std: Don't deadlock/panic on recursive prints

Previously a panic was generated for recursive prints due to a double-borrow of
a `RefCell`. This was solved by the second borrow's output being directed
towards the global stdout instead of the per-thread stdout (still experimental
functionality).

After this functionality was altered, however, recursive prints still deadlocked
due to the overridden `write_fmt` method which locked itself first and then
wrote all the data. This was fixed by removing the override of the `write_fmt`
method. This means that unlocked usage of `write!` on a `Stdout`/`Stderr` may be
slower due to acquiring more locks, but it's easy to make more performant with a
call to `.lock()`.

Closes #23781

3 files changed, 52 insertions, 11 deletions

diff --git a/src/libstd/io/stdio.rs b/src/libstd/io/stdio.rs
index 53f67766ea6..d361f17cbe4 100644
--- a/src/libstd/io/stdio.rs
+++ b/src/libstd/io/stdio.rs
@@ -11,7 +11,7 @@
 use prelude::v1::*;
 use io::prelude::*;
 
-use cell::RefCell;
+use cell::{RefCell, BorrowState};
 use cmp;
 use fmt;
 use io::lazy::Lazy;
@@ -264,9 +264,8 @@ impl Write for Stdout {
     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
         self.lock().write_all(buf)
     }
-    fn write_fmt(&mut self, fmt: fmt::Arguments) -> io::Result<()> {
-        self.lock().write_fmt(fmt)
-    }
+    // Don't override write_fmt as it's possible to run arbitrary code during a
+    // write_fmt, allowing the possibility of a recursive lock (aka deadlock)
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a> Write for StdoutLock<'a> {
@@ -334,9 +333,7 @@ impl Write for Stderr {
     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
         self.lock().write_all(buf)
     }
-    fn write_fmt(&mut self, fmt: fmt::Arguments) -> io::Result<()> {
-        self.lock().write_fmt(fmt)
-    }
+    // Don't override write_fmt for the same reasons as Stdout
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a> Write for StderrLock<'a> {
@@ -395,10 +392,15 @@ pub fn set_print(sink: Box<Write + Send>) -> Option<Box<Write + Send>> {
            reason = "implementation detail which may disappear or be replaced at any time")]
 #[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
-    if let Err(e) = LOCAL_STDOUT.with(|s| match s.borrow_mut().as_mut() {
-        Some(w) => w.write_fmt(args),
-        None => stdout().write_fmt(args)
-    }) {
+    let result = LOCAL_STDOUT.with(|s| {
+        if s.borrow_state() == BorrowState::Unused {
+            if let Some(w) = s.borrow_mut().as_mut() {
+                return w.write_fmt(args);
+            }
+        }
+        stdout().write_fmt(args)
+    });
+    if let Err(e) = result {
         panic!("failed printing to stdout: {}", e);
     }
 }
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 8de6e5257ec..5f5f2fed567 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -127,6 +127,7 @@
 #![feature(str_char)]
 #![feature(into_cow)]
 #![feature(slice_patterns)]
+#![feature(std_misc)]
 #![cfg_attr(test, feature(test, rustc_private, std_misc))]
 
 // Don't link to std. We are std.
diff --git a/src/test/run-pass/issue-23781.rs b/src/test/run-pass/issue-23781.rs
new file mode 100644
index 00000000000..23ac8d2b782
--- /dev/null
+++ b/src/test/run-pass/issue-23781.rs
@@ -0,0 +1,38 @@
+// Copyright 2015 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.
+
+use std::fmt;
+
+struct Foo;
+impl fmt::Debug for Foo {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        println!("<Foo as Debug>::fmt()");
+
+        write!(fmt, "")
+    }
+}
+
+fn test1() {
+    let foo_str = format!("{:?}", Foo);
+
+    println!("{}", foo_str);
+}
+
+fn test2() {
+    println!("{:?}", Foo);
+}
+
+fn main() {
+    // This works fine
+    test1();
+
+    // This fails
+    test2();
+}