🤖 fix: use atomic writes for session files to prevent corruption

Non-atomic fs.writeFile can leave truncated/malformed JSON if the app
crashes mid-write. This causes 'Unexpected end of JSON input' errors
when reading partial.json on the next stream start, leading to amnesia
(messages being lost).

Use write-file-atomic which writes to a temp file then renames,
ensuring readers always see either the old complete file or the new
complete file, never a partial write.

Fixed in:
- partialService.ts (partial.json)
- historyService.ts (chat.jsonl rewrites)
- sessionFile.ts (generic session file utility)

Fixes #803

Author: ammar-agent

src/node/services/historyService.ts

@@ -1,5 +1,6 @@
 import * as fs from "fs/promises";
 import * as path from "path";
+import writeFileAtomic from "write-file-atomic";
 import type { Result } from "@/common/types/result";
 import { Ok, Err } from "@/common/types/result";
 import type { MuxMessage } from "@/common/types/message";
@@ -235,7 +236,8 @@ export class HistoryService {
           .map((msg) => JSON.stringify({ ...msg, workspaceId }) + "\n")
           .join("");
 
-        await fs.writeFile(historyPath, historyEntries);
+        // Atomic write prevents corruption if app crashes mid-write
+        await writeFileAtomic(historyPath, historyEntries);
         return Ok(undefined);
       } catch (error) {
         const message = error instanceof Error ? error.message : String(error);
@@ -272,7 +274,8 @@ export class HistoryService {
           .map((msg) => JSON.stringify({ ...msg, workspaceId }) + "\n")
           .join("");
 
-        await fs.writeFile(historyPath, historyEntries);
+        // Atomic write prevents corruption if app crashes mid-write
+        await writeFileAtomic(historyPath, historyEntries);
 
         // Update sequence counter to continue from where we truncated
         if (truncatedMessages.length > 0) {
@@ -399,7 +402,8 @@ export class HistoryService {
           .map((msg) => JSON.stringify({ ...msg, workspaceId }) + "\n")
           .join("");
 
-        await fs.writeFile(historyPath, historyEntries);
+        // Atomic write prevents corruption if app crashes mid-write
+        await writeFileAtomic(historyPath, historyEntries);
 
         // Update sequence counter to continue from where we are
         if (remainingMessages.length > 0) {
@@ -455,7 +459,8 @@ export class HistoryService {
           .map((msg) => JSON.stringify({ ...msg, workspaceId: newWorkspaceId }) + "\n")
           .join("");
 
-        await fs.writeFile(newHistoryPath, historyEntries);
+        // Atomic write prevents corruption if app crashes mid-write
+        await writeFileAtomic(newHistoryPath, historyEntries);
 
         // Transfer sequence counter to new workspace ID
         const oldCounter = this.sequenceCounters.get(oldWorkspaceId) ?? 0;

src/node/services/partialService.ts

@@ -1,5 +1,6 @@
 import * as fs from "fs/promises";
 import * as path from "path";
+import writeFileAtomic from "write-file-atomic";
 import type { Result } from "@/common/types/result";
 import { Ok, Err } from "@/common/types/result";
 import type { MuxMessage } from "@/common/types/message";
@@ -80,7 +81,9 @@ export class PartialService {
           },
         };
 
-        await fs.writeFile(partialPath, JSON.stringify(partialMessage, null, 2));
+        // Atomic write: writes to temp file then renames, preventing corruption
+        // if app crashes mid-write (prevents "Unexpected end of JSON input" on read)
+        await writeFileAtomic(partialPath, JSON.stringify(partialMessage, null, 2));
         return Ok(undefined);
       } catch (error) {
         const message = error instanceof Error ? error.message : String(error);

src/node/utils/sessionFile.ts

@@ -1,5 +1,6 @@
 import * as fs from "fs/promises";
 import * as path from "path";
+import writeFileAtomic from "write-file-atomic";
 import type { Result } from "@/common/types/result";
 import { Ok, Err } from "@/common/types/result";
 import type { Config } from "@/node/config";
@@ -55,7 +56,8 @@ export class SessionFileManager<T> {
         const sessionDir = this.config.getSessionDir(workspaceId);
         await fs.mkdir(sessionDir, { recursive: true });
         const filePath = this.getFilePath(workspaceId);
-        await fs.writeFile(filePath, JSON.stringify(data, null, 2));
+        // Atomic write prevents corruption if app crashes mid-write
+        await writeFileAtomic(filePath, JSON.stringify(data, null, 2));
         return Ok(undefined);
       } catch (error) {
         const message = error instanceof Error ? error.message : String(error);