[Aschenbath]

Closes #37105

structuredClone(value, { transfer }) had side effects when it threw, and checked the transfer list too late:

• op_serialize detached the transferred ArrayBuffers right after write_value, before checking whether serialization succeeded. A getter that throws, or a value that can't be cloned, left the caller's buffers detached.
• serializeJsMessageData rejected entries that aren't transferable (Value not transferable) only after core.serialize, so getters on the value ran first, and a getter's exception replaced the DataCloneError. A SharedArrayBuffer in the list had the same problem.
• Duplicate entries weren't rejected. MessagePort.prototype.postMessage has its own duplicate check, but structuredClone doesn't go through it.

StructuredSerializeWithTransfer rejects duplicate and non-transferable entries first, then serializes, and transfers only after serialization succeeds.

Changes:

• ext/web/13_message_port.js: the loop in serializeJsMessageData that already runs before core.serialize now also throws DataCloneError for a duplicate entry and for an entry that is neither an ArrayBuffer nor a host object. The message for the second is the same Value not transferable as before, so the existing SharedArrayBuffer test still matches.
• libs/core/ops_builtin_v8.rs: op_serialize detaches the buffers only when write_value succeeded and nothing was caught. On failure it leaves them attached and takes their backing stores back out of shared_array_buffer_store.

With this change, the reproduction in the issue prints exactly the expected output:

getter throws: Error 4
function value: DataCloneError 4
invalid entry after buffer: DataCloneError 4
invalid entry: DataCloneError 0
invalid entry, getter throws: DataCloneError
duplicate entry: DataCloneError 0 4

Testing

• Added two tests to tests/unit/structured_clone_test.ts, covering the cases from the issue plus a successful transfer. Both fail on main and pass with this change (deno rebuilt for each run).
• unit::structured_clone_test, unit::message_channel_test, unit::worker_test and unit_node::worker_threads_test pass.
• tools/lint.js --js and dprint check pass on the changed files, and cargo clippy -p deno_core -- -D warnings is clean.
• Built and tested on Windows.

AI disclosure

This PR was written primarily by Claude Code (Claude Opus 5.5), including the change, the tests and this description. It has not yet been reviewed by a human.

🤖 Generated with Claude Code

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