[Webkit-unassigned] [Bug 184012] Unable to use HashMap<WebCore::SecurityOriginData, Ref<WebSWServerToContextConnection>> type in StorageProcess.h

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=184012

--- Comment #3 from Chris Dumez <cdumez at apple.com> ---
(In reply to Chris Dumez from comment #2)
> Using HashMap<RefPtr<WebCore::SecurityOrigin>,
> Ref<WebSWServerToContextConnection>> works. I believe the different is that
> emptyValueIsZero for both the Key and the Value.
> 
> emptyValueIsZero is false when the key is a SecurityOriginData. So
> KeyValuePairHashTraits::emptyValueIsZero goes from true to false and it
> causes failures.

As a result, it ends up using a different template specialization for HashTableBucketInitializer:
    template<bool emptyValueIsZero> struct HashTableBucketInitializer;

    template<> struct HashTableBucketInitializer<false> {
        template<typename Traits, typename Value> static void initialize(Value& bucket)
        {
            new (NotNull, std::addressof(bucket)) Value(Traits::emptyValue());
        }
    };

    template<> struct HashTableBucketInitializer<true> {
        template<typename Traits, typename Value> static void initialize(Value& bucket)
        {
            // This initializes the bucket without copying the empty value.
            // That makes it possible to use this with types that don't support copying.
            // The memset to 0 looks like a slow operation but is optimized by the compilers.
            memset(std::addressof(bucket), 0, sizeof(bucket));
        }
    };

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