Rollup merge of #40516 - alexcrichton:no-cache-handles, r=aturon

std: Don't cache stdio handles on Windows

This alters the stdio code on Windows to always call `GetStdHandle` whenever the
stdio read/write functions are called as this allows us to track changes to the
value over time (such as if a process calls `SetStdHandle` while it's running).

Closes #40490

src/test/run-pass-fulldeps/switch-stdout.rs

@@ -0,0 +1,64 @@
+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(rustc_private)]
+
+extern crate rustc_back;
+
+use std::fs::File;
+use std::io::{Read, Write};
+
+use rustc_back::tempdir::TempDir;
+
+#[cfg(unix)]
+fn switch_stdout_to(file: File) {
+    use std::os::unix::prelude::*;
+
+    extern {
+        fn dup2(old: i32, new: i32) -> i32;
+    }
+
+    unsafe {
+        assert_eq!(dup2(file.as_raw_fd(), 1), 1);
+    }
+}
+
+#[cfg(windows)]
+fn switch_stdout_to(file: File) {
+    use std::os::windows::prelude::*;
+
+    extern "system" {
+        fn SetStdHandle(nStdHandle: u32, handle: *mut u8) -> i32;
+    }
+
+    const STD_OUTPUT_HANDLE: u32 = (-11i32) as u32;
+
+    unsafe {
+        let rc = SetStdHandle(STD_OUTPUT_HANDLE,
+                              file.into_raw_handle() as *mut _);
+        assert!(rc != 0);
+    }
+}
+
+fn main() {
+    let td = TempDir::new("foo").unwrap();
+    let path = td.path().join("bar");
+    let f = File::create(&path).unwrap();
+
+    println!("foo");
+    std::io::stdout().flush().unwrap();
+    switch_stdout_to(f);
+    println!("bar");
+    std::io::stdout().flush().unwrap();
+
+    let mut contents = String::new();
+    File::open(&path).unwrap().read_to_string(&mut contents).unwrap();
+    assert_eq!(contents, "bar\n");
+}