From 1aaca5f29069a05e1aed73e33dd314c069fde756 Mon Sep 17 00:00:00 2001
From: Steven Allen <steven@stebalien.com>
Date: Sat, 19 Nov 2016 09:53:54 -0800
Subject: [PATCH] Add a method for setting permissions directly on an open
 file.

On unix like systems, the underlying file corresponding to any given path may
change at any time. This function makes it possible to set the permissions of
the a file corresponding to a `File` object even if its path changes.
---
 src/libstd/fs.rs             | 53 ++++++++++++++++++++++++++++++++++++
 src/libstd/sys/unix/fs.rs    |  5 ++++
 src/libstd/sys/windows/c.rs  |  9 ++++++
 src/libstd/sys/windows/fs.rs | 18 ++++++++++++
 4 files changed, 85 insertions(+)

diff --git a/src/libstd/fs.rs b/src/libstd/fs.rs
index df5741d00a2..e91e808c548 100644
--- a/src/libstd/fs.rs
+++ b/src/libstd/fs.rs
@@ -348,6 +348,41 @@ impl File {
             inner: self.inner.duplicate()?
         })
     }
+
+    /// Changes the permissions on the underlying file.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// This function currently corresponds to the `fchmod` function on Unix and
+    /// the `SetFileInformationByHandle` function on Windows. Note that, this
+    /// [may change in the future][changes].
+    ///
+    /// [changes]: ../io/index.html#platform-specific-behavior
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if the user lacks permission change
+    /// attributes on the underlying file. It may also return an error in other
+    /// os-specific unspecified cases.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(set_permissions_atomic)]
+    /// # fn foo() -> std::io::Result<()> {
+    /// use std::fs::File;
+    ///
+    /// let file = File::open("foo.txt")?;
+    /// let mut perms = file.metadata()?.permissions();
+    /// perms.set_readonly(true);
+    /// file.set_permissions(perms)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[unstable(feature = "set_permissions_atomic", issue="37916")]
+    pub fn set_permissions(&self, perm: Permissions) -> io::Result<()> {
+        self.inner.set_permissions(perm.0)
+    }
 }
 
 impl AsInner<fs_imp::File> for File {
@@ -2469,6 +2504,24 @@ mod tests {
         check!(fs::set_permissions(&file, p));
     }
 
+    #[test]
+    fn fchmod_works() {
+        let tmpdir = tmpdir();
+        let path = tmpdir.join("in.txt");
+
+        let file = check!(File::create(&path));
+        let attr = check!(fs::metadata(&path));
+        assert!(!attr.permissions().readonly());
+        let mut p = attr.permissions();
+        p.set_readonly(true);
+        check!(file.set_permissions(p.clone()));
+        let attr = check!(fs::metadata(&path));
+        assert!(attr.permissions().readonly());
+
+        p.set_readonly(false);
+        check!(file.set_permissions(p));
+    }
+
     #[test]
     fn sync_doesnt_kill_anything() {
         let tmpdir = tmpdir();
diff --git a/src/libstd/sys/unix/fs.rs b/src/libstd/sys/unix/fs.rs
index 0b43fd2ac8c..9ee0458b5da 100644
--- a/src/libstd/sys/unix/fs.rs
+++ b/src/libstd/sys/unix/fs.rs
@@ -526,6 +526,11 @@ impl File {
     pub fn fd(&self) -> &FileDesc { &self.0 }
 
     pub fn into_fd(self) -> FileDesc { self.0 }
+
+    pub fn set_permissions(&self, perm: FilePermissions) -> io::Result<()> {
+        cvt_r(|| unsafe { libc::fchmod(self.0.raw(), perm.mode) })?;
+        Ok(())
+    }
 }
 
 impl DirBuilder {
diff --git a/src/libstd/sys/windows/c.rs b/src/libstd/sys/windows/c.rs
index dfc998b88e3..1a563127f7f 100644
--- a/src/libstd/sys/windows/c.rs
+++ b/src/libstd/sys/windows/c.rs
@@ -389,6 +389,15 @@ pub enum FILE_INFO_BY_HANDLE_CLASS {
     MaximumFileInfoByHandlesClass
 }
 
+#[repr(C)]
+pub struct FILE_BASIC_INFO {
+    pub CreationTime: LARGE_INTEGER,
+    pub LastAccessTime: LARGE_INTEGER,
+    pub LastWriteTime: LARGE_INTEGER,
+    pub ChangeTime: LARGE_INTEGER,
+    pub FileAttributes: DWORD,
+}
+
 #[repr(C)]
 pub struct FILE_END_OF_FILE_INFO {
     pub EndOfFile: LARGE_INTEGER,
diff --git a/src/libstd/sys/windows/fs.rs b/src/libstd/sys/windows/fs.rs
index 98fd15f863b..7d7d78bbd87 100644
--- a/src/libstd/sys/windows/fs.rs
+++ b/src/libstd/sys/windows/fs.rs
@@ -417,6 +417,24 @@ impl File {
             Ok(PathBuf::from(OsString::from_wide(subst)))
         }
     }
+
+    pub fn set_permissions(&self, perm: FilePermissions) -> io::Result<()> {
+        let mut info = c::FILE_BASIC_INFO {
+            CreationTime: 0,
+            LastAccessTime: 0,
+            LastWriteTime: 0,
+            ChangeTime: 0,
+            FileAttributes: perm.attrs,
+        };
+        let size = mem::size_of_val(&info);
+        cvt(unsafe {
+            c::SetFileInformationByHandle(self.handle.raw(),
+                                          c::FileBasicInfo,
+                                          &mut info as *mut _ as *mut _,
+                                          size as c::DWORD)
+        })?;
+        Ok(())
+    }
 }
 
 impl FromInner<c::HANDLE> for File {