std: Always check for EDEADLK in rwlocks on unix

Apparently implementations are allowed to return EDEADLK instead of blocking
forever, in which case this can lead to unsafety in the `RwLock` primitive
exposed by the standard library. A debug-build of the standard library would
have caught this error (due to the debug assert), but we don't ship debug
builds right now.

This commit adds explicit checks for the EDEADLK error code and triggers a panic
to ensure the call does not succeed.

Closes #25012

src/libstd/sys/unix/rwlock.rs

@@ -10,6 +10,7 @@
 
 use prelude::v1::*;
 
+use libc;
 use cell::UnsafeCell;
 use sys::sync as ffi;
 
@@ -26,7 +27,23 @@ impl RWLock {
     #[inline]
     pub unsafe fn read(&self) {
         let r = ffi::pthread_rwlock_rdlock(self.inner.get());
-        debug_assert_eq!(r, 0);
+
+        // According to the pthread_rwlock_rdlock spec, this function **may**
+        // fail with EDEADLK if a deadlock is detected. On the other hand
+        // pthread mutexes will *never* return EDEADLK if they are initialized
+        // as the "fast" kind (which ours always are). As a result, a deadlock
+        // situation may actually return from the call to pthread_rwlock_rdlock
+        // instead of blocking forever (as mutexes and Windows rwlocks do). Note
+        // that not all unix implementations, however, will return EDEADLK for
+        // their rwlocks.
+        //
+        // We roughly maintain the deadlocking behavior by panicking to ensure
+        // that this lock acquisition does not succeed.
+        if r == libc::EDEADLK {
+            panic!("rwlock read lock would result in deadlock");
+        } else {
+            debug_assert_eq!(r, 0);
+        }
     }
     #[inline]
     pub unsafe fn try_read(&self) -> bool {
@@ -35,7 +52,12 @@ pub unsafe fn try_read(&self) -> bool {
     #[inline]
     pub unsafe fn write(&self) {
         let r = ffi::pthread_rwlock_wrlock(self.inner.get());
-        debug_assert_eq!(r, 0);
+        // see comments above for why we check for EDEADLK
+        if r == libc::EDEADLK {
+            panic!("rwlock write lock would result in deadlock");
+        } else {
+            debug_assert_eq!(r, 0);
+        }
     }
     #[inline]
     pub unsafe fn try_write(&self) -> bool {