enum_lock.patch

diff --git src/Perl6/Metamodel/EnumHOW.nqp src/Perl6/Metamodel/EnumHOW.nqp
index 8cf0c042b..2ecd32fbc 100644
--- src/Perl6/Metamodel/EnumHOW.nqp
+++ src/Perl6/Metamodel/EnumHOW.nqp
@@ -43,6 +43,8 @@ class Perl6::Metamodel::EnumHOW
     # Exportation callback for enum symbols, if any.
     has $!export_callback;
 
+    my $lock := NQPLock.new;
+
     my $archetypes := Perl6::Metamodel::Archetypes.new( :nominal(1), :composalizable(1),
                                                         :augmentable(1) );
     method archetypes() {
@@ -70,6 +72,15 @@ class Perl6::Metamodel::EnumHOW
     method add_enum_value($obj, $value) {
         %!values{nqp::unbox_s($value.key)} := $value.value;
         @!enum_value_list[+@!enum_value_list] := $value;
+        my %value_to_enum := %!value_to_enum;
+        $lock.protect: {
+            if %value_to_enum {
+                %value_to_enum{$value.value} := $value.key;
+            }
+            nqp::scwbdisable();
+            %!value_to_enum := %value_to_enum;
+            nqp::scwbenable();
+        };
     }
 
     method set_export_callback($obj, $callback) {
@@ -85,11 +96,17 @@ class Perl6::Metamodel::EnumHOW
     }
 
     method enum_from_value($obj, $value) {
-        unless %!value_to_enum {
-            for @!enum_value_list {
-                %!value_to_enum{$_.value} := $_;
+        my %value_to_enum := %!value_to_enum;
+        $lock.protect: {
+            unless %value_to_enum {
+                for @!enum_value_list {
+                    %value_to_enum{$_.value} := $_;
+                }
+                nqp::scwbdisable();
+                %!value_to_enum := %value_to_enum;
+                nqp::scwbenable();
             }
-        }
+        };
         nqp::existskey(%!value_to_enum, $value)
             ?? %!value_to_enum{$value}
             !! nqp::null()