fix(usage): write usage cache snapshots and sidecars owner-only (0600) (#31027)

Usage caches and their source sidecars hold session ids and project
paths, but their durable temp files were created with the process
default mode, so a replace under umask 022 published them as 0644.
Thread an optional creation mode through writeFileDurable /
writeFileDurableIfCurrent and pass 0o600 from UsageCacheSnapshotWriter
and the usage source sidecar writer. Other durable writers are unchanged.

Fixes #31026

src/main/durable-file-write.ts

@@ -79,9 +79,10 @@ export async function writeTempFileDurable(
 export async function writeFileDurable(
   tmpPath: string,
   finalPath: string,
-  payload: string | Uint8Array
+  payload: string | Uint8Array,
+  mode?: number
 ): Promise<void> {
-  await writeFileDurableIfCurrent(tmpPath, finalPath, payload, () => true)
+  await writeFileDurableIfCurrent(tmpPath, finalPath, payload, () => true, mode)
 }
 
 /** Veto superseded writes before rename and remove their temp files on every uncommitted path. */
@@ -89,12 +90,14 @@ export async function writeFileDurableIfCurrent(
   tmpPath: string,
   finalPath: string,
   payload: string | Uint8Array,
-  isCurrent: () => boolean
+  isCurrent: () => boolean,
+  /** Creation mode for the new temp file, e.g. 0o600 for state other users must not read. */
+  mode?: number
 ): Promise<boolean> {
   let renamed = false
   try {
     // Fsync before publication so power loss cannot expose an empty file.
-    await writeTempFileDurable(tmpPath, payload)
+    await writeTempFileDurable(tmpPath, payload, mode)
     if (!(await renameFileWithWindowsRetryAsync(tmpPath, finalPath, isCurrent))) {
       return false
     }

src/main/usage-cache-snapshot-writer.test.ts

@@ -0,0 +1,25 @@
+import { chmodSync, mkdtempSync, readFileSync, rmSync, statSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { expect, it } from 'vitest'
+import { UsageCacheSnapshotWriter } from './usage-cache-snapshot-writer'
+
+it.skipIf(process.platform === 'win32')(
+  'replaces a world-readable usage cache with an owner-only snapshot',
+  async () => {
+    const directory = mkdtempSync(join(tmpdir(), 'orca-usage-cache-mode-'))
+    try {
+      const file = join(directory, 'orca-claude-usage.json')
+      writeFileSync(file, '{}')
+      chmodSync(file, 0o644)
+      const writer = new UsageCacheSnapshotWriter('[test]', () => file)
+
+      await writer.write(() => '{"sessionId":"s"}')
+
+      expect(readFileSync(file, 'utf8')).toBe('{"sessionId":"s"}')
+      expect(statSync(file).mode & 0o777).toBe(0o600)
+    } finally {
+      rmSync(directory, { recursive: true, force: true })
+    }
+  }
+)

src/main/usage-cache-snapshot-writer.ts

@@ -47,7 +47,9 @@ export class UsageCacheSnapshotWriter {
       durableWriteTempPath(usageFile),
       usageFile,
       payload,
-      () => generation === this.generation
+      () => generation === this.generation,
+      // Usage caches hold session ids and project paths.
+      0o600
     )
   }
 }

src/main/usage/usage-source-cache-file.test.ts

@@ -1,4 +1,4 @@
-import { existsSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
+import { existsSync, mkdtempSync, readFileSync, rmSync, statSync, writeFileSync } from 'node:fs'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { afterEach, beforeEach, describe, expect, it } from 'vitest'
@@ -40,6 +40,12 @@ describe('usage source cache file', () => {
     await expect(readUsageSourceCache(ref)).resolves.toEqual([{ path: 'a.jsonl' }])
   })
 
+  it.skipIf(process.platform === 'win32')('writes the sidecar owner-only', async () => {
+    await writeUsageSourceCache(ref, [{ path: 'a.jsonl' }])
+
+    expect(statSync(ref.path).mode & 0o777).toBe(0o600)
+  })
+
   it('rejects attribution from a sidecar committed ahead of its report', async () => {
     await writeUsageSourceCache({ ...ref, worktreeFingerprint: 'new-worktrees' }, [
       { path: 'a.jsonl', repoId: 'new-repo' }

src/main/usage/usage-source-cache-file.ts

@@ -78,7 +78,8 @@ export async function writeUsageSourceCacheData(
   await writeFileDurable(
     durableWriteTempPath(ref.path, threadId > 0 ? String(threadId) : undefined),
     ref.path,
-    data
+    data,
+    0o600
   )
 }