From 00e195c1ba2648094fab8a48035cb565c67879cf Mon Sep 17 00:00:00 2001
From: Diego Imbert <diego@windmill.dev>
Date: Mon, 1 Jun 2026 20:30:34 +0200
Subject: [PATCH] readLastSyncMap

---
 frontend/src/lib/userDraftDbSyncer.svelte.ts | 94 +++++++++++++++++++-
 1 file changed, 92 insertions(+), 2 deletions(-)

diff --git a/frontend/src/lib/userDraftDbSyncer.svelte.ts b/frontend/src/lib/userDraftDbSyncer.svelte.ts
index 33610fef21..7b391fb455 100644
--- a/frontend/src/lib/userDraftDbSyncer.svelte.ts
+++ b/frontend/src/lib/userDraftDbSyncer.svelte.ts
@@ -1,5 +1,70 @@
 import { DraftService, type UserDraftItemKind } from './gen'
 
+/**
+ * Per-draft last-sync map persisted under a single localStorage key. Lets
+ * the next `save_draft` (or a downstream UI) know the server's clock at
+ * our most recent successful sync for `(workspace, itemKind, path)` —
+ * without doing a network round-trip first.
+ *
+ * Shape: `Record<draftKey, { lastSync: ISO-8601 string }>`. The map is
+ * intentionally one storage slot for the whole tab; reading is one
+ * `localStorage.getItem` + `JSON.parse`, not one per entry.
+ */
+const DRAFT_LAST_SYNC_KEY = 'userdraft/draftLastSync'
+
+type DraftLastSyncEntry = { lastSync: string }
+type DraftLastSyncMap = Record<string, DraftLastSyncEntry>
+
+/** Must match `mapKey` in `userDraft.svelte.ts` so the two files agree
+ *  on what identifies a draft. */
+function draftKey(workspace: string, itemKind: UserDraftItemKind, path: string): string {
+	return `${workspace}/${itemKind}/${path}`
+}
+
+function readLastSyncMap(): DraftLastSyncMap {
+	try {
+		const raw = localStorage.getItem(DRAFT_LAST_SYNC_KEY)
+		if (!raw) return {}
+		const parsed = JSON.parse(raw)
+		// Defensive: a corrupt slot (wrong type, array, null) shouldn't
+		// crash the syncer — reset to an empty map.
+		if (parsed && typeof parsed === 'object' && !Array.isArray(parsed)) {
+			return parsed as DraftLastSyncMap
+		}
+		return {}
+	} catch (e) {
+		console.error('UserDraftDbSyncer: draftLastSync read failed', e)
+		return {}
+	}
+}
+
+function writeLastSyncMap(map: DraftLastSyncMap): void {
+	try {
+		localStorage.setItem(DRAFT_LAST_SYNC_KEY, JSON.stringify(map))
+	} catch (e) {
+		console.error('UserDraftDbSyncer: draftLastSync write failed', e)
+	}
+}
+
+function setLastSync(
+	workspace: string,
+	itemKind: UserDraftItemKind,
+	path: string,
+	lastSync: string
+): void {
+	// Read-modify-write is fine: each step is synchronous, so two saves
+	// for different keys can't interleave their updates within a tab.
+	const map = readLastSyncMap()
+	map[draftKey(workspace, itemKind, path)] = { lastSync }
+	writeLastSyncMap(map)
+}
+
+function clearLastSync(workspace: string, itemKind: UserDraftItemKind, path: string): void {
+	const map = readLastSyncMap()
+	delete map[draftKey(workspace, itemKind, path)]
+	writeLastSyncMap(map)
+}
+
 export type UserDraftDbSyncerSaveOpts = {
 	workspace: string
 	itemKind: UserDraftItemKind
@@ -9,6 +74,12 @@ export type UserDraftDbSyncerSaveOpts = {
 	value: unknown | null
 }
 
+export type UserDraftLastSyncQuery = {
+	workspace: string
+	itemKind: UserDraftItemKind
+	path: string
+}
+
 /**
  * Server-side persistence for `UserDraft`. `UserDraft` owns the localStorage
  * cache; this module forwards each write through to `POST /drafts/save_draft`
@@ -21,12 +92,23 @@ export type UserDraftDbSyncerSaveOpts = {
  *
  * NOTE: every save currently uses `force: true`, so the server copy is
  * unconditionally overwritten. This is intentional for the first cut —
- * we'll thread `last_sync` through once the client side is settled.
+ * we'll thread `last_sync` through (using `getLastSync` below) once the
+ * client side is settled.
  */
 export const UserDraftDbSyncer = {
+	/**
+	 * Read the recorded last-sync timestamp (ISO string) for a draft.
+	 * Returns `undefined` when we've never successfully synced this
+	 * `(workspace, itemKind, path)` — a fresh save should be treated as
+	 * a first-time push.
+	 */
+	getLastSync(opts: UserDraftLastSyncQuery): string | undefined {
+		return readLastSyncMap()[draftKey(opts.workspace, opts.itemKind, opts.path)]?.lastSync
+	},
+
 	async save(opts: UserDraftDbSyncerSaveOpts): Promise<void> {
 		try {
-			await DraftService.saveDraft({
+			const resp = await DraftService.saveDraft({
 				workspace: opts.workspace,
 				kind: opts.itemKind,
 				path: opts.path,
@@ -35,6 +117,14 @@ export const UserDraftDbSyncer = {
 					force: true
 				}
 			})
+			// On a successful delete, drop the recorded last-sync so the
+			// next save starts fresh. On an upsert, remember the server's
+			// timestamp as our baseline for future conflict checks.
+			if (opts.value === null) {
+				clearLastSync(opts.workspace, opts.itemKind, opts.path)
+			} else {
+				setLastSync(opts.workspace, opts.itemKind, opts.path, resp.current_timestamp)
+			}
 		} catch (e) {
 			console.error('UserDraftDbSyncer.save failed', e)
 		}