From 8512e6d8a51fc4e1d1e03366ba3e9c53913806a9 Mon Sep 17 00:00:00 2001
From: Lawrence Chen <54008264+lawrencecchen@users.noreply.github.com>
Date: Wed, 18 Mar 2026 03:24:40 -0700
Subject: [PATCH] fix: skip identical session autosave writes (#1732)

* test: cover repeated identical session saves

* fix: skip identical session snapshot writes

---------

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>
---
 Sources/SessionPersistence.swift        | 13 +++++++++---
 cmuxTests/SessionPersistenceTests.swift | 27 +++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/Sources/SessionPersistence.swift b/Sources/SessionPersistence.swift
index b0303d53..188833d4 100644
--- a/Sources/SessionPersistence.swift
+++ b/Sources/SessionPersistence.swift
@@ -377,9 +377,10 @@ enum SessionPersistenceStore {
         let directory = fileURL.deletingLastPathComponent()
         do {
             try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true, attributes: nil)
-            let encoder = JSONEncoder()
-            encoder.outputFormatting = [.sortedKeys]
-            let data = try encoder.encode(snapshot)
+            let data = try encodedSnapshotData(snapshot)
+            if let existingData = try? Data(contentsOf: fileURL), existingData == data {
+                return true
+            }
             try data.write(to: fileURL, options: .atomic)
             return true
         } catch {
@@ -387,6 +388,12 @@ enum SessionPersistenceStore {
         }
     }
 
+    private static func encodedSnapshotData(_ snapshot: AppSessionSnapshot) throws -> Data {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.sortedKeys]
+        return try encoder.encode(snapshot)
+    }
+
     static func removeSnapshot(fileURL: URL? = nil) {
         guard let fileURL = fileURL ?? defaultSnapshotFileURL() else { return }
         try? FileManager.default.removeItem(at: fileURL)
diff --git a/cmuxTests/SessionPersistenceTests.swift b/cmuxTests/SessionPersistenceTests.swift
index 7d04db1d..8c00c0c1 100644
--- a/cmuxTests/SessionPersistenceTests.swift
+++ b/cmuxTests/SessionPersistenceTests.swift
@@ -86,6 +86,28 @@ final class SessionPersistenceTests: XCTestCase {
         )
     }
 
+    func testSaveSkipsRewritingIdenticalSnapshotData() throws {
+        let tempDir = FileManager.default.temporaryDirectory
+            .appendingPathComponent("cmux-session-tests-\(UUID().uuidString)", isDirectory: true)
+        try FileManager.default.createDirectory(at: tempDir, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: tempDir) }
+
+        let snapshotURL = tempDir.appendingPathComponent("session.json", isDirectory: false)
+        let snapshot = makeSnapshot(version: SessionSnapshotSchema.currentVersion)
+
+        XCTAssertTrue(SessionPersistenceStore.save(snapshot, fileURL: snapshotURL))
+        let firstFileNumber = try fileNumber(for: snapshotURL)
+
+        XCTAssertTrue(SessionPersistenceStore.save(snapshot, fileURL: snapshotURL))
+        let secondFileNumber = try fileNumber(for: snapshotURL)
+
+        XCTAssertEqual(
+            secondFileNumber,
+            firstFileNumber,
+            "Saving identical session data should not replace the snapshot file"
+        )
+    }
+
     func testWorkspaceCustomColorDecodeSupportsMissingLegacyField() throws {
         var snapshot = makeSnapshot(version: SessionSnapshotSchema.currentVersion)
         snapshot.windows[0].tabManager.workspaces[0].customColor = nil
@@ -780,6 +802,11 @@ final class SessionPersistenceTests: XCTestCase {
             windows: [window]
         )
     }
+
+    private func fileNumber(for fileURL: URL) throws -> Int {
+        let attributes = try FileManager.default.attributesOfItem(atPath: fileURL.path)
+        return try XCTUnwrap(attributes[.systemFileNumber] as? Int)
+    }
 }
 
 final class SocketListenerAcceptPolicyTests: XCTestCase {