`exportStub()` always allocates a new export ID, even for duplicate capabilities

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調査の方向性

Trace exportStub() in src/rpc.ts and the devaluateHook, getHookForRpcTarget(), and getHookForWritableStream() paths in src/serialize.ts. Compare hook duplication with reverseExports and the intentional non-deduplication in exportPromise. Done should ensure repeated exportable capabilities can reuse export IDs without changing promise behavior or reintroducing the remoteRefcount issue in #140.

索引モデルが issue の本文から書いたものです。

説明

exportStub() deduplication by hook identity never fires.

https://github.com/cloudflare/capnweb/blob/c2bb17b940b23eb8ab89be1e85538493cb4552ad/src/rpc.ts#L369-L383

reverseExports is a Map<StubHook, ExportId>, so dedup is by object identity.

https://github.com/cloudflare/capnweb/blob/c2bb17b940b23eb8ab89be1e85538493cb4552ad/src/serialize.ts#L275-L281

devaluateHook routes to either exportStub or exportPromise depending on whether the type is "export"/"writable" or "promise". The promise path is intentionally never deduplicated (exportPromise comment: "Promises always use a new ID because otherwise the recipient could miss the resolution."), so the issue only concerns the paths that reach exportStub:

"stub"/"rpc-promise" https://github.com/cloudflare/capnweb/blob/c2bb17b940b23eb8ab89be1e85538493cb4552ad/src/serialize.ts#L218-L224 hook is set to the result of hook.dup() if pathIfPromise is falsy (the only time exportStub will be called).

"function"/"rpc-target" https://github.com/cloudflare/capnweb/blob/c2bb17b940b23eb8ab89be1e85538493cb4552ad/src/serialize.ts#L233-L234
getHookForRpcTarget() returns hook.dup() for "return" payloads (after correctly deduplicating in rpcTargets by target object identity), or a fresh TargetStubHook.create() each time for "params" payloads. Either way, a fresh object reaches exportStub.

"writable" https://github.com/cloudflare/capnweb/blob/c2bb17b940b23eb8ab89be1e85538493cb4552ad/src/serialize.ts#L251-L252
getHookForWritableStream() uses identical logic to getHookForRpcTarget() for our purposes, it deduplicates in rpcTargets for "return" payloads but returns hook.dup() each time, fresh WritableStreamStubHook each time for "params".

In all three cases, dup() always returns a new object for every hook type that can be exported (RpcImportHook, TargetStubHook, WritableStreamStubHook, ReadableStreamStubHook, PayloadStubHook). The two types where dup() returns this (ErrorStubHook and MapVariableHook) are never exported.

Because the stub fed to exportStub is always a newly created object, it will never be found in reverseExports, so we will always create a new export.

This means the same capability appearing twice always gets separate export IDs. This is what prevents the remoteRefcount bug (#140) from being triggered currently

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