Keep polling a job after a failed poll

A failed GET /api/v1/jobs/{id} no longer ends polling. useJob exposes the
request's error, keeps the last known status and result, and schedules the
next poll on the same 500 ms to 2 s backoff. Polling stops only at a
terminal status, a changed job id or unmount.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/api/useJob.test.tsx

@@ -85,16 +85,54 @@ describe('useJob', () => {
     expect(fetchMock).toHaveBeenCalledTimes(2)
   })
 
-  it('exposes a failed poll as the error and stops', async () => {
-    const fetchMock = stubFetch(job('running', { dinners: ['Tacos'] }), errorResponse(404, 'not_found'))
+  it('exposes a failed poll as the error and keeps polling to the terminal status', async () => {
+    const fetchMock = stubFetch(
+      job('running', { dinners: ['Tacos'] }),
+      errorResponse(503, 'unavailable', true),
+      job('succeeded', { dinners: ['Tacos', 'Chili'] }),
+    )
     const { result } = renderJob('j1')
 
     await advance(0)
     await advance(500)
-    expect(result.current).toMatchObject({ status: 'running', result: { dinners: ['Tacos'] } })
-    expect(result.current.error).toMatchObject({ status: 404, code: 'not_found' })
-    expect(vi.getTimerCount()).toBe(0)
     expect(fetchMock).toHaveBeenCalledTimes(2)
+    expect(result.current).toMatchObject({ status: 'running', result: { dinners: ['Tacos'] } })
+    expect(result.current.error).toMatchObject({ status: 503, code: 'unavailable' })
+    expect(vi.getTimerCount()).toBe(1)
+
+    // The backoff carries on from the failed poll: the next one is 1 s later.
+    await advance(999)
+    expect(fetchMock).toHaveBeenCalledTimes(2)
+    await advance(1)
+    expect(fetchMock).toHaveBeenCalledTimes(3)
+    expect(result.current).toEqual({ status: 'succeeded', result: { dinners: ['Tacos', 'Chili'] }, error: null })
+    expect(vi.getTimerCount()).toBe(0)
+  })
+
+  it('stops polling on unmount after a failed poll', async () => {
+    const fetchMock = stubFetch(errorResponse(503, 'unavailable', true))
+    const { result, unmount } = renderJob('j1')
+
+    await advance(0)
+    expect(result.current).toMatchObject({ status: undefined, result: null })
+    expect(result.current.error).toMatchObject({ status: 503 })
+    expect(vi.getTimerCount()).toBe(1)
+    unmount()
+    expect(vi.getTimerCount()).toBe(0)
+    await advance(10_000)
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+  })
+
+  it('stops polling the old job when the job id changes', async () => {
+    const fetchMock = stubFetch(job('running', null), job('running', null))
+    const { result, rerender } = renderJob('j1')
+
+    await advance(0)
+    rerender({ id: 'j2' })
+    expect(result.current).toEqual({ status: undefined, result: null, error: null })
+    await advance(0)
+    expect(request(fetchMock, 1).path).toBe('/api/v1/jobs/j2')
+    expect(vi.getTimerCount()).toBe(1)
   })
 
   it('stops polling on unmount', async () => {

web/src/api/useJob.ts

@@ -18,7 +18,8 @@ export type JobState<R> = {
   status: JobStatus | undefined
   // The latest result, partial while the job runs.
   result: R | null
-  // The job's failure, or the polling request's own failure.
+  // The job's failure, or the latest polling request's own failure (cleared
+  // by the next poll that answers).
   error: Error | null
 }
 
@@ -33,8 +34,9 @@ function isTerminal(status: JobStatus): boolean {
 }
 
 // useJob polls GET /api/v1/jobs/{jobId} until the job succeeds or fails,
-// backing off from 500 ms to 2 s. Polling stops at a terminal status, when a
-// poll fails and on unmount. A null jobId polls nothing.
+// backing off from 500 ms to 2 s. A failed poll is exposed as the error and
+// polling continues. Polling stops only at a terminal status, when jobId
+// changes and on unmount. A null jobId polls nothing.
 export function useJob<R>(jobId: string | null): JobState<R> {
   const { apiFetch } = useApi()
   const [state, setState] = useState<{ jobId: string | null; job: JobState<R> }>({ jobId, job: idle })
@@ -45,6 +47,11 @@ export function useJob<R>(jobId: string | null): JobState<R> {
     let timer: ReturnType<typeof setTimeout> | undefined
     let delay = FIRST_DELAY_MS
 
+    function next() {
+      timer = setTimeout(poll, delay)
+      delay = Math.min(delay * 2, MAX_DELAY_MS)
+    }
+
     async function poll() {
       try {
         const job = await apiFetch<Job<R>>(`/api/v1/jobs/${encodeURIComponent(jobId!)}`)
@@ -52,15 +59,16 @@ export function useJob<R>(jobId: string | null): JobState<R> {
         const error = job.status === 'failed' ? new Error(job.error ?? `job ${job.id} failed without an error message`) : null
         setState({ jobId, job: { status: job.status, result: job.result, error } })
         if (isTerminal(job.status)) return
-        timer = setTimeout(poll, delay)
-        delay = Math.min(delay * 2, MAX_DELAY_MS)
       } catch (error) {
         if (!active) return
+        // A failed poll is not the job's end: keep the last known status and
+        // result, expose the request's error, and poll again.
         setState((prev) => ({
           jobId,
           job: { ...(prev.jobId === jobId ? prev.job : idle), error: error instanceof Error ? error : new Error(String(error)) },
         }))
       }
+      next()
     }
 
     void poll()