fix(lore-0046): answer rpc_unavailable when /settle's checks get no answer

apps/facilitator/README.md

@@ -407,7 +407,7 @@ unlocks one response.
 | `settle_exact_stellar_payer_busy`                    | —                       | another payment from this payer in this asset is settling; nothing was recorded for this one         | send the same payment again                                                                 |
 | `settle_exact_stellar_interrupted`                   | —                       | the facilitator restarted before anything was signed; nothing was or will be                         | repeat the same payment while `extra.retryable` (see below)                                 |
 | `settle_exact_stellar_shutting_down`                 | an earlier hash, if any | the facilitator was stopping; nothing was applied, and nothing more will be sent                     | repeat the same payment while `extra.retryable` (see below)                                 |
-| `settle_exact_stellar_rpc_unavailable`               | an earlier hash, if any | no RPC endpoint answered (refused, failed or timed out)                                              | repeat the same payment while `extra.retryable` (see below)                                 |
+| `settle_exact_stellar_rpc_unavailable`               | an earlier hash, if any | no RPC endpoint answered (refused, failed or timed out); in the checks, a timeout only               | repeat the same payment while `extra.retryable` (see below)                                 |
 | `settle_exact_stellar_no_channel_in_time`            | —                       | every channel was busy until the deadline                                                            | repeat the same payment while `extra.retryable` (see below)                                 |
 | `settle_exact_stellar_no_channels`                   | —                       | every channel is out of the pool                                                                     | repeat the same payment while `extra.retryable` (see below)                                 |
 | `settle_exact_stellar_lease_lost`                    | an earlier hash, if any | this process lost the channel lease before it sent; another process holds it, or none does           | repeat the same payment while `extra.retryable` (see below)                                 |

apps/facilitator/src/http/app.ts

@@ -237,8 +237,9 @@ export function createApp({
     });
     res.json(response);
   });
-  // Like `/verify`: any settle outcome is a 200 with an x402 body (AD-6). A thrown error, such as
-  // upstream verify failing or the store down before the claim, is a 500.
+  // Like `/verify`: any settle outcome is a 200 with an x402 body (AD-6), an RPC that gave the checks
+  // no answer included. A thrown error, such as upstream's `unexpected_verify_error` or the store
+  // down before the claim, is a 500.
   app.post(
     "/settle",
     leased,

apps/facilitator/src/settlement/settle.ts

@@ -49,6 +49,7 @@ import {
 import {
   PAYER_AUTH_REASON,
   UNSUPPORTED_CREDENTIAL_REASON,
+  UpstreamTimeoutError,
   payerExpiration,
   payerNonce,
   type Check,
@@ -962,6 +963,11 @@ export function createSettler(deps: SettlerDeps): Settle & {
       checked = await bounded(deps.check(payload, requirements), left());
     } catch (error) {
       if (error instanceof OutOfTimeError) return unclaimed();
+      // An RPC that gave no answer judged nothing, as `/verify`'s 503 says: nothing is claimed, so
+      // the same payment may be sent again as it is.
+      if (error instanceof UpstreamTimeoutError || error instanceof RpcUnavailableError) {
+        return { ...untouched, response: failure(SETTLE_REASONS.rpcUnavailable, undefined) };
+      }
       throw error;
     }
     const { response: verdict, verified } = checked;

apps/facilitator/test/settle.test.ts

@@ -58,6 +58,7 @@ import {
 } from "../src/settlement/settle.js";
 import {
   PAYER_AUTH_REASON,
+  UpstreamTimeoutError,
   createChecker,
   payerNonce,
   type Check,
@@ -1974,6 +1975,26 @@ describe("createSettler", () => {
       expect(await store.get(key)).toBeUndefined();
     });
 
+    it.each([
+      new UpstreamTimeoutError("upstream verify gave no answer within 10000 ms"),
+      new RpcUnavailableError("every RPC endpoint failed"),
+    ])(
+      "answers rpc_unavailable without claiming when the checks' RPC gave no answer: $name",
+      async (error) => {
+        const check: Check = async () => {
+          throw error;
+        };
+        const { settle, store, submit } = setup(landing(), { check });
+        expect((await settle(payload(), requirements)).response).toMatchObject({
+          success: false,
+          errorReason: SETTLE_REASONS.rpcUnavailable,
+        });
+        expect(submit).not.toHaveBeenCalled();
+        // Nothing is recorded, so the same payment can be sent again as it is.
+        expect(await store.get(key)).toBeUndefined();
+      },
+    );
+
     it("answers a timeout without claiming when the checks end just as the budget does", async () => {
       let time = 0;
       const check: Check = async (p, r) => {