Merge pull request #22 from FenrirWolf/stdio

Implement synchronized stdio

1 files changed, 722 insertions, 0 deletions

diff --git a/ctr-std/src/io/stdio.rs b/ctr-std/src/io/stdio.rs
new file mode 100644
index 0000000..e16e801
--- /dev/null
+++ b/ctr-std/src/io/stdio.rs
@@ -0,0 +1,722 @@
+// 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 io::prelude::*;
+
+use cell::RefCell;
+use fmt;
+use io::lazy::Lazy;
+use io::{self, BufReader, LineWriter};
+use sync::{Arc, Mutex, MutexGuard};
+use sys::stdio;
+use sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
+use thread::LocalKeyState;
+
+/// Stdout used by print! and println! macros
+thread_local! {
+    static LOCAL_STDOUT: RefCell<Option<Box<Write + Send>>> = {
+        RefCell::new(None)
+    }
+}
+
+/// A handle to a raw instance of the standard input stream of this process.
+///
+/// This handle is not synchronized or buffered in any fashion. Constructed via
+/// the `std::io::stdio::stdin_raw` function.
+struct StdinRaw(stdio::Stdin);
+
+/// A handle to a raw instance of the standard output stream of this process.
+///
+/// This handle is not synchronized or buffered in any fashion. Constructed via
+/// the `std::io::stdio::stdout_raw` function.
+struct StdoutRaw(stdio::Stdout);
+
+/// A handle to a raw instance of the standard output stream of this process.
+///
+/// This handle is not synchronized or buffered in any fashion. Constructed via
+/// the `std::io::stdio::stderr_raw` function.
+struct StderrRaw(stdio::Stderr);
+
+/// Constructs a new raw handle to the standard input of this process.
+///
+/// The returned handle does not interact with any other handles created nor
+/// handles returned by `std::io::stdin`. Data buffered by the `std::io::stdin`
+/// handles is **not** available to raw handles returned from this function.
+///
+/// The returned handle has no external synchronization or buffering.
+fn stdin_raw() -> io::Result<StdinRaw> { stdio::Stdin::new().map(StdinRaw) }
+
+/// Constructs a new raw handle to the standard output stream of this process.
+///
+/// The returned handle does not interact with any other handles created nor
+/// handles returned by `std::io::stdout`. Note that data is buffered by the
+/// `std::io::stdout` handles so writes which happen via this raw handle may
+/// appear before previous writes.
+///
+/// The returned handle has no external synchronization or buffering layered on
+/// top.
+fn stdout_raw() -> io::Result<StdoutRaw> { stdio::Stdout::new().map(StdoutRaw) }
+
+/// Constructs a new raw handle to the standard error stream of this process.
+///
+/// The returned handle does not interact with any other handles created nor
+/// handles returned by `std::io::stderr`.
+///
+/// The returned handle has no external synchronization or buffering layered on
+/// top.
+fn stderr_raw() -> io::Result<StderrRaw> { stdio::Stderr::new().map(StderrRaw) }
+
+impl Read for StdinRaw {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { self.0.read(buf) }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        self.0.read_to_end(buf)
+    }
+}
+impl Write for StdoutRaw {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> { self.0.write(buf) }
+    fn flush(&mut self) -> io::Result<()> { self.0.flush() }
+}
+impl Write for StderrRaw {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> { self.0.write(buf) }
+    fn flush(&mut self) -> io::Result<()> { self.0.flush() }
+}
+
+enum Maybe<T> {
+    Real(T),
+    Fake,
+}
+
+impl<W: io::Write> io::Write for Maybe<W> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        match *self {
+            Maybe::Real(ref mut w) => handle_ebadf(w.write(buf), buf.len()),
+            Maybe::Fake => Ok(buf.len())
+        }
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        match *self {
+            Maybe::Real(ref mut w) => handle_ebadf(w.flush(), ()),
+            Maybe::Fake => Ok(())
+        }
+    }
+}
+
+impl<R: io::Read> io::Read for Maybe<R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        match *self {
+            Maybe::Real(ref mut r) => handle_ebadf(r.read(buf), 0),
+            Maybe::Fake => Ok(0)
+        }
+    }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        match *self {
+            Maybe::Real(ref mut r) => handle_ebadf(r.read_to_end(buf), 0),
+            Maybe::Fake => Ok(0)
+        }
+    }
+}
+
+fn handle_ebadf<T>(r: io::Result<T>, default: T) -> io::Result<T> {
+    use sys::stdio::EBADF_ERR;
+
+    match r {
+        Err(ref e) if e.raw_os_error() == Some(EBADF_ERR) => Ok(default),
+        r => r
+    }
+}
+
+/// A handle to the standard input stream of a process.
+///
+/// Each handle is a shared reference to a global buffer of input data to this
+/// process. A handle can be `lock`'d to gain full access to [`BufRead`] methods
+/// (e.g. `.lines()`). Reads to this handle are otherwise locked with respect
+/// to other reads.
+///
+/// This handle implements the `Read` trait, but beware that concurrent reads
+/// of `Stdin` must be executed with care.
+///
+/// Created by the [`io::stdin`] method.
+///
+/// [`io::stdin`]: fn.stdin.html
+/// [`BufRead`]: trait.BufRead.html
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Stdin {
+    inner: Arc<Mutex<BufReader<Maybe<StdinRaw>>>>,
+}
+
+/// A locked reference to the `Stdin` handle.
+///
+/// This handle implements both the [`Read`] and [`BufRead`] traits, and
+/// is constructed via the [`Stdin::lock`] method.
+///
+/// [`Read`]: trait.Read.html
+/// [`BufRead`]: trait.BufRead.html
+/// [`Stdin::lock`]: struct.Stdin.html#method.lock
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct StdinLock<'a> {
+    inner: MutexGuard<'a, BufReader<Maybe<StdinRaw>>>,
+}
+
+/// Constructs a new handle to the standard input of the current process.
+///
+/// Each handle returned is a reference to a shared global buffer whose access
+/// is synchronized via a mutex. If you need more explicit control over
+/// locking, see the [`lock() method`][lock].
+///
+/// [lock]: struct.Stdin.html#method.lock
+///
+/// # Examples
+///
+/// Using implicit synchronization:
+///
+/// ```
+/// use std::io::{self, Read};
+///
+/// # fn foo() -> io::Result<String> {
+/// let mut buffer = String::new();
+/// io::stdin().read_to_string(&mut buffer)?;
+/// # Ok(buffer)
+/// # }
+/// ```
+///
+/// Using explicit synchronization:
+///
+/// ```
+/// use std::io::{self, Read};
+///
+/// # fn foo() -> io::Result<String> {
+/// let mut buffer = String::new();
+/// let stdin = io::stdin();
+/// let mut handle = stdin.lock();
+///
+/// handle.read_to_string(&mut buffer)?;
+/// # Ok(buffer)
+/// # }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn stdin() -> Stdin {
+    static INSTANCE: Lazy<Mutex<BufReader<Maybe<StdinRaw>>>> = Lazy::new(stdin_init);
+    return Stdin {
+        inner: INSTANCE.get().expect("cannot access stdin during shutdown"),
+    };
+
+    fn stdin_init() -> Arc<Mutex<BufReader<Maybe<StdinRaw>>>> {
+        let stdin = match stdin_raw() {
+            Ok(stdin) => Maybe::Real(stdin),
+            _ => Maybe::Fake
+        };
+
+        Arc::new(Mutex::new(BufReader::with_capacity(stdio::STDIN_BUF_SIZE, stdin)))
+    }
+}
+
+impl Stdin {
+    /// Locks this handle to the standard input stream, returning a readable
+    /// guard.
+    ///
+    /// The lock is released when the returned lock goes out of scope. The
+    /// returned guard also implements the [`Read`] and [`BufRead`] traits for
+    /// accessing the underlying data.
+    ///
+    /// [`Read`]: trait.Read.html
+    /// [`BufRead`]: trait.BufRead.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{self, Read};
+    ///
+    /// # fn foo() -> io::Result<String> {
+    /// let mut buffer = String::new();
+    /// let stdin = io::stdin();
+    /// let mut handle = stdin.lock();
+    ///
+    /// handle.read_to_string(&mut buffer)?;
+    /// # Ok(buffer)
+    /// # }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn lock(&self) -> StdinLock {
+        StdinLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
+    }
+
+    /// Locks this handle and reads a line of input into the specified buffer.
+    ///
+    /// For detailed semantics of this method, see the documentation on
+    /// [`BufRead::read_line`].
+    ///
+    /// [`BufRead::read_line`]: trait.BufRead.html#method.read_line
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    ///
+    /// let mut input = String::new();
+    /// match io::stdin().read_line(&mut input) {
+    ///     Ok(n) => {
+    ///         println!("{} bytes read", n);
+    ///         println!("{}", input);
+    ///     }
+    ///     Err(error) => println!("error: {}", error),
+    /// }
+    /// ```
+    ///
+    /// You can run the example one of two ways:
+    ///
+    /// - Pipe some text to it, e.g. `printf foo | path/to/executable`
+    /// - Give it text interactively by running the executable directly,
+    ///   in which case it will wait for the Enter key to be pressed before
+    ///   continuing
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn read_line(&self, buf: &mut String) -> io::Result<usize> {
+        self.lock().read_line(buf)
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for Stdin {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Stdin { .. }")
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Read for Stdin {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.lock().read(buf)
+    }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        self.lock().read_to_end(buf)
+    }
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        self.lock().read_to_string(buf)
+    }
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        self.lock().read_exact(buf)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a> Read for StdinLock<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.inner.read(buf)
+    }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        self.inner.read_to_end(buf)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a> BufRead for StdinLock<'a> {
+    fn fill_buf(&mut self) -> io::Result<&[u8]> { self.inner.fill_buf() }
+    fn consume(&mut self, n: usize) { self.inner.consume(n) }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<'a> fmt::Debug for StdinLock<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("StdinLock { .. }")
+    }
+}
+
+/// A handle to the global standard output stream of the current process.
+///
+/// Each handle shares a global buffer of data to be written to the standard
+/// output stream. Access is also synchronized via a lock and explicit control
+/// over locking is available via the [`lock()`] method.
+///
+/// Created by the [`io::stdout`] method.
+///
+/// [`lock()`]: #method.lock
+/// [`io::stdout`]: fn.stdout.html
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Stdout {
+    // FIXME: this should be LineWriter or BufWriter depending on the state of
+    //        stdout (tty or not). Note that if this is not line buffered it
+    //        should also flush-on-panic or some form of flush-on-abort.
+    inner: Arc<ReentrantMutex<RefCell<LineWriter<Maybe<StdoutRaw>>>>>,
+}
+
+/// A locked reference to the `Stdout` handle.
+///
+/// This handle implements the [`Write`] trait, and is constructed via
+/// the [`Stdout::lock`] method.
+///
+/// [`Write`]: trait.Write.html
+/// [`Stdout::lock`]: struct.Stdout.html#method.lock
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct StdoutLock<'a> {
+    inner: ReentrantMutexGuard<'a, RefCell<LineWriter<Maybe<StdoutRaw>>>>,
+}
+
+/// Constructs a new handle to the standard output of the current process.
+///
+/// Each handle returned is a reference to a shared global buffer whose access
+/// is synchronized via a mutex. If you need more explicit control over
+/// locking, see the [Stdout::lock] method.
+///
+/// [Stdout::lock]: struct.Stdout.html#method.lock
+///
+/// # Examples
+///
+/// Using implicit synchronization:
+///
+/// ```
+/// use std::io::{self, Write};
+///
+/// # fn foo() -> io::Result<()> {
+/// io::stdout().write(b"hello world")?;
+///
+/// # Ok(())
+/// # }
+/// ```
+///
+/// Using explicit synchronization:
+///
+/// ```
+/// use std::io::{self, Write};
+///
+/// # fn foo() -> io::Result<()> {
+/// let stdout = io::stdout();
+/// let mut handle = stdout.lock();
+///
+/// handle.write(b"hello world")?;
+///
+/// # Ok(())
+/// # }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn stdout() -> Stdout {
+    static INSTANCE: Lazy<ReentrantMutex<RefCell<LineWriter<Maybe<StdoutRaw>>>>>
+        = Lazy::new(stdout_init);
+    return Stdout {
+        inner: INSTANCE.get().expect("cannot access stdout during shutdown"),
+    };
+
+    fn stdout_init() -> Arc<ReentrantMutex<RefCell<LineWriter<Maybe<StdoutRaw>>>>> {
+        let stdout = match stdout_raw() {
+            Ok(stdout) => Maybe::Real(stdout),
+            _ => Maybe::Fake,
+        };
+        Arc::new(ReentrantMutex::new(RefCell::new(LineWriter::new(stdout))))
+    }
+}
+
+impl Stdout {
+    /// Locks this handle to the standard output stream, returning a writable
+    /// guard.
+    ///
+    /// The lock is released when the returned lock goes out of scope. The
+    /// returned guard also implements the `Write` trait for writing data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{self, Write};
+    ///
+    /// # fn foo() -> io::Result<()> {
+    /// let stdout = io::stdout();
+    /// let mut handle = stdout.lock();
+    ///
+    /// handle.write(b"hello world")?;
+    ///
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn lock(&self) -> StdoutLock {
+        StdoutLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for Stdout {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Stdout { .. }")
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Write for Stdout {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.lock().write(buf)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.lock().flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.lock().write_all(buf)
+    }
+    fn write_fmt(&mut self, args: fmt::Arguments) -> io::Result<()> {
+        self.lock().write_fmt(args)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a> Write for StdoutLock<'a> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.inner.borrow_mut().write(buf)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.inner.borrow_mut().flush()
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<'a> fmt::Debug for StdoutLock<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("StdoutLock { .. }")
+    }
+}
+
+/// A handle to the standard error stream of a process.
+///
+/// For more information, see the [`io::stderr`] method.
+///
+/// [`io::stderr`]: fn.stderr.html
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Stderr {
+    inner: Arc<ReentrantMutex<RefCell<Maybe<StderrRaw>>>>,
+}
+
+/// A locked reference to the `Stderr` handle.
+///
+/// This handle implements the `Write` trait and is constructed via
+/// the [`Stderr::lock`] method.
+///
+/// [`Stderr::lock`]: struct.Stderr.html#method.lock
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct StderrLock<'a> {
+    inner: ReentrantMutexGuard<'a, RefCell<Maybe<StderrRaw>>>,
+}
+
+/// Constructs a new handle to the standard error of the current process.
+///
+/// This handle is not buffered.
+///
+/// # Examples
+///
+/// Using implicit synchronization:
+///
+/// ```
+/// use std::io::{self, Write};
+///
+/// # fn foo() -> io::Result<()> {
+/// io::stderr().write(b"hello world")?;
+///
+/// # Ok(())
+/// # }
+/// ```
+///
+/// Using explicit synchronization:
+///
+/// ```
+/// use std::io::{self, Write};
+///
+/// # fn foo() -> io::Result<()> {
+/// let stderr = io::stderr();
+/// let mut handle = stderr.lock();
+///
+/// handle.write(b"hello world")?;
+///
+/// # Ok(())
+/// # }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn stderr() -> Stderr {
+    static INSTANCE: Lazy<ReentrantMutex<RefCell<Maybe<StderrRaw>>>> = Lazy::new(stderr_init);
+    return Stderr {
+        inner: INSTANCE.get().expect("cannot access stderr during shutdown"),
+    };
+
+    fn stderr_init() -> Arc<ReentrantMutex<RefCell<Maybe<StderrRaw>>>> {
+        let stderr = match stderr_raw() {
+            Ok(stderr) => Maybe::Real(stderr),
+            _ => Maybe::Fake,
+        };
+        Arc::new(ReentrantMutex::new(RefCell::new(stderr)))
+    }
+}
+
+impl Stderr {
+    /// Locks this handle to the standard error stream, returning a writable
+    /// guard.
+    ///
+    /// The lock is released when the returned lock goes out of scope. The
+    /// returned guard also implements the `Write` trait for writing data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{self, Write};
+    ///
+    /// fn foo() -> io::Result<()> {
+    ///     let stderr = io::stderr();
+    ///     let mut handle = stderr.lock();
+    ///
+    ///     handle.write(b"hello world")?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn lock(&self) -> StderrLock {
+        StderrLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for Stderr {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Stderr { .. }")
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Write for Stderr {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.lock().write(buf)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.lock().flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.lock().write_all(buf)
+    }
+    fn write_fmt(&mut self, args: fmt::Arguments) -> io::Result<()> {
+        self.lock().write_fmt(args)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a> Write for StderrLock<'a> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.inner.borrow_mut().write(buf)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.inner.borrow_mut().flush()
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<'a> fmt::Debug for StderrLock<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("StderrLock { .. }")
+    }
+}
+
+/// Resets the thread-local stderr handle to the specified writer
+///
+/// This will replace the current thread's stderr handle, returning the old
+/// handle. All future calls to `panic!` and friends will emit their output to
+/// this specified handle.
+///
+/// Note that this does not need to be called for all new threads; the default
+/// output handle is to the process's stderr stream.
+#[unstable(feature = "set_stdio",
+           reason = "this function may disappear completely or be replaced \
+                     with a more general mechanism",
+           issue = "0")]
+#[doc(hidden)]
+pub fn set_panic(sink: Option<Box<Write + Send>>) -> Option<Box<Write + Send>> {
+    use panicking::LOCAL_STDERR;
+    use mem;
+    LOCAL_STDERR.with(move |slot| {
+        mem::replace(&mut *slot.borrow_mut(), sink)
+    }).and_then(|mut s| {
+        let _ = s.flush();
+        Some(s)
+    })
+}
+
+/// Resets the thread-local stdout handle to the specified writer
+///
+/// This will replace the current thread's stdout handle, returning the old
+/// handle. All future calls to `print!` and friends will emit their output to
+/// this specified handle.
+///
+/// Note that this does not need to be called for all new threads; the default
+/// output handle is to the process's stdout stream.
+#[unstable(feature = "set_stdio",
+           reason = "this function may disappear completely or be replaced \
+                     with a more general mechanism",
+           issue = "0")]
+#[doc(hidden)]
+pub fn set_print(sink: Option<Box<Write + Send>>) -> Option<Box<Write + Send>> {
+    use mem;
+    LOCAL_STDOUT.with(move |slot| {
+        mem::replace(&mut *slot.borrow_mut(), sink)
+    }).and_then(|mut s| {
+        let _ = s.flush();
+        Some(s)
+    })
+}
+
+#[unstable(feature = "print",
+           reason = "implementation detail which may disappear or be replaced at any time",
+           issue = "0")]
+#[doc(hidden)]
+pub fn _print(args: fmt::Arguments) {
+    // As an implementation of the `println!` macro, we want to try our best to
+    // not panic wherever possible and get the output somewhere. There are
+    // currently two possible vectors for panics we take care of here:
+    //
+    // 1. If the TLS key for the local stdout has been destroyed, accessing it
+    //    would cause a panic. Note that we just lump in the uninitialized case
+    //    here for convenience, we're not trying to avoid a panic.
+    // 2. If the local stdout is currently in use (e.g. we're in the middle of
+    //    already printing) then accessing again would cause a panic.
+    //
+    // If, however, the actual I/O causes an error, we do indeed panic.
+    let result = match LOCAL_STDOUT.state() {
+        LocalKeyState::Uninitialized |
+        LocalKeyState::Destroyed => stdout().write_fmt(args),
+        LocalKeyState::Valid => {
+            LOCAL_STDOUT.with(|s| {
+                if let Ok(mut borrowed) = s.try_borrow_mut() {
+                    if let Some(w) = borrowed.as_mut() {
+                        return w.write_fmt(args);
+                    }
+                }
+                stdout().write_fmt(args)
+            })
+        }
+    };
+    if let Err(e) = result {
+        panic!("failed printing to stdout: {}", e);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use thread;
+    use super::*;
+
+    #[test]
+    #[cfg_attr(target_os = "emscripten", ignore)]
+    fn panic_doesnt_poison() {
+        thread::spawn(|| {
+            let _a = stdin();
+            let _a = _a.lock();
+            let _a = stdout();
+            let _a = _a.lock();
+            let _a = stderr();
+            let _a = _a.lock();
+            panic!();
+        }).join().unwrap_err();
+
+        let _a = stdin();
+        let _a = _a.lock();
+        let _a = stdout();
+        let _a = _a.lock();
+        let _a = stderr();
+        let _a = _a.lock();
+    }
+}