From 630f831cd06d24732c9a422b8d3d3e2ce9e1a879 Mon Sep 17 00:00:00 2001
From: Chris Denton <christophersdenton@gmail.com>
Date: Thu, 1 Sep 2022 04:17:36 +0100
Subject: [PATCH] Use `FILE_ATTRIBUTE_TAG_INFO` to get reparse tag

This avoid unnecessarily getting the full reparse data when all we need is the tag.
---
 library/std/src/sys/windows/c.rs  |  6 ++++++
 library/std/src/sys/windows/fs.rs | 26 ++++++++++++++++++--------
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/library/std/src/sys/windows/c.rs b/library/std/src/sys/windows/c.rs
index c99c8efe4367f..b4db77700aabd 100644
--- a/library/std/src/sys/windows/c.rs
+++ b/library/std/src/sys/windows/c.rs
@@ -454,6 +454,12 @@ pub enum FILE_INFO_BY_HANDLE_CLASS {
     MaximumFileInfoByHandlesClass,
 }
 
+#[repr(C)]
+pub struct FILE_ATTRIBUTE_TAG_INFO {
+    pub FileAttributes: DWORD,
+    pub ReparseTag: DWORD,
+}
+
 #[repr(C)]
 pub struct FILE_DISPOSITION_INFO {
     pub DeleteFile: BOOLEAN,
diff --git a/library/std/src/sys/windows/fs.rs b/library/std/src/sys/windows/fs.rs
index 98c8834d38403..c2ad592dfea73 100644
--- a/library/std/src/sys/windows/fs.rs
+++ b/library/std/src/sys/windows/fs.rs
@@ -326,10 +326,15 @@ impl File {
             cvt(c::GetFileInformationByHandle(self.handle.as_raw_handle(), &mut info))?;
             let mut reparse_tag = 0;
             if info.dwFileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
-                let mut b =
-                    Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
-                if let Ok((_, buf)) = self.reparse_point(&mut b) {
-                    reparse_tag = (*buf).ReparseTag;
+                let mut attr_tag: c::FILE_ATTRIBUTE_TAG_INFO = mem::zeroed();
+                cvt(c::GetFileInformationByHandleEx(
+                    self.handle.as_raw_handle(),
+                    c::FileAttributeTagInfo,
+                    ptr::addr_of_mut!(attr_tag).cast(),
+                    mem::size_of::<c::FILE_ATTRIBUTE_TAG_INFO>().try_into().unwrap(),
+                ))?;
+                if attr_tag.FileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+                    reparse_tag = attr_tag.ReparseTag;
                 }
             }
             Ok(FileAttr {
@@ -390,10 +395,15 @@ impl File {
             attr.file_size = info.AllocationSize as u64;
             attr.number_of_links = Some(info.NumberOfLinks);
             if attr.file_type().is_reparse_point() {
-                let mut b =
-                    Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
-                if let Ok((_, buf)) = self.reparse_point(&mut b) {
-                    attr.reparse_tag = (*buf).ReparseTag;
+                let mut attr_tag: c::FILE_ATTRIBUTE_TAG_INFO = mem::zeroed();
+                cvt(c::GetFileInformationByHandleEx(
+                    self.handle.as_raw_handle(),
+                    c::FileAttributeTagInfo,
+                    ptr::addr_of_mut!(attr_tag).cast(),
+                    mem::size_of::<c::FILE_ATTRIBUTE_TAG_INFO>().try_into().unwrap(),
+                ))?;
+                if attr_tag.FileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+                    reparse_tag = attr_tag.ReparseTag;
                 }
             }
             Ok(attr)