[GROOVY-5316] @Lazy (LazyASTTransformation) not multi-thread safe

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Not A Problem
• Affects Version/s: 2.0-beta-2
• Fix Version/s: 2.0-beta-3, 1.8.7
• Component/s: Compiler
• Assignee: Paul King
• Reporter: thurston n
• Created: 17/Feb/12 12:08
• Updated: 13/May/12 03:30
• Resolved: 19/Feb/12 05:14

Description

I checked out the latest code from git on the master branch, and looking at LazyASTTransformation, it doesn't appear to be multi-thread safe in the case of an instance variable annotated with @Lazy.
specifically:

private void create(FieldNode fieldNode, final Expression initExpr) {
        final BlockStatement body = new BlockStatement();
        if (fieldNode.isStatic()) {
            addHolderClassIdiomBody(body, fieldNode, initExpr);
        } else if (isVolatile(fieldNode)) {
            addNonThreadSafeBody(body, fieldNode, initExpr);
        } else {
            addDoubleCheckedLockingBody(body, fieldNode, initExpr);
        }
        ...

But the JMM makes DCL-locking work with volatile, i.e. it doesn't obviate the need for DCL in the case of volatile instance variables. As written now, it seems the resulting code isn't multi-thread safe in either case (the source declares the field volatile or not)
Interestingly enough, @Singleton("lazy") (SingletonASTTransformation) does this correctly, by declaring a volatile variable (named "instance", although static) and then applying the DCL idiom (see #GROOVY-4782).

So it looks like the issue is understood but somehow isn't in @Lazy.
The fix seems rather straightforward:
modify the @Lazy FieldNode to always be volatile (whether declared volatile or not in the source)
and then create becomes:

private void create(FieldNode fieldNode, final Expression initExpr) {
        final BlockStatement body = new BlockStatement();
        if (fieldNode.isStatic()) {
            addHolderClassIdiomBody(body, fieldNode, initExpr);
        else
            addDoubleCheckedLockingBody(body, fieldNode, initExpr);
        ...