From acffe20683042e8567eae3bf014d36c0edf9b710 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Thu, 16 May 2024 20:37:56 +0200
Subject: [PATCH] [3.13] GH-118943: Fix a race condition when generating
 jit_stencils.h (GH-119101)

(cherry picked from commit 4702b7b5bdc07d046576b4126cf4e4f5f7145abb)
---
 ...-05-11-15-11-30.gh-issue-118943.VI_MnY.rst |  3 +++
 Tools/jit/_targets.py                         | 19 ++++++++++++-------
 2 files changed, 15 insertions(+), 7 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Build/2024-05-11-15-11-30.gh-issue-118943.VI_MnY.rst

diff --git a/Misc/NEWS.d/next/Build/2024-05-11-15-11-30.gh-issue-118943.VI_MnY.rst b/Misc/NEWS.d/next/Build/2024-05-11-15-11-30.gh-issue-118943.VI_MnY.rst
new file mode 100644
index 00000000000000..4e886be034fb82
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2024-05-11-15-11-30.gh-issue-118943.VI_MnY.rst
@@ -0,0 +1,3 @@
+Fix a possible race condition affecting parallel builds configured with
+``--enable-experimental-jit``, in which compilation errors could be caused
+by an incompletely-generated header file.
diff --git a/Tools/jit/_targets.py b/Tools/jit/_targets.py
index b020f49cf4a2c1..5604c429bcf8ad 100644
--- a/Tools/jit/_targets.py
+++ b/Tools/jit/_targets.py
@@ -212,13 +212,18 @@ def build(
         ):
             return
         stencil_groups = asyncio.run(self._build_stencils())
-        with jit_stencils.open("w") as file:
-            file.write(digest)
-            if comment:
-                file.write(f"// {comment}\n\n")
-            file.write("")
-            for line in _writer.dump(stencil_groups):
-                file.write(f"{line}\n")
+        jit_stencils_new = out / "jit_stencils.h.new"
+        try:
+            with jit_stencils_new.open("w") as file:
+                file.write(digest)
+                if comment:
+                    file.write(f"// {comment}\n")
+                file.write("\n")
+                for line in _writer.dump(stencil_groups):
+                    file.write(f"{line}\n")
+            jit_stencils_new.replace(jit_stencils)
+        finally:
+            jit_stencils_new.unlink(missing_ok=True)
 
 
 class _COFF(