[Webkit-unassigned] [Bug 276224] New: [GStreamer] gstStructureGet<T> may return optionals with unitialized values on Clang 18

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=276224

            Bug ID: 276224
           Summary: [GStreamer] gstStructureGet<T> may return optionals
                    with unitialized values on Clang 18
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Platform
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: cadubentzen at igalia.com

On bug 275609 we introduced the gstStructureGet<T> helper. After using it in more places, we found what may be a compiler bug (reproduced it with Clang 18 so far).

Given a GstStructure named structure with "a-structure, width=10". If we try:

auto height = gstStructureGet<int>(structure, "height"_s);
if (height)
    GST_LOG("height = %d", *height);

although the branch shouldn't be executed since there's no "height" field. It is executed with an unitialized value.

After debugging what's happening, it seems to be actually a compiler issue. Fixed it by using if(UNLIKELY(gst_structure_get_<type>)) in gstStructureGet branches. Without that, the branches were being executed and returning std::optional with unitialized values when they shouldn't.

Patch for the fix in WebKit incoming. Pending on a minimal reproducer outside WebKit to confirm it's a bug in Clang...

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