Fix destroy assertions in mutex/rwlock/condvar

On DragonFly pthread_{mutex,rwlock,condvar}_destroy() returns EINVAL
when called on a pthread_{mutex,rwlock,condvar}_t that was just
initialized via PTHREAD_{MUTEX,RWLOCK,CONDVAR}_INITIALIZER and not used
in the meantime or initialized via pthread_{mutex,rwlock,condvar}_init().
Change the code to treat a return value of EINVAL on DragonFly as success.

src/libstd/sys/unix/condvar.rs

@@ -76,8 +76,20 @@ impl Condvar {
     }
 
     #[inline]
+    #[cfg(not(target_os = "dragonfly"))]
     pub unsafe fn destroy(&self) {
         let r = ffi::pthread_cond_destroy(self.inner.get());
         debug_assert_eq!(r, 0);
     }
+
+    #[inline]
+    #[cfg(target_os = "dragonfly")]
+    pub unsafe fn destroy(&self) {
+        let r = ffi::pthread_cond_destroy(self.inner.get());
+        // On DragonFly pthread_cond_destroy() returns EINVAL if called on
+        // a condvar that was just initialized with
+        // ffi::PTHREAD_COND_INITIALIZER. Once it is used or
+        // pthread_cond_init() is called, this behaviour no longer occurs.
+        debug_assert!(r == 0 || r == libc::EINVAL);
+    }
 }

src/libstd/sys/unix/mutex.rs

@@ -48,8 +48,20 @@ impl Mutex {
         ffi::pthread_mutex_trylock(self.inner.get()) == 0
     }
     #[inline]
+    #[cfg(not(target_os = "dragonfly"))]
     pub unsafe fn destroy(&self) {
         let r = ffi::pthread_mutex_destroy(self.inner.get());
         debug_assert_eq!(r, 0);
     }
+    #[inline]
+    #[cfg(target_os = "dragonfly")]
+    pub unsafe fn destroy(&self) {
+        use libc;
+        let r = ffi::pthread_mutex_destroy(self.inner.get());
+        // On DragonFly pthread_mutex_destroy() returns EINVAL if called on a
+        // mutex that was just initialized with ffi::PTHREAD_MUTEX_INITIALIZER.
+        // Once it is used (locked/unlocked) or pthread_mutex_init() is called,
+        // this behaviour no longer occurs.
+        debug_assert!(r == 0 || r == libc::EINVAL);
+    }
 }

src/libstd/sys/unix/rwlock.rs

@@ -50,8 +50,21 @@ impl RWLock {
     #[inline]
     pub unsafe fn write_unlock(&self) { self.read_unlock() }
     #[inline]
+    #[cfg(not(target_os = "dragonfly"))]
     pub unsafe fn destroy(&self) {
         let r = ffi::pthread_rwlock_destroy(self.inner.get());
         debug_assert_eq!(r, 0);
     }
+
+    #[inline]
+    #[cfg(target_os = "dragonfly")]
+    pub unsafe fn destroy(&self) {
+        use libc;
+        let r = ffi::pthread_rwlock_destroy(self.inner.get());
+        // On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a
+        // rwlock that was just initialized with
+        // ffi::PTHREAD_RWLOCK_INITIALIZER. Once it is used (locked/unlocked)
+        // or pthread_rwlock_init() is called, this behaviour no longer occurs.
+        debug_assert!(r == 0 || r == libc::EINVAL);
+    }
 }