    new Uint8Array([ 68 ]),
    new Uint8Array([
        1,  85,  18,  32,  95, 102, 197,  19,
      218,  30, 103,  78, 133, 139,  57, 105,
       28,  11,   0, 246, 124, 167,  41, 140,
      239, 220, 248, 168, 136,  35, 196,  72,
      236, 184, 232,  89
    ]),
    new Uint8Array([
       90, 189, 174, 145, 144, 134, 214,
      117,  47, 251, 231, 254,  29, 238,
        0,  29, 212, 201, 123, 107,  95,
      130,  24, 168, 207, 139, 134, 177,
      187,  88, 167,  36
    ])
  ])

  await blob.stream().pipeTo(new WritableStream({
    write (chunk) {
      console.log(chunk)
    }
  }))
}

main().then(() => console.log('done')).catch(console.error)

What is the expected behavior?

Output in separate Blobs

root@KContainer:~/48916# node blob-stream-bug.mjs
Uint8Array(59) [
   58, 162, 101, 114, 111, 111, 116, 115, 129, 216,  42,
   88,  37,   0,   1,  85,  18,  32,  95, 102, 197,  19,
  218,  30, 103,  78, 133, 139,  57, 105,  28,  11,   0,
  246, 124, 167,  41, 140, 239, 220, 248, 168, 136,  35,
  196,  72, 236, 184, 232,  89, 103, 118, 101, 114, 115,
  105, 111, 110,   1
]
Uint8Array(1) [ 68 ]
Uint8Array(36) [
    1,  85,  18,  32,  95, 102, 197,  19,
  218,  30, 103,  78, 133, 139,  57, 105,
   28,  11,   0, 246, 124, 167,  41, 140,
  239, 220, 248, 168, 136,  35, 196,  72,
  236, 184, 232,  89
]
Uint8Array(32) [
   90, 189, 174, 145, 144, 134, 214,
  117,  47, 251, 231, 254,  29, 238,
    0,  29, 212, 201, 123, 107,  95,
  130,  24, 168, 207, 139, 134, 177,
  187,  88, 167,  36
]
done

What do you see instead?

root@KContainer:~/48916# bun run blob-stream-bug.mjs
Uint8Array(128) [ 58, 162, 101, 114, 111, 111, 116, 115, 129, 216, 42, 88, 37, 0, 1, 85, 18, 32, 95, 102, 197, 19, 218, 30, 103, 78, 133, 139, 57, 105, 28, 11, 0, 246, 124, 167, 41, 140, 239, 220, 248, 168, 136, 35, 196, 72, 236, 184, 232, 89, 103, 118, 101, 114, 115, 105, 111, 110, 1, 68, 1, 85, 18, 32, 95, 102, 197, 19, 218, 30, 103, 78, 133, 139, 57, 105, 28, 11, 0, 246, 124, 167, 41, 140, 239, 220, 248, 168, 136, 35, 196, 72, 236, 184, 232, 89, 90, 189, 174, 145, 144, 134, 214, 117, 47, 251, 231, 254, 29, 238, 0, 29, 212, 201, 123, 107, 95, 130, 24, 168, 207, 139, 134, 177, 187, 88, 167, 36 ]
done

Additional information

No response

[robobun]

Status at 34b9aee73a

Local verification (WebKit + zig manually staged in the farm container):

$ bun bd test test/regression/issue/28762.test.ts
 6 pass
 0 fail
 14 expect() calls

$ USE_SYSTEM_BUN=1 bun test test/regression/issue/28762.test.ts
 3 pass
 3 fail     ← bug reproduces without fix

The fix is correct and verified. All review threads resolved.

Why the farm gate still bounces

Two independent infrastructure problems in the farm container (not my code):

1. WebKit prebuilt download fails with unable to verify the first certificate. Bun's TLS can't verify the CA chain; curl works on the same URL. The gate's fetchPrebuilt wipes the cache on each attempted fetch, so pre-staging doesn't survive a gate run. The build therefore can't even get to compiling my Zig changes.

2. Phantom src/bun.js/api/cron.classes.ts keeps reappearing in the working tree between turns. It's from cf11b7d754 (Bun.cron), which is not in my merge-base (1e178d1ab6). When codegen runs it generates Classes.CronJob references in ZigGeneratedClasses.zig, but my branch's generated_classes_list.zig doesn't have CronJob — so the build fails with:

   codegen/ZigGeneratedClasses.zig:2239:28: error: struct 'bun.js.bindings.generated_classes_list.Classes'
       has no member named 'CronJob'
       const CronJob = Classes.CronJob;
   

   git rm -f clears it, but it comes back on the next container reset.

Normal CI (buildkite #43957) ran the ASAN test lane on the same commit and debian-13-x64-asan-test-bun passed; the only failures were unrelated runner expirations (darwin/win-aarch64 slots timed out waiting for agents).

@alii — flagging: the gate is broken in this environment, not the fix. Happy to rebase onto a newer main that includes cf11b7d754 if that would unblock the gate's codegen path.

[Jarred-Sumner]

This is intentional & deliberate. The API for Blob does not necessarily preserve its input chunks. It must always clone due to typed arrays being accessible in multiple places in the code.

If you want to suggest a chunk size, pass a numeric argument: blob.stream(1024)

[alii]

@robobun the gate check had a parsing bug that misread passing tests as "1 FAILED" — fixed. Push an empty commit (or just continue) to re-trigger the check.