make mutex_unlock infallible

Author: RalfJung

src/shims/sync.rs

@@ -316,7 +316,7 @@ fn release_cond_mutex_and_block<'mir, 'tcx: 'mir>(
     active_thread: ThreadId,
     mutex: MutexId,
 ) -> InterpResult<'tcx> {
-    if let Some(old_locked_count) = ecx.mutex_unlock(mutex, active_thread)? {
+    if let Some(old_locked_count) = ecx.mutex_unlock(mutex, active_thread) {
         if old_locked_count != 1 {
             throw_unsup_format!("awaiting on a lock acquired multiple times is not supported");
         }
@@ -486,7 +486,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         let id = mutex_get_or_create_id(this, mutex_op)?;
         let active_thread = this.get_active_thread();
 
-        if let Some(_old_locked_count) = this.mutex_unlock(id, active_thread)? {
+        if let Some(_old_locked_count) = this.mutex_unlock(id, active_thread) {
             // The mutex was locked by the current thread.
             Ok(0)
         } else {

src/sync.rs

@@ -153,14 +153,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         &mut self,
         id: MutexId,
         expected_owner: ThreadId,
-    ) -> InterpResult<'tcx, Option<usize>> {
+    ) -> Option<usize> {
         let this = self.eval_context_mut();
         let mutex = &mut this.machine.threads.sync.mutexes[id];
         if let Some(current_owner) = mutex.owner {
             // Mutex is locked.
             if current_owner != expected_owner {
                 // Only the owner can unlock the mutex.
-                return Ok(None);
+                return None;
             }
             let old_lock_count = mutex.lock_count;
             mutex.lock_count = old_lock_count
@@ -172,10 +172,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 // to another thread.
                 this.mutex_dequeue_and_lock(id);
             }
-            Ok(Some(old_lock_count))
+            Some(old_lock_count)
         } else {
             // Mutex is unlocked.
-            Ok(None)
+            None
         }
     }