From 141b685ebd72d8168098248757d234e76648be8b Mon Sep 17 00:00:00 2001
From: Vladimir Shiryaev <w@tagolog.com>
Date: Sun, 4 Oct 2026 15:34:59 -0700
Subject: [PATCH] [Support] Check whether a file is local by its handle on
 Windows

---
 llvm/lib/Support/Windows/Path.inc | 75 ++++++++++++++++++++++++-------
 1 file changed, 59 insertions(+), 16 deletions(-)

diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
index fe316d11e1b8..ffe5df2e3a21 100644
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -29,6 +29,7 @@
 #include <shellapi.h>
 #include <shlobj.h>
 #include <winioctl.h>
+#include <winternl.h>
 
 #undef max
 
@@ -43,6 +44,32 @@ typedef int errno_t;
 #pragma comment(lib, "ole32.lib")    // This provides CoTaskMemFree
 #endif
 
+#ifndef __MINGW32__
+// The Windows SDK does not declare these types.
+// The WDK declares the types in `wdm.h` and `NtQueryVolumeInformationFile`
+// in `ntifs.h`.
+// MinGW-w64 declares all three in `winternl.h`.
+typedef enum _FSINFOCLASS { FileFsDeviceInformation = 4 } FS_INFORMATION_CLASS;
+
+typedef struct _FILE_FS_DEVICE_INFORMATION {
+  DEVICE_TYPE DeviceType;
+  ULONG Characteristics;
+} FILE_FS_DEVICE_INFORMATION;
+
+// Exported by `ntdll.dll`.
+extern "C" NTSYSAPI NTSTATUS NTAPI NtQueryVolumeInformationFile(
+    HANDLE FileHandle, PIO_STATUS_BLOCK IoStatusBlock, PVOID FsInformation,
+    ULONG Length, FS_INFORMATION_CLASS FsInformationClass);
+#endif
+
+// Defined only in `wdm.h` in WDK and MinGW-w64.
+#ifndef FILE_REMOVABLE_MEDIA
+#define FILE_REMOVABLE_MEDIA 0x00000001
+#endif
+#ifndef FILE_REMOTE_DEVICE
+#define FILE_REMOTE_DEVICE 0x00000010
+#endif
+
 using namespace llvm;
 
 using llvm::sys::windows::CurCPToUTF16;
@@ -549,18 +576,41 @@ static std::error_code realPathFromHandle(HANDLE H,
   return std::error_code();
 }
 
-std::error_code is_local(int FD, bool &Result) {
-  sandbox::violationIfEnabled();
+/// Returns true if the volume holding \p Handle is known to be a fixed local
+/// disk. Returns false if it is not or if that could not be determined.
+static bool isLocalByQueryVolumeInformationFile(HANDLE Handle) {
+  IO_STATUS_BLOCK IoStatus;
+  FILE_FS_DEVICE_INFORMATION Info;
+  if (::NtQueryVolumeInformationFile(Handle, &IoStatus, &Info, sizeof(Info),
+                                     FileFsDeviceInformation) < 0)
+    return false;
 
-  SmallVector<wchar_t, 128> FinalPath;
-  HANDLE Handle = reinterpret_cast<HANDLE>(_get_osfhandle(FD));
+  return (Info.DeviceType == FILE_DEVICE_DISK ||
+          Info.DeviceType == FILE_DEVICE_DISK_FILE_SYSTEM) &&
+         !(Info.Characteristics & (FILE_REMOVABLE_MEDIA | FILE_REMOTE_DEVICE));
+}
+
+static std::error_code is_local_internal(HANDLE Handle, bool &Result) {
+  // Fast path for the common case: an ordinary local disk, one system call on
+  // the handle. Everything else goes through the path-based check below.
+  if (isLocalByQueryVolumeInformationFile(Handle)) {
+    Result = true;
+    return std::error_code();
+  }
 
+  SmallVector<wchar_t, 128> FinalPath;
   if (std::error_code EC = realPathFromHandle(Handle, FinalPath))
     return EC;
-
   return is_local_internal(FinalPath, Result);
 }
 
+std::error_code is_local(int FD, bool &Result) {
+  sandbox::violationIfEnabled();
+
+  HANDLE Handle = reinterpret_cast<HANDLE>(_get_osfhandle(FD));
+  return is_local_internal(Handle, Result);
+}
+
 static std::error_code setDeleteDisposition(HANDLE Handle, bool Delete) {
   // Clear the FILE_DISPOSITION_INFO flag first, before checking if it's a
   // network file. On Windows 7 the function realPathFromHandle() below fails
@@ -577,12 +627,8 @@ static std::error_code setDeleteDisposition(HANDLE Handle, bool Delete) {
   // Check if the file is on a network (non-local) drive. If so, don't
   // continue when DeleteFile is true, since it prevents opening the file for
   // writes.
-  SmallVector<wchar_t, 128> FinalPath;
-  if (std::error_code EC = realPathFromHandle(Handle, FinalPath))
-    return EC;
-
   bool IsLocal;
-  if (std::error_code EC = is_local_internal(FinalPath, IsLocal))
+  if (std::error_code EC = is_local_internal(Handle, IsLocal))
     return EC;
 
   if (!IsLocal)
@@ -1171,12 +1217,9 @@ void mapped_file_region::unmapImpl() {
         // cases where we do the flushing. Only do the flushing if we aren't
         // sure we are on a local file system.
         bool IsLocal = false;
-        SmallVector<wchar_t, 128> FinalPath;
-        if (!realPathFromHandle(FileHandle, FinalPath)) {
-          // Not checking the return value here - if the check fails, assume the
-          // file isn't local.
-          is_local_internal(FinalPath, IsLocal);
-        }
+        // Not checking the return value here - if the check fails, assume the
+        // file isn't local.
+        is_local_internal(FileHandle, IsLocal);
         DoFlush = !IsLocal;
       }
       if (DoFlush)