From b437ccd2428267ccba4d5a5d5b4cf3fa94d94a47 Mon Sep 17 00:00:00 2001
From: Nazar Mokrynskyi <nazar@mokrynskyi.com>
Date: Sat, 5 Sep 2026 07:54:00 +0300
Subject: [PATCH] Avoid truncating event.json while other steps read it

---
 src/Runner.Worker/ExecutionContext.cs | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/src/Runner.Worker/ExecutionContext.cs b/src/Runner.Worker/ExecutionContext.cs
index 0f9410821c6..0547ad3dce8 100644
--- a/src/Runner.Worker/ExecutionContext.cs
+++ b/src/Runner.Worker/ExecutionContext.cs
@@ -1330,11 +1330,36 @@ public void WriteWebhookPayload()
             {
                 var workflowFile = Path.Combine(workflowDirectory, "event.json");
                 Trace.Info($"Write event payload to {workflowFile}");
-                File.WriteAllText(workflowFile, gitHubEvent, new UTF8Encoding(false));
+                WriteWebhookPayloadFile(workflowFile, gitHubEvent);
                 SetGitHubContext("event_path", workflowFile);
             }
         }
 
+        // Every step writes the very same webhook payload to the very same file before it runs, and
+        // steps can run concurrently (parallel/background steps). This implements an atomic write
+        // that avoids file corruption.
+        private static void WriteWebhookPayloadFile(string workflowFile, string gitHubEvent)
+        {
+            if (File.Exists(workflowFile))
+            {
+                return;
+            }
+
+            var tempFile = $"{workflowFile}.{Guid.NewGuid():N}.tmp";
+            File.WriteAllText(tempFile, gitHubEvent, new UTF8Encoding(false));
+            try
+            {
+                File.Move(tempFile, workflowFile);
+            }
+            catch (IOException) when (File.Exists(workflowFile))
+            {
+            }
+            finally
+            {
+                File.Delete(tempFile);
+            }
+        }
+
         private void InitializeTimelineRecord(Guid timelineId, Guid timelineRecordId, Guid? parentTimelineRecordId, string recordType, string displayName, string refName, int? order, bool embedded = false)
         {
             _mainTimelineId = timelineId;