attempt to reduce the scope of the Rlock for mutex check

I have not convinced myself that this is safe yet (particularly the
change to field.go, I think the view change is likely fine).

It does seem to stop the issue... I want to see if tests run in CI.

field.go

@@ -1235,9 +1235,14 @@ func (f *Field) MutexCheck(ctx context.Context, qcx *Qcx, details bool, limit in
 	if f.Type() != FieldTypeMutex {
 		return nil, errors.New("mutex check only valid for mutex fields")
 	}
+
+	// Rather than deferring the unlock, we grab the standard view
+	// from the field's viewMap and unlock immediately. This avoids
+	// holding the rlock for a potentially long time which blocks any
+	// write lock, and pending write locks block other read locks.
 	f.mu.RLock()
-	defer f.mu.RUnlock()
 	standard := f.viewMap[viewStandard]
+	f.mu.RUnlock()
 	if standard == nil {
 		// no standard view present means we've never needed to create it,
 		// so it has no bits set, so it has no extra bits set.

view.go

@@ -301,8 +301,8 @@ func (v *view) Fragment(shard uint64) *fragment {
 
 // allFragments returns a list of all fragments in the view.
 func (v *view) allFragments() []*fragment {
-	v.mu.Lock()
-	defer v.mu.Unlock()
+	v.mu.RLock()
+	defer v.mu.RUnlock()
 
 	other := make([]*fragment, 0, len(v.fragments))
 	for _, fragment := range v.fragments {