Auto merge of #51529 - nodakai:improve-sys_common-mutex, r=oli-obk

libstd: add an RAII utility for sys_common::mutex::Mutex

It is indeed debatable whether or not we should introduce more sophistication like this to the lowest layer of a system library. In fact, `Drop::drop()` cannot be `unsafe` (IIRC there was a discussion on introducing an unsafe variant of `Drop` whose entire scope must be within `unsafe`)

Author: bors

src/libstd/io/lazy.rs

@@ -20,6 +20,9 @@ pub struct Lazy<T> {
     init: fn() -> Arc<T>,
 }
 
+#[inline]
+const fn done<T>() -> *mut Arc<T> { 1_usize as *mut _ }
+
 unsafe impl<T> Sync for Lazy<T> {}
 
 impl<T: Send + Sync + 'static> Lazy<T> {
@@ -33,17 +36,15 @@ impl<T: Send + Sync + 'static> Lazy<T> {
 
     pub fn get(&'static self) -> Option<Arc<T>> {
         unsafe {
-            self.lock.lock();
+            let _guard = self.lock.lock();
             let ptr = self.ptr.get();
-            let ret = if ptr.is_null() {
+            if ptr.is_null() {
                 Some(self.init())
-            } else if ptr as usize == 1 {
+            } else if ptr == done() {
                 None
             } else {
                 Some((*ptr).clone())
-            };
-            self.lock.unlock();
-            return ret
+            }
         }
     }
 
@@ -53,10 +54,10 @@ impl<T: Send + Sync + 'static> Lazy<T> {
         // the at exit handler). Otherwise we just return the freshly allocated
         // `Arc`.
         let registered = sys_common::at_exit(move || {
-            self.lock.lock();
-            let ptr = self.ptr.get();
-            self.ptr.set(1 as *mut _);
-            self.lock.unlock();
+            let ptr = {
+                let _guard = self.lock.lock();
+                self.ptr.replace(done())
+            };
             drop(Box::from_raw(ptr))
         });
         let ret = (self.init)();

src/libstd/sync/mutex.rs

@@ -227,7 +227,7 @@ impl<T: ?Sized> Mutex<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn lock(&self) -> LockResult<MutexGuard<T>> {
         unsafe {
-            self.inner.lock();
+            self.inner.raw_lock();
             MutexGuard::new(self)
         }
     }
@@ -454,7 +454,7 @@ impl<'a, T: ?Sized> Drop for MutexGuard<'a, T> {
     fn drop(&mut self) {
         unsafe {
             self.__lock.poison.done(&self.__poison);
-            self.__lock.inner.unlock();
+            self.__lock.inner.raw_unlock();
         }
     }
 }

src/libstd/sys/redox/args.rs

@@ -73,17 +73,15 @@ mod imp {
             CStr::from_ptr(*argv.offset(i) as *const libc::c_char).to_bytes().to_vec()
         }).collect();
 
-        LOCK.lock();
+        let _guard = LOCK.lock();
         let ptr = get_global_ptr();
         assert!((*ptr).is_none());
         (*ptr) = Some(box args);
-        LOCK.unlock();
     }
 
     pub unsafe fn cleanup() {
-        LOCK.lock();
+        let _guard = LOCK.lock();
         *get_global_ptr() = None;
-        LOCK.unlock();
     }
 
     pub fn args() -> Args {
@@ -96,16 +94,14 @@ mod imp {
 
     fn clone() -> Option<Vec<Vec<u8>>> {
         unsafe {
-            LOCK.lock();
+            let _guard = LOCK.lock();
             let ptr = get_global_ptr();
-            let ret = (*ptr).as_ref().map(|s| (**s).clone());
-            LOCK.unlock();
-            return ret
+            (*ptr).as_ref().map(|s| (**s).clone())
         }
     }
 
-    fn get_global_ptr() -> *mut Option<Box<Vec<Vec<u8>>>> {
-        unsafe { mem::transmute(&GLOBAL_ARGS_PTR) }
+    unsafe fn get_global_ptr() -> *mut Option<Box<Vec<Vec<u8>>>> {
+        mem::transmute(&GLOBAL_ARGS_PTR)
     }
 
 }

src/libstd/sys/unix/args.rs

@@ -82,17 +82,15 @@ mod imp {
     static LOCK: Mutex = Mutex::new();
 
     pub unsafe fn init(argc: isize, argv: *const *const u8) {
-        LOCK.lock();
+        let _guard = LOCK.lock();
         ARGC = argc;
         ARGV = argv;
-        LOCK.unlock();
     }
 
     pub unsafe fn cleanup() {
-        LOCK.lock();
+        let _guard = LOCK.lock();
         ARGC = 0;
         ARGV = ptr::null();
-        LOCK.unlock();
     }
 
     pub fn args() -> Args {
@@ -104,13 +102,11 @@ mod imp {
 
     fn clone() -> Vec<OsString> {
         unsafe {
-            LOCK.lock();
-            let ret = (0..ARGC).map(|i| {
+            let _guard = LOCK.lock();
+            (0..ARGC).map(|i| {
                 let cstr = CStr::from_ptr(*ARGV.offset(i) as *const libc::c_char);
                 OsStringExt::from_vec(cstr.to_bytes().to_vec())
-            }).collect();
-            LOCK.unlock();
-            return ret
+            }).collect()
         }
     }
 }

src/libstd/sys/unix/os.rs

@@ -409,10 +409,9 @@ pub unsafe fn environ() -> *mut *const *const c_char {
 /// environment variables of the current process.
 pub fn env() -> Env {
     unsafe {
-        ENV_LOCK.lock();
+        let _guard = ENV_LOCK.lock();
         let mut environ = *environ();
         if environ == ptr::null() {
-            ENV_LOCK.unlock();
             panic!("os::env() failure getting env string from OS: {}",
                    io::Error::last_os_error());
         }
@@ -423,12 +422,10 @@ pub fn env() -> Env {
             }
             environ = environ.offset(1);
         }
-        let ret = Env {
+        return Env {
             iter: result.into_iter(),
             _dont_send_or_sync_me: PhantomData,
-        };
-        ENV_LOCK.unlock();
-        return ret
+        }
     }
 
     fn parse(input: &[u8]) -> Option<(OsString, OsString)> {
@@ -452,15 +449,14 @@ pub fn getenv(k: &OsStr) -> io::Result<Option<OsString>> {
     // always None as well
     let k = CString::new(k.as_bytes())?;
     unsafe {
-        ENV_LOCK.lock();
+        let _guard = ENV_LOCK.lock();
         let s = libc::getenv(k.as_ptr()) as *const libc::c_char;
         let ret = if s.is_null() {
             None
         } else {
             Some(OsStringExt::from_vec(CStr::from_ptr(s).to_bytes().to_vec()))
         };
-        ENV_LOCK.unlock();
-        return Ok(ret)
+        Ok(ret)
     }
 }
 
@@ -469,21 +465,17 @@ pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     let v = CString::new(v.as_bytes())?;
 
     unsafe {
-        ENV_LOCK.lock();
-        let ret = cvt(libc::setenv(k.as_ptr(), v.as_ptr(), 1)).map(|_| ());
-        ENV_LOCK.unlock();
-        return ret
+        let _guard = ENV_LOCK.lock();
+        cvt(libc::setenv(k.as_ptr(), v.as_ptr(), 1)).map(|_| ())
     }
 }
 
 pub fn unsetenv(n: &OsStr) -> io::Result<()> {
     let nbuf = CString::new(n.as_bytes())?;
 
     unsafe {
-        ENV_LOCK.lock();
-        let ret = cvt(libc::unsetenv(nbuf.as_ptr())).map(|_| ());
-        ENV_LOCK.unlock();
-        return ret
+        let _guard = ENV_LOCK.lock();
+        cvt(libc::unsetenv(nbuf.as_ptr())).map(|_| ())
     }
 }
 

src/libstd/sys_common/at_exit_imp.rs

@@ -14,6 +14,7 @@
 
 use boxed::FnBox;
 use ptr;
+use mem;
 use sys_common::mutex::Mutex;
 
 type Queue = Vec<Box<FnBox()>>;
@@ -25,6 +26,8 @@ type Queue = Vec<Box<FnBox()>>;
 static LOCK: Mutex = Mutex::new();
 static mut QUEUE: *mut Queue = ptr::null_mut();
 
+const DONE: *mut Queue = 1_usize as *mut _;
+
 // The maximum number of times the cleanup routines will be run. While running
 // the at_exit closures new ones may be registered, and this count is the number
 // of times the new closures will be allowed to register successfully. After
@@ -35,7 +38,7 @@ unsafe fn init() -> bool {
     if QUEUE.is_null() {
         let state: Box<Queue> = box Vec::new();
         QUEUE = Box::into_raw(state);
-    } else if QUEUE as usize == 1 {
+    } else if QUEUE == DONE {
         // can't re-init after a cleanup
         return false
     }
@@ -44,18 +47,18 @@ unsafe fn init() -> bool {
 }
 
 pub fn cleanup() {
-    for i in 0..ITERS {
+    for i in 1..=ITERS {
         unsafe {
-            LOCK.lock();
-            let queue = QUEUE;
-            QUEUE = if i == ITERS - 1 {1} else {0} as *mut _;
-            LOCK.unlock();
+            let queue = {
+                let _guard = LOCK.lock();
+                mem::replace(&mut QUEUE, if i == ITERS { DONE } else { ptr::null_mut() })
+            };
 
             // make sure we're not recursively cleaning up
-            assert!(queue as usize != 1);
+            assert!(queue != DONE);
 
             // If we never called init, not need to cleanup!
-            if queue as usize != 0 {
+            if !queue.is_null() {
                 let queue: Box<Queue> = Box::from_raw(queue);
                 for to_run in *queue {
                     to_run();
@@ -66,15 +69,13 @@ pub fn cleanup() {
 }
 
 pub fn push(f: Box<FnBox()>) -> bool {
-    let mut ret = true;
     unsafe {
-        LOCK.lock();
+        let _guard = LOCK.lock();
         if init() {
             (*QUEUE).push(f);
+            true
         } else {
-            ret = false;
+            false
         }
-        LOCK.unlock();
     }
-    ret
 }

src/libstd/sys_common/mutex.rs

@@ -37,7 +37,15 @@ impl Mutex {
     /// Behavior is undefined if the mutex has been moved between this and any
     /// previous function call.
     #[inline]
-    pub unsafe fn lock(&self) { self.0.lock() }
+    pub unsafe fn raw_lock(&self) { self.0.lock() }
+
+    /// Calls raw_lock() and then returns an RAII guard to guarantee the mutex
+    /// will be unlocked.
+    #[inline]
+    pub unsafe fn lock(&self) -> MutexGuard {
+        self.raw_lock();
+        MutexGuard(&self.0)
+    }
 
     /// Attempts to lock the mutex without blocking, returning whether it was
     /// successfully acquired or not.
@@ -51,8 +59,11 @@ impl Mutex {
     ///
     /// Behavior is undefined if the current thread does not actually hold the
     /// mutex.
+    ///
+    /// Consider switching from the pair of raw_lock() and raw_unlock() to
+    /// lock() whenever possible.
     #[inline]
-    pub unsafe fn unlock(&self) { self.0.unlock() }
+    pub unsafe fn raw_unlock(&self) { self.0.unlock() }
 
     /// Deallocates all resources associated with this mutex.
     ///
@@ -64,3 +75,14 @@ impl Mutex {
 
 // not meant to be exported to the outside world, just the containing module
 pub fn raw(mutex: &Mutex) -> &imp::Mutex { &mutex.0 }
+
+#[must_use]
+/// A simple RAII utility for the above Mutex without the poisoning semantics.
+pub struct MutexGuard<'a>(&'a imp::Mutex);
+
+impl<'a> Drop for MutexGuard<'a> {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe { self.0.unlock(); }
+    }
+}

src/libstd/sys_common/thread_local.rs

@@ -162,13 +162,12 @@ impl StaticKey {
         // we just simplify the whole branch.
         if imp::requires_synchronized_create() {
             static INIT_LOCK: Mutex = Mutex::new();
-            INIT_LOCK.lock();
+            let _guard = INIT_LOCK.lock();
             let mut key = self.key.load(Ordering::SeqCst);
             if key == 0 {
                 key = imp::create(self.dtor) as usize;
                 self.key.store(key, Ordering::SeqCst);
             }
-            INIT_LOCK.unlock();
             rtassert!(key != 0);
             return key
         }

src/libstd/thread/mod.rs

@@ -935,20 +935,17 @@ impl ThreadId {
         static mut COUNTER: u64 = 0;
 
         unsafe {
-            GUARD.lock();
+            let _guard = GUARD.lock();
 
             // If we somehow use up all our bits, panic so that we're not
             // covering up subtle bugs of IDs being reused.
             if COUNTER == ::u64::MAX {
-                GUARD.unlock();
                 panic!("failed to generate unique thread ID: bitspace exhausted");
             }
 
             let id = COUNTER;
             COUNTER += 1;
 
-            GUARD.unlock();
-
             ThreadId(id)
         }
     }