Add a method for setting permissions directly on an open file.

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();

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 {

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,

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 {