fix(sync): let backend activation complete when a referenced attachment blob is gone from the candidate (#1119)

Author: dongdongbh

docs/release-notes/unreleased.md

@@ -179,3 +179,4 @@ Changes collected after `v1.2.1` and before the next version tag.
 - Mobile: the quick "Add task" sheet's More panel now starts with a Note field, so a longer thought can be captured without typing the /note: token; a typed field and a /note: token merge instead of one overwriting the other. (#1118)
 - Deleting a section now says up front that its tasks are kept and move to No Section, instead of a bare "are you sure". Desktop and mobile. (#1011)
 - Desktop Timeline: redesigned around a fixed name column — every task's title now sits in a sticky left column with its project, bars no longer carry floating labels, single-dated tasks draw as small dots on their day, and the card ends after its last row instead of stretching a gridded empty canvas. Feedback welcome on #1111. 
+- Switching to a sync location that references an attachment whose file is genuinely gone from that location no longer fails forever with "Candidate attachment proof incomplete": the missing attachment is marked unrecoverable, the same way an already-connected device handles it, and the switch completes. An attachment silently disappearing during the check still refuses the switch. (#1119)

packages/core/src/sync-run.test.ts

@@ -2294,3 +2294,71 @@ describe('normalizeRemoteWriteResult', () => {
         expect(normalizeRemoteWriteResult('cloud', null)).toEqual({ fingerprint: null, serverMergedRemoteData: false });
     });
 });
+
+describe('activation proof with unrecoverable attachments (#1119)', () => {
+    const fileAttachment = (id: string, title: string) => ({
+        id,
+        kind: 'file' as const,
+        title,
+        uri: '',
+        cloudKey: `attachments/${id}.txt`,
+        localStatus: 'missing' as const,
+        createdAt: STAMP,
+        updatedAt: STAMP,
+    });
+
+    it('accepts activation when the trial tombstones a blob that is gone from the candidate', async () => {
+        const remoteTask = createTask('t-two-attachments', 'Two attachments');
+        remoteTask.attachments = [fileAttachment('attachment-ok', 'Fine'), fileAttachment('attachment-404', 'Gone')];
+        const syncAttachments = vi.fn(async (data: AppData) => {
+            for (const attachment of data.tasks[0]?.attachments ?? []) {
+                if (attachment.id === 'attachment-ok') {
+                    attachment.localStatus = 'available';
+                } else {
+                    // What markAttachmentUnrecoverable does on a terminal 404.
+                    attachment.cloudKey = undefined;
+                    attachment.fileHash = undefined;
+                    attachment.localStatus = 'missing';
+                    attachment.deletedAt = '2026-07-02T00:00:00.000Z';
+                }
+            }
+            return data;
+        });
+        const { run } = createHarness({
+            local: createData(),
+            remote: createData([remoteTask]),
+            activationProbe: true,
+            io: { syncAttachments },
+        });
+
+        const result = await run();
+
+        expect(result.success).toBe(true);
+    });
+
+    it('still refuses activation when an expected attachment vanishes without a tombstone', async () => {
+        const remoteTask = createTask('t-two-attachments', 'Two attachments');
+        remoteTask.attachments = [fileAttachment('attachment-ok', 'Fine'), fileAttachment('attachment-dropped', 'Dropped')];
+        const syncAttachments = vi.fn(async (data: AppData) => {
+            const task = data.tasks[0]!;
+            for (const attachment of task.attachments ?? []) {
+                attachment.localStatus = 'available';
+            }
+            task.attachments = (task.attachments ?? []).filter((attachment) => attachment.id !== 'attachment-dropped');
+            return data;
+        });
+        const { run } = createHarness({
+            local: createData(),
+            remote: createData([remoteTask]),
+            activationProbe: true,
+            io: { syncAttachments },
+        });
+
+        const result = await run();
+
+        expect(result).toMatchObject({
+            success: false,
+            error: '[cloud] Candidate attachment proof incomplete: expected 2, proved 1',
+        });
+    });
+});

packages/core/src/sync-run.ts

@@ -175,15 +175,17 @@ const mergedContentMustReplaceCandidateBlob = (
 const prepareActivationAttachmentSnapshot = (
     data: AppData,
     candidateRemoteData: AppData | null,
-): { data: AppData; count: number } => {
+): { data: AppData; count: number; expectedIds: Set<string> } => {
     const candidateAttachments = new Map<string, Attachment>();
     if (candidateRemoteData) {
         visitLiveFileAttachments(candidateRemoteData, (attachment) => {
             candidateAttachments.set(attachment.id, attachment);
         });
     }
     const candidate = cloneAppData(data);
+    const expectedIds = new Set<string>();
     const count = visitLiveFileAttachments(candidate, (attachment) => {
+        expectedIds.add(attachment.id);
         const candidateAttachment = candidateAttachments.get(attachment.id);
         const mustReplaceCandidateBlob = Boolean(
             candidateAttachment?.cloudKey
@@ -207,23 +209,36 @@ const prepareActivationAttachmentSnapshot = (
         }
         attachment.localStatus = 'missing';
     });
-    return { data: candidate, count };
+    return { data: candidate, count, expectedIds };
 };
 
-const assertActivationAttachmentsProven = (data: AppData, expectedCount: number): void => {
-    let provenCount = 0;
-    visitLiveFileAttachments(data, (attachment) => {
-        if (
-            !attachment.cloudKey
-            || attachment.localStatus !== 'available'
-            || attachment.pendingContentUpload === true
-        ) {
-            throw new Error(`Candidate attachment proof failed for ${attachment.id}`);
+const assertActivationAttachmentsProven = (data: AppData, expectedIds: ReadonlySet<string>): void => {
+    const resolved = new Set<string>();
+    for (const owner of [...data.tasks, ...data.projects]) {
+        for (const attachment of owner.attachments ?? []) {
+            if (attachment.kind !== 'file') continue;
+            if (owner.deletedAt || attachment.deletedAt) {
+                // Tombstoned during the trial itself (a 404'd blob is marked
+                // unrecoverable, #1119): that is a PROVEN outcome - the same one an
+                // established device converges to - not a gap in the proof. Only ids
+                // the snapshot expected count; a pre-existing tombstone never does.
+                if (expectedIds.has(attachment.id)) resolved.add(attachment.id);
+                continue;
+            }
+            if (
+                !attachment.cloudKey
+                || attachment.localStatus !== 'available'
+                || attachment.pendingContentUpload === true
+            ) {
+                throw new Error(`Candidate attachment proof failed for ${attachment.id}`);
+            }
+            if (expectedIds.has(attachment.id)) resolved.add(attachment.id);
         }
-        provenCount += 1;
-    });
-    if (provenCount !== expectedCount) {
-        throw new Error(`Candidate attachment proof incomplete: expected ${expectedCount}, proved ${provenCount}`);
+    }
+    // An expected attachment that vanished without a tombstone is a silent drop
+    // and still refuses the activation.
+    if (resolved.size !== expectedIds.size) {
+        throw new Error(`Candidate attachment proof incomplete: expected ${expectedIds.size}, proved ${resolved.size}`);
     }
 };
 
@@ -874,7 +889,7 @@ class SharedSyncRunMachine {
             const provenData = result && typeof result === 'object'
                 ? result
                 : activationSnapshot.data;
-            assertActivationAttachmentsProven(provenData, activationSnapshot.count);
+            assertActivationAttachmentsProven(provenData, activationSnapshot.expectedIds);
             this.ensureLocalSnapshotFresh();
             this.notifier.onDiagnostic?.({
                 event: 'attachments-prepare-complete',