Flush stdout on read from stdin again

This is possible now since we have reentrant mutexes. Fixes #25555.

Author: nagisa

src/libstd/io/stdio.rs

@@ -165,6 +165,9 @@ impl Stdin {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Read for Stdin {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        // Flush stdout so that weird issues like a print!'d prompt not being
+        // shown until after the user hits enter.
+        drop(stdout().flush());
         self.lock().read(buf)
     }
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {

src/test/run-pass/print-read-flushes.rs

@@ -0,0 +1,34 @@
+use std::env;
+use std::io;
+use std::io::{Read, Write};
+use std::process::{Command, Stdio};
+
+fn main(){
+    if env::args().count() > 1 && env::args().nth(1) == Some("child".to_string()) {
+        child()
+    } else {
+        let mut p = Command::new(env::args().nth(0).unwrap())
+                .arg("child")
+                .stdin(Stdio::piped())
+                .stdout(Stdio::piped())
+                .stderr(Stdio::inherit())
+                .spawn().unwrap();
+        {
+            let mut buf = [0; 1];
+            assert!(p.stdout.as_mut().unwrap().read(&mut buf).unwrap() >= 1);
+            assert_eq!(buf[0], b'>');
+            assert!(p.stdin.as_mut().unwrap().write(b"abcd\n").unwrap() >= 1);
+        }
+        // FIXME: timeout and fail on timeout
+        assert!(p.wait().unwrap().success());
+    }
+}
+
+fn child(){
+    let stdout = io::stdout();
+    let lstdout = stdout.lock();
+    let mut stdin = io::stdin();
+    print!(">");
+    let mut letter = [0; 1];
+    stdin.read(&mut letter).unwrap();
+}