badge-service: verify App Store and Google Play purchases (#7654)

docs/protocol/badges-rpc.md

@@ -34,12 +34,27 @@ The client never states what is signed. The tier is that of whatever funded the
 - `redeemBadgeCode` → `badgeCredential` — redeems a code, records the credit, and issues the first credential, in one round trip. It carries `masterKey` and `code` and no `badgeRequest`: a code states no tier and no expiry, so the credential is what reports them. Errors: `code_invalid` for an unknown or malformed code, `code_used` when other keys have taken all its uses (a code has one use unless issued with more), `code_expired` past a redemption deadline.
 - `purchaseBadge` → `badgeCredential` — verifies the funding (`apple` JWS offline; `google` product id and token via the Publisher API; `invoice` against webhook-confirmed settlement, `payment_pending` until it lands; `receipt`), records the credit, and issues the first credential, in one round trip. It carries `masterKey` and no `badgeRequest`. The response `receipt` is the recovery bearer secret (model § recovery); the service stores its hash; lifetime badges receive none.
   - A store transaction is claimed by its own id, read from the evidence before verification, and credited only when the verified transaction carries that same id. A receipt already credited to the signing key is answered from the record, without asking the store.
-  - Errors: `receipt_invalid`, `payment_pending`, `provider_unavailable` — see the classes below; `receipt_used` when another key was credited with it; `product_unavailable` for a product this service does not price; `provider_not_configured` when this deployment has no verifier for the store, terminal for the request since retrying cannot deploy one. `receipt_invalid` and `receipt_used` are the only answers on which the client drops the keys it signed with and finishes the store transaction. Every other answer records nothing and leaves the purchase with its keys, to be presented again at the next trigger.
-  - **Which class an answer belongs to.** The two mistakes do not cost the same, so when it is not certain, answer unreachable. `receipt_invalid` tells the client the purchase is dead: it drops the keys it signed with and finishes the store transaction — consumed on Play, finished on StoreKit — and nothing can present it again, so a buyer answered this by mistake has paid for a badge they can never receive. Answering unreachable by mistake costs one more verification request each time the app presents the purchase again, and a Play purchase left unacknowledged for three days is refunded to the buyer.
-    - **Terminal**, `receipt_invalid` — the store answered about this purchase and cannot change its mind: an Apple signature or certificate chain that does not verify, another app's bundle id, a test purchase (Apple Sandbox, Play `purchaseType` test), a refunded or revoked purchase, Play `purchaseState` canceled.
-    - **Pending**, `payment_pending` — Play `purchaseState` pending.
-    - **Unreachable**, `provider_unavailable` — the store was not asked or did not answer: network, timeout, 5xx, quota, this service's own auth or permission failures, and every Play error response, a 404 from `purchases.products.get` or `purchases.subscriptionsv2.get` and a 400 or 410 calling the token invalid included. A 404 decides nothing: Play answers it both for a token it has just issued and not yet recorded, and for one that was never a purchase. Only a purchase record is a verdict. A token whose characters this service will not send is never sent, so Play never answers about it — and Play documents no token grammar, so that check is our guess, not its verdict. A product id outside the characters Play does document is terminal, since no product of this app can have one.
-    - Apple is verified offline, so it has no "not yet" and a negative answer about the signed evidence is terminal. Two things are not the store's answer and leave the client holding the purchase: the verifier's own failure, such as a root certificate it could not load, which is `internal`; and a product this service does not price, which is `product_unavailable` — the verifier vouched for the transaction either way.
+  - Errors: `receipt_invalid`, `test_purchase`, `payment_pending`, `provider_unavailable` — see the classes below; `receipt_used` when another key was credited with it; `product_unavailable` for a product this service does not price; `provider_not_configured` when this deployment has no verifier for the store. `receipt_invalid`, `test_purchase` and `receipt_used` are the only answers on which the client drops the keys it signed with and finishes the store transaction. Every other answer records nothing and leaves the purchase with its keys, to be presented again at the next trigger.
+  - **Which class an answer belongs to.** The two mistakes do not cost the same, so when it is not certain, answer unreachable. `receipt_invalid` tells the client the purchase is dead: it drops the keys it signed with and finishes the store transaction — consumed on Play, finished on StoreKit — and nothing can present it again, so a buyer answered this by mistake has paid for a badge they can never receive. Answering unreachable by mistake costs one more verification request each time the app presents the purchase again, and a Play purchase left unacknowledged for three days is refunded to the buyer. It also locks the buyer out: the app offers no Buy while a purchase is open, so until that receipt settles the profile holding it cannot buy any badge, and an answer that never changes leaves it that way for good.
+    - **Terminal**, `receipt_invalid` — only a verdict no later attempt could change: the store answered about this purchase and cannot change its mind, or the evidence can never verify. Anything that might answer differently later — the store's lag, an outage, this service's own misconfiguration or bug, a shape of evidence this service does not know yet — is one of the other classes.
+    - **Terminal and named**, `test_purchase` — the store vouched for a purchase that cost nothing: Apple's `Sandbox`, or Play's `purchaseType` test. It is separated from `receipt_invalid` for one reason: the device that made the purchase already knows it was a test one, so naming it reveals nothing a forger could iterate on, and it lets the app say why instead of reporting a generic failure. Every other refusal stays lumped together for exactly that reason.
+    - **Pending**, `payment_pending` — a real purchase the store has not settled.
+    - **Unreachable**, `provider_unavailable` — the store was not asked or did not answer.
+    - **Internal**, `internal` — this service's own failure, never the store's answer.
+    - **App Store.** Apple is verified offline, so it has no "not yet".
+      - `receipt_invalid`: a JWS that is not three base64url parts or names no `transactionId`; an `x5c` certificate that is not base64; a JWS signature that is not 64 bytes; a leaf whose signature does not verify against the intermediate; a JWS signature that does not verify against the leaf; a transaction with a `revocationDate`, refunded or revoked.
+      - `test_purchase`: refused by the service after the verifier vouched for it, a `Sandbox` transaction.
+      - `internal`: whatever this service's own configuration decides — an intermediate whose signature does not verify against the configured root, and another app's `bundleId`, since the configured root or bundle id may be the wrong one as likely as the receipt forged; a transaction from Xcode's StoreKit testing, which Xcode signs and not the App Store, so the configured root does not verify it; evidence in a shape this service does not know — a header that does not name `ES256` or carries no `x5c` of exactly three certificates, a certificate that does not decode, a leaf signature the intermediate's key cannot check, an algorithm or key type the library does not support, a chain without Apple's receipt-signing marker extensions, a leaf key that is not P-256, a payload with a missing or mistyped field or an unknown `environment`; the verifier's own failure or overrun; and, refused by the service, a verified transaction other than the one claimed, or a quantity other than 1.
+      - `product_unavailable`: a product this service does not price — the verifier vouched for the transaction either way.
+      - `provider_not_configured`: the deployment has no `[apple]` section.
+    - **Google Play.** Only a purchase record is a verdict.
+      - `receipt_invalid`: `purchaseState` canceled, which a purchase token never leaves, and a product id outside the characters Play documents, since no product of this app can have one.
+      - `test_purchase`: refused by the service after the verifier vouched for it, `purchaseType` test, a license tester's purchase.
+      - `payment_pending`: `purchaseState` pending.
+      - `provider_unavailable`: network failure or timeout, and every error response from `purchases.products.get` but 401 and 403 — a 404 included, which Play may answer both for a token it has just issued and not yet recorded and for one that was never a purchase, and a 400 or 410 calling the token invalid; a 5xx or 429 from the token endpoint; and a token this service will not send — whose characters fall outside the grammar it accepts, which is our guess since Play documents none, or which is only dots, which a URL path would resolve away.
+      - `internal`: Google refusing this service's credentials or permissions — a 401 or 403 from `purchases.products.get`, and any other refusal from the token endpoint; a response that cannot be read, is too large, or names another product; a `purchaseState` this service does not know; and a `purchaseType` other than test or promo — a promo code, which only this app's developer issues, is credited like a purchase.
+      - `product_unavailable` and `provider_not_configured` as for the App Store, the latter without a `[google]` section.
+      - Before crediting a purchase of a product it prices, the service acknowledges it, unless Play already reports it acknowledged or consumed. It acknowledges and never consumes, so the app's purchase query still lists the purchase. The app may not be honest, and only acknowledgement stops Play refunding the purchase after three days, so nothing is credited unacknowledged. A failed acknowledgement credits nothing and is answered like a failed read — `provider_unavailable` when Play does not answer, `internal` when it refuses this service, both logged with "acknowledging" in the reason — and the app presents the purchase again. A deployment whose service account lacks the permission to acknowledge therefore answers `internal` to every Play purchase the app has not already acknowledged itself.
   - Funding by `receipt` is a transfer (post-MVP): the unissued months of the purchase that receipt belongs to move to the signing key, recorded as `debit(transferOut)` on the source and `credit(transferIn)` on the new purchase, and the presented receipt is retired for a fresh one. The transferred period's issuance debits a month like any other. Lifetime badges hold no receipt, so support handles them.
 - `upgradeBadgeSubscription` → `badgeCredential` — the app-led store subscription change, on the same key: verifies the store evidence of the replaced subscription and records the new plan; an immediate upgrade returns the new credential, a deferred change returns none. Its `badgeRequest` is to be dropped (see above).
 - `issueBadge` → `badgeCredential` — issues the next period from the balance, the only source of issuance. It carries `balance` alone: the credential is signed with the purchase's stored master key, for the type the balance funds, expiring at the `sundayAfter` of the period issued. The ledger is advanced first; the credential is signed before the `debit(badge)` and issuance rows are written, in one transaction. An exhausted balance yields no `credential`; the `statement` shows why. Issuing on a paused badge resumes it (model 2.13).
@@ -70,4 +85,4 @@ An assertion that names an entry the service holds is a prefix: the service proc
 
 ## Errors
 
-`retryAfter` marks the transient codes: `payment_pending`, `provider_unavailable`, `rate_limited`. `offer_disabled` calls for a catalog refresh. `code_invalid` covers unknown, malformed and revoked codes alike, so a guesser learns nothing from the difference; `code_used` — all its uses taken by other keys; `code_expired` — past its redemption deadline. `receipt_invalid` covers forged, malformed, another app's, refunded and test receipts alike, so a guesser learns nothing from the difference; `receipt_used` — credited to another key; `provider_not_configured` — no verifier for that store is deployed, so the receipt is neither credited nor refused. All other codes are terminal for the attempted command.
+`retryAfter` marks the transient codes: `payment_pending`, `provider_unavailable`, `rate_limited`. `offer_disabled` calls for a catalog refresh. `code_invalid` covers unknown, malformed and revoked codes alike, so a guesser learns nothing from the difference; `code_used` — all its uses taken by other keys; `code_expired` — past its redemption deadline. `receipt_invalid` covers forged, malformed and refunded receipts alike, so a guesser learns nothing from the difference; `test_purchase` is named apart from them because the device that bought it already knows; `receipt_used` — credited to another key; `provider_not_configured` — no verifier for that store is deployed, so the receipt is neither credited nor refused. All other codes are terminal for the attempted command.