[Webkit-unassigned] [Bug 175602] New: [GTK] ASSERTION FAILED: !HashTranslator::equal(KeyTraits::emptyValue(), key) when dragging file into webview

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=175602

            Bug ID: 175602
           Summary: [GTK] ASSERTION FAILED:
                    !HashTranslator::equal(KeyTraits::emptyValue(), key)
                    when dragging file into webview
           Product: WebKit
           Version: Other
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit Gtk
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: mcatanzaro at igalia.com
                CC: bugs-noreply at webkitgtk.org

Created attachment 318181

  --> https://bugs.webkit.org/attachment.cgi?id=318181&action=review

Backtrace

I tried dragging a file into Epiphany. It usually works but I guess it is racy as today (using WebKit trunk) I hit this assertion:

ASSERTION FAILED: !HashTranslator::equal(KeyTraits::emptyValue(), key)
../../Source/WTF/wtf/HashTable.h(587) : void WTF::HashTable<Key, Value, Extractor, HashFunctions, Traits, KeyTraits>::checkKey(const T&) [with HashTranslator = WTF::IdentityHashTranslator<WTF::HashMap<_GdkDragContext*, std::unique_ptr<WebKit::DragAndDropHandler::DroppingContext> >::KeyValuePairTraits, WTF::PtrHash<_GdkDragContext*> >; T = _GdkDragContext*; Key = _GdkDragContext*; Value = WTF::KeyValuePair<_GdkDragContext*, std::unique_ptr<WebKit::DragAndDropHandler::DroppingContext> >; Extractor = WTF::KeyValuePairKeyExtractor<WTF::KeyValuePair<_GdkDragContext*, std::unique_ptr<WebKit::DragAndDropHandler::DroppingContext> > >; HashFunctions = WTF::PtrHash<_GdkDragContext*>; Traits = WTF::HashMap<_GdkDragContext*, std::unique_ptr<WebKit::DragAndDropHandler::DroppingContext> >::KeyValuePairTraits; KeyTraits = WTF::HashTraits<_GdkDragContext*>]

Detailed backtrace attached. Unfortunately I can't reproduce it. Here's the function it's crashing in:

    template<typename Key, typename Value, typename Extractor, typename HashFunctions, typename Traits, typename KeyTraits>
    template<typename HashTranslator, typename T>
    void HashTable<Key, Value, Extractor, HashFunctions, Traits, KeyTraits>::checkKey(const T& key)
    {
        if (!HashFunctions::safeToCompareToEmptyOrDeleted)
            return;
        ASSERT(!HashTranslator::equal(KeyTraits::emptyValue(), key));
        // ...
    }

Clearly HashFunctions::safeToCompareToEmptyOrDeleted is returning true, so the function does not return early. Then the assertion fails. It's not clear to me why it is impermissible for a HashMap to contain an empty-valued key, but OK.

A naive speculative fix would add a null-check for the gdkContext in DragAndDropHandler.cpp, like this:

void DragAndDropHandler::dragLeave(GdkDragContext* context)
{
    DroppingContext* droppingContext = m_droppingContexts.get(context);
    if (!droppingContext || !droppingContext->gdkContext)
        return;

    // ...
}

But I suspect that's not the right answer. It's not clear to me why the GdkDragContext would ever be null anyway.

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