Auto merge of #24029 - nagisa:print-locking, r=<try>

write_fmt calls write for each formatted field. The default implementation of write_fmt is used,
which will call write on not-yet-locked stdout (and write locking after), therefore making print!
in multithreaded environment still interleave contents of two separate prints.

I’m not sure whether we want to do this change, though, because it has the same deadlock hazard which we tried to avoid by not locking inside write_fmt itself (see [this comment](https://github.com/rust-lang/rust/blob/80def6c2447d23a624e611417f24cf0ab2a5a676/src/libstd/io/stdio.rs#L267)).

Spotted on [reddit].

cc @alexcrichton 

[reddit]: http://www.reddit.com/r/rust/comments/31comh/println_with_multiple_threads/

Author: bors

src/libstd/io/stdio.rs

@@ -16,7 +16,7 @@ use cmp;
 use fmt;
 use io::lazy::Lazy;
 use io::{self, BufReader, LineWriter};
-use sync::{Arc, Mutex, MutexGuard};
+use sync::{Arc, ReentrantMutex, ReentrantMutexGuard};
 use sys::stdio;
 
 /// Stdout used by print! and println! macros
@@ -96,7 +96,7 @@ impl Write for StderrRaw {
 /// of `Stdin` must be executed with care.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stdin {
-    inner: Arc<Mutex<BufReader<StdinRaw>>>,
+    inner: Arc<ReentrantMutex<BufReader<StdinRaw>>>,
 }
 
 /// A locked reference to the a `Stdin` handle.
@@ -105,7 +105,7 @@ pub struct Stdin {
 /// constructed via the `lock` method on `Stdin`.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct StdinLock<'a> {
-    inner: MutexGuard<'a, BufReader<StdinRaw>>,
+    inner: ReentrantMutexGuard<'a, BufReader<StdinRaw>>,
 }
 
 /// Create a new handle to the global standard input stream of this process.
@@ -119,17 +119,17 @@ pub struct StdinLock<'a> {
 /// locked version, `StdinLock`, implements both `Read` and `BufRead`, however.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stdin() -> Stdin {
-    static INSTANCE: Lazy<Mutex<BufReader<StdinRaw>>> = lazy_init!(stdin_init);
+    static INSTANCE: Lazy<ReentrantMutex<BufReader<StdinRaw>>> = lazy_init!(stdin_init);
     return Stdin {
         inner: INSTANCE.get().expect("cannot access stdin during shutdown"),
     };
 
-    fn stdin_init() -> Arc<Mutex<BufReader<StdinRaw>>> {
+    fn stdin_init() -> Arc<ReentrantMutex<BufReader<StdinRaw>>> {
         // The default buffer capacity is 64k, but apparently windows
         // doesn't like 64k reads on stdin. See #13304 for details, but the
         // idea is that on windows we use a slightly smaller buffer that's
         // been seen to be acceptable.
-        Arc::new(Mutex::new(if cfg!(windows) {
+        Arc::new(ReentrantMutex::new(if cfg!(windows) {
             BufReader::with_capacity(8 * 1024, stdin_raw())
         } else {
             BufReader::new(stdin_raw())
@@ -210,7 +210,7 @@ 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<Mutex<LineWriter<StdoutRaw>>>,
+    inner: Arc<ReentrantMutex<LineWriter<StdoutRaw>>>,
 }
 
 /// A locked reference to the a `Stdout` handle.
@@ -219,7 +219,7 @@ pub struct Stdout {
 /// method on `Stdout`.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct StdoutLock<'a> {
-    inner: MutexGuard<'a, LineWriter<StdoutRaw>>,
+    inner: ReentrantMutexGuard<'a, LineWriter<StdoutRaw>>,
 }
 
 /// Constructs a new reference to the standard output of the current process.
@@ -231,13 +231,13 @@ pub struct StdoutLock<'a> {
 /// The returned handle implements the `Write` trait.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stdout() -> Stdout {
-    static INSTANCE: Lazy<Mutex<LineWriter<StdoutRaw>>> = lazy_init!(stdout_init);
+    static INSTANCE: Lazy<ReentrantMutex<LineWriter<StdoutRaw>>> = lazy_init!(stdout_init);
     return Stdout {
         inner: INSTANCE.get().expect("cannot access stdout during shutdown"),
     };
 
-    fn stdout_init() -> Arc<Mutex<LineWriter<StdoutRaw>>> {
-        Arc::new(Mutex::new(LineWriter::new(stdout_raw())))
+    fn stdout_init() -> Arc<ReentrantMutex<LineWriter<StdoutRaw>>> {
+        Arc::new(ReentrantMutex::new(LineWriter::new(stdout_raw())))
     }
 }
 
@@ -264,8 +264,9 @@ impl Write for Stdout {
     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
         self.lock().write_all(buf)
     }
-    // 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)
+    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> {
@@ -280,7 +281,7 @@ impl<'a> Write for StdoutLock<'a> {
 /// For more information, see `stderr`
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stderr {
-    inner: Arc<Mutex<StderrRaw>>,
+    inner: Arc<ReentrantMutex<StderrRaw>>,
 }
 
 /// A locked reference to the a `Stderr` handle.
@@ -289,7 +290,7 @@ pub struct Stderr {
 /// method on `Stderr`.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct StderrLock<'a> {
-    inner: MutexGuard<'a, StderrRaw>,
+    inner: ReentrantMutexGuard<'a, StderrRaw>,
 }
 
 /// Constructs a new reference to the standard error stream of a process.
@@ -300,13 +301,13 @@ pub struct StderrLock<'a> {
 /// The returned handle implements the `Write` trait.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stderr() -> Stderr {
-    static INSTANCE: Lazy<Mutex<StderrRaw>> = lazy_init!(stderr_init);
+    static INSTANCE: Lazy<ReentrantMutex<StderrRaw>> = lazy_init!(stderr_init);
     return Stderr {
         inner: INSTANCE.get().expect("cannot access stderr during shutdown"),
     };
 
-    fn stderr_init() -> Arc<Mutex<StderrRaw>> {
-        Arc::new(Mutex::new(stderr_raw()))
+    fn stderr_init() -> Arc<ReentrantMutex<StderrRaw>> {
+        Arc::new(ReentrantMutex::new(stderr_raw()))
     }
 }
 
@@ -333,7 +334,9 @@ impl Write for Stderr {
     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
         self.lock().write_all(buf)
     }
-    // Don't override write_fmt for the same reasons as Stdout
+    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> {

src/libstd/sync/mod.rs

@@ -20,15 +20,16 @@
 pub use alloc::arc::{Arc, Weak};
 pub use core::atomic;
 
-pub use self::mutex::{Mutex, MutexGuard, StaticMutex};
-pub use self::mutex::MUTEX_INIT;
-pub use self::rwlock::{RwLock, StaticRwLock, RW_LOCK_INIT};
-pub use self::rwlock::{RwLockReadGuard, RwLockWriteGuard};
+pub use self::barrier::{Barrier, BarrierWaitResult};
 pub use self::condvar::{Condvar, StaticCondvar, CONDVAR_INIT};
+pub use self::mutex::MUTEX_INIT;
+pub use self::mutex::{Mutex, MutexGuard, StaticMutex};
 pub use self::once::{Once, ONCE_INIT};
-pub use self::semaphore::{Semaphore, SemaphoreGuard};
-pub use self::barrier::{Barrier, BarrierWaitResult};
 pub use self::poison::{PoisonError, TryLockError, TryLockResult, LockResult};
+pub use self::remutex::{ReentrantMutex, ReentrantMutexGuard};
+pub use self::rwlock::{RwLockReadGuard, RwLockWriteGuard};
+pub use self::rwlock::{RwLock, StaticRwLock, RW_LOCK_INIT};
+pub use self::semaphore::{Semaphore, SemaphoreGuard};
 
 pub use self::future::Future;
 
@@ -40,5 +41,6 @@ mod future;
 mod mutex;
 mod once;
 mod poison;
+mod remutex;
 mod rwlock;
 mod semaphore;

src/libstd/sync/mutex.rs

@@ -212,7 +212,7 @@ impl<T> Mutex<T> {
 
     /// Attempts to acquire this lock.
     ///
-    /// If the lock could not be acquired at this time, then `None` is returned.
+    /// If the lock could not be acquired at this time, then `Err` is returned.
     /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
     /// guard is dropped.
     ///