Fix race condition in WriteLinesToFile transactional mode (#13323)

src/Tasks.UnitTests/WriteLinesToFile_Tests.cs

@@ -453,7 +453,7 @@ public void TransactionalModeHandlesConcurrentWritesSuccessfully()
         }
 
         [Fact]
-        public void TransactionalModePreservesAllData()
+        public void TransactionalModeSucceedsWithConcurrentOverwrites()
         {
             using (var testEnv = TestEnvironment.Create(_output))
             {

src/Tasks/FileIO/WriteLinesToFile.cs

@@ -217,9 +217,55 @@ private bool SaveAtomically(AbsolutePath filePath, string contentsAsString, Enco
                     }
                     catch (IOException moveEx)
                     {
-                        // Move failed, log and return
-                        string lockedFileMessage = LockCheck.GetLockedFileMessage(filePath);
-                        Log.LogErrorWithCodeFromResources("WriteLinesToFile.ErrorOrWarning", filePath.OriginalValue, moveEx.Message, lockedFileMessage);
+                        // Only retry with Replace if the destination now exists (concurrent write race).
+                        if (System.IO.File.Exists(filePath))
+                        {
+                            // Retry Replace a few times with a small delay, mirroring the later Replace retry logic.
+                            bool TryReplaceWithRetry(string sourcePath, string destinationPath, out IOException lastReplaceException)
+                            {
+                                lastReplaceException = null;
+
+                                for (int retry = 0; retry < 3; retry++)
+                                {
+                                    try
+                                    {
+                                        if (retry > 0)
+                                        {
+                                            System.Threading.Thread.Sleep(10);
+                                        }
+
+                                        System.IO.File.Replace(sourcePath, destinationPath, null, true);
+                                        return true;
+                                    }
+                                    catch (IOException replaceEx)
+                                    {
+                                        lastReplaceException = replaceEx;
+                                        // Continue to next retry.
+                                    }
+                                }
+
+                                return false;
+                            }
+
+                            if (TryReplaceWithRetry(temporaryFilePath, filePath, out IOException lastReplaceException))
+                            {
+                                temporaryFilePath = null; // Mark as successfully replaced
+                                return !Log.HasLoggedErrors;
+                            }
+                            // All Replace retries failed; log the original move error as the root cause.
+                            string lockedFileMessage = LockCheck.GetLockedFileMessage(filePath);
+                            string replaceMessage = lastReplaceException != null ? lastReplaceException.Message : string.Empty;
+                            Log.LogErrorWithCodeFromResources(
+                                "WriteLinesToFile.ErrorOrWarning",
+                                filePath.OriginalValue,
+                                moveEx.Message + (string.IsNullOrEmpty(replaceMessage) ? string.Empty : " " + replaceMessage),
+                                lockedFileMessage);
+                            return !Log.HasLoggedErrors;
+                        }
+
+                        // Destination doesn't exist; move failed for a different reason.
+                        string lockedFileDiagnostics = LockCheck.GetLockedFileMessage(filePath);
+                        Log.LogErrorWithCodeFromResources("WriteLinesToFile.ErrorOrWarning", filePath.OriginalValue, moveEx.Message, lockedFileDiagnostics);
                         return !Log.HasLoggedErrors;
                     }
                 }