revert: keep backfill cursor decoder throwing on malformed input

Reverts 525e363 (graceful legacy-cursor handling). The legacy bare-id cursor
only matters for a backfill in flight across the single deploy that changed the
cursor format, and that backfill is internal and admin-triggered, so the
one-time transition is handled operationally: do not run a backfill during the
rollout deploy. Keeping decodeBackfillCursor throwing surfaces genuine cursor
corruption loudly instead of silently restarting the window. The old keyset was
id-ordered and the new one createdAt-ordered, so a stale cursor cannot be safely
resumed anyway, only restarted.

Author: d-cs

apps/webapp/app/services/runsBackfiller.server.ts

@@ -47,15 +47,6 @@ export class RunsBackfillerService {
       // different ranges. RunStore merges the two tables only on a time-based
       // key, so order by createdAt and tiebreak on id within a timestamp.
       const keyset = cursor ? decodeBackfillCursor(cursor) : undefined;
-      if (cursor && !keyset) {
-        // Legacy/corrupt cursor: ignore it and restart the window (idempotent
-        // re-backfill). Self-recovers a backfill in flight across the cursor
-        // format change instead of throwing on every batch.
-        this.logger.warn(
-          "RunsBackfillerService: unparsable backfill cursor, restarting from window start",
-          { cursor }
-        );
-      }
 
       const runs = await runStore.findRuns(
         {
@@ -131,19 +122,15 @@ export function encodeBackfillCursor(createdAt: Date, id: string): string {
   return `${createdAt.toISOString()}${BACKFILL_CURSOR_SEPARATOR}${id}`;
 }
 
-export function decodeBackfillCursor(cursor: string): { createdAt: Date; id: string } | undefined {
+export function decodeBackfillCursor(cursor: string): { createdAt: Date; id: string } {
   const separatorIndex = cursor.indexOf(BACKFILL_CURSOR_SEPARATOR);
   const createdAt = separatorIndex === -1 ? new Date(NaN) : new Date(cursor.slice(0, separatorIndex));
   const id = separatorIndex === -1 ? "" : cursor.slice(separatorIndex + 1);
 
-  // A cursor with no separator is the pre-(createdAt, id) format (a bare run id,
-  // e.g. a backfill that was in flight across this change), or otherwise corrupt.
-  // The old id-only keyset can't be translated to the new (createdAt, id) order,
-  // so return undefined and let the caller restart the window. Re-backfilling is
-  // idempotent (ClickHouse ReplacingMergeTree keyed by run id), so the only cost
-  // is redoing the already-done portion once.
   if (Number.isNaN(createdAt.getTime()) || id.length === 0) {
-    return undefined;
+    throw new Error(
+      `RunsBackfillerService: malformed cursor "${cursor}" (expected "<createdAt>_<id>")`
+    );
   }
 
   return { createdAt, id };

apps/webapp/test/runsBackfiller.test.ts

@@ -9,38 +9,13 @@ vi.mock("~/db.server", () => ({
 import { ClickHouse } from "@internal/clickhouse";
 import { replicationContainerTest } from "@internal/testcontainers";
 import { z } from "zod";
-import {
-  RunsBackfillerService,
-  decodeBackfillCursor,
-  encodeBackfillCursor,
-} from "~/services/runsBackfiller.server";
+import { RunsBackfillerService } from "~/services/runsBackfiller.server";
 import { RunsReplicationService } from "~/services/runsReplicationService.server";
 import { createInMemoryTracing } from "./utils/tracing";
 import { TestReplicationClickhouseFactory } from "./utils/testReplicationClickhouseFactory";
 
 vi.setConfig({ testTimeout: 60_000 });
 
-describe("backfill cursor", () => {
-  it("round-trips createdAt + id", () => {
-    const createdAt = new Date("2026-06-23T00:00:00.000Z");
-    const decoded = decodeBackfillCursor(encodeBackfillCursor(createdAt, "cmqpwioyy0009unul63v3mxw2"));
-    expect(decoded?.createdAt.toISOString()).toBe(createdAt.toISOString());
-    expect(decoded?.id).toBe("cmqpwioyy0009unul63v3mxw2");
-  });
-
-  it("treats a legacy bare-id cursor (no separator) as undefined so the window restarts", () => {
-    // Pre-(createdAt, id) format: a bare run id. Decoding must not throw — it
-    // returns undefined so an in-flight backfill restarts the window instead of
-    // failing every batch after the cursor-format change.
-    expect(decodeBackfillCursor("cmqpwioyy0009unul63v3mxw2")).toBeUndefined();
-  });
-
-  it("returns undefined for a corrupt cursor instead of throwing", () => {
-    expect(decodeBackfillCursor("not-a-date_cmqpwioyy0009unul63v3mxw2")).toBeUndefined();
-    expect(decodeBackfillCursor("_cmqpwioyy0009unul63v3mxw2")).toBeUndefined();
-  });
-});
-
 describe("RunsBackfillerService", () => {
   replicationContainerTest(
     "should backfill completed runs to clickhouse",