From c04f8acfcb3026fcc9e2ce6f41645c2ec42686e0 Mon Sep 17 00:00:00 2001
From: Dmitry Vasilyev <dvassiliev@accesssoftek.com>
Date: Fri, 9 Oct 2026 16:58:59 +0400
Subject: [PATCH] [Windows][Support] Force FlushViewOfFile() for sparse files
 before unmap()

This patch fixes swiftlang/llvm-project / issues / 12605 (avoid ref for now).
We have reproduced the issue on x86_64 too running 2000 processes simultaneously.
It is hard to create a stable test for this patch.
---
 llvm/lib/CAS/MappedFileRegionArena.cpp | 8 ++++++++
 llvm/lib/Support/Windows/Path.inc      | 4 ++++
 2 files changed, 12 insertions(+)

diff --git a/llvm/lib/CAS/MappedFileRegionArena.cpp b/llvm/lib/CAS/MappedFileRegionArena.cpp
index bb098d4a2258..69a8b96b6f6d 100644
--- a/llvm/lib/CAS/MappedFileRegionArena.cpp
+++ b/llvm/lib/CAS/MappedFileRegionArena.cpp
@@ -332,6 +332,14 @@ void MappedFileRegionArena::destroyImpl() {
     }
   }
 
+#ifdef _WIN32
+  // Always Unmap to 
+  // - avoid address space leak;
+  // - avoid data loss for sparse files.
+  if (Region)
+    Region.unmap();
+#endif
+
   auto Close = [](std::optional<int> &FD) {
     if (FD) {
       sys::fs::file_t File = sys::fs::convertFDToNativeFile(*FD);
diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
index 7e78626f2852..e92273388c6e 100644
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -1142,6 +1142,10 @@ static bool isEXE(StringRef Magic) {
 
 void mapped_file_region::unmapImpl() {
   if (Mapping) {
+    BY_HANDLE_FILE_INFORMATION FI;
+    if (::GetFileInformationByHandle(FileHandle.get(), &FI) &&
+        (FI.dwFileAttributes & FILE_ATTRIBUTE_SPARSE_FILE) != 0)
+      ::FlushViewOfFile(Mapping, Size);
 
     bool Exe = isEXE(StringRef((char *)Mapping, Size));