FIX: stop the findings export when a page offset repeats

frontend/src/components/Operations/FindingExportDialog.test.tsx

@@ -137,6 +137,17 @@ describe('FindingExportDialog', () => {
     expect(jest.mocked(operationsApi.exportFindings).mock.calls[0][1].finding_ids).toEqual([ALPHA.id, GAMMA.id])
   })
 
+  it('stops with an error when the server repeats the same page offset', async () => {
+    ;(operationsApi.listFindings as jest.Mock).mockImplementation(
+      async () => ({ items: [ALPHA], has_more: true, next_offset: 0 }),
+    )
+    const user = userEvent.setup()
+    renderDialog()
+    const dialog = await openDialog(user)
+    expect(await within(dialog).findByText(/did not advance/)).toBeInTheDocument()
+    expect(operationsApi.listFindings).toHaveBeenCalledTimes(1)
+  })
+
   it('lists and exports once a finding that a later page repeats', async () => {
     const user = userEvent.setup()
     // A finding created while pages load shifts the next page, so it can start with the previous page's last row.

frontend/src/components/Operations/FindingExportDialog.tsx

@@ -89,8 +89,16 @@ export default function FindingExportDialog({ operation }: FindingExportDialogPr
     let ignore = false
     const loadAllPages = async (): Promise<void> => {
       const loaded = new Map<string, FindingListItem>()
+      const seenOffsets = new Set<number>()
       let offset: number | null = 0
       while (offset !== null) {
+        // A repeated offset means the server is not advancing the page, and
+        // fetching it again would loop forever, so stop with an error the
+        // same way the shared pagination helper treats a cyclic cursor.
+        if (seenOffsets.has(offset)) {
+          throw new Error('The findings page offset did not advance.')
+        }
+        seenOffsets.add(offset)
         const page = await operationsApi.listFindings(operation.id, { limit: PAGE_SIZE, offset })
         if (ignore) return
         // A finding created while pages load shifts later pages, so a page can repeat an earlier row.