LibWeb: Implement HTMLImageElement::decode with a few FIXMEs

Implements enough of HTMLImageElement::decode for it to not break
websites and actually load an image :)

(cherry picked from commit 862fc91b2cf9bee9b7f180cdf930c6c99b5dd053)

Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp

@@ -26,9 +26,11 @@
 #include <LibWeb/HTML/ListOfAvailableImages.h>
 #include <LibWeb/HTML/Parser/HTMLParser.h>
 #include <LibWeb/HTML/PotentialCORSRequest.h>
+#include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
 #include <LibWeb/Layout/ImageBox.h>
 #include <LibWeb/Loader/ResourceLoader.h>
 #include <LibWeb/Painting/PaintableBox.h>
+#include <LibWeb/Platform/EventLoopPlugin.h>
 #include <LibWeb/Platform/ImageCodecPlugin.h>
 #include <LibWeb/SVG/SVGDecodedImageData.h>
 
@@ -276,6 +278,86 @@ String HTMLImageElement::current_src() const
     return MUST(m_current_request->current_url().to_string());
 }
 
+// https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-decode
+WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::Promise>> HTMLImageElement::decode() const
+{
+    auto& realm = this->realm();
+
+    // 1. Let promise be a new promise.
+    auto promise = WebIDL::create_promise(realm);
+
+    // 2. Queue a microtask to perform the following steps:
+    queue_a_microtask(&document(), JS::create_heap_function(realm.heap(), [this, promise, &realm]() mutable {
+        auto reject_if_document_not_fully_active = [this, promise, &realm]() -> bool {
+            if (this->document().is_fully_active())
+                return false;
+
+            auto exception = WebIDL::EncodingError::create(realm, "Node document not fully active"_fly_string);
+            WebIDL::reject_promise(realm, promise, exception);
+            return true;
+        };
+
+        auto reject_if_current_request_state_broken = [this, promise, &realm]() {
+            if (this->current_request().state() != ImageRequest::State::Broken)
+                return false;
+
+            auto exception = WebIDL::EncodingError::create(realm, "Current request state is broken"_fly_string);
+            WebIDL::reject_promise(realm, promise, exception);
+            return true;
+        };
+
+        // 2.1 If any of the following are true:
+        // 2.1.1 this's node document is not fully active;
+        // 2.1.1 then reject promise with an "EncodingError" DOMException.
+        if (reject_if_document_not_fully_active())
+            return;
+
+        // 2.1.2  or this's current request's state is broken,
+        // 2.1.2 then reject promise with an "EncodingError" DOMException.
+        if (reject_if_current_request_state_broken())
+            return;
+
+        // 2.2 Otherwise, in parallel wait for one of the following cases to occur, and perform the corresponding actions:
+        Platform::EventLoopPlugin::the().deferred_invoke([this, promise, &realm, reject_if_document_not_fully_active, reject_if_current_request_state_broken] {
+            Platform::EventLoopPlugin::the().spin_until([&] {
+                auto state = this->current_request().state();
+
+                return !this->document().is_fully_active() || state == ImageRequest::State::Broken || state == ImageRequest::State::CompletelyAvailable;
+            });
+
+            // 2.2.1 This img element's node document stops being fully active
+            // 2.2.1 Reject promise with an "EncodingError" DOMException.
+            if (reject_if_document_not_fully_active())
+                return;
+
+            // FIXME: 2.2.2 This img element's current request changes or is mutated
+            // FIXME: 2.2.2 Reject promise with an "EncodingError" DOMException.
+
+            // 2.2.3 This img element's current request's state becomes broken
+            // 2.2.3 Reject promise with an "EncodingError" DOMException.
+            if (reject_if_current_request_state_broken())
+                return;
+
+            // 2.2.4 This img element's current request's state becomes completely available
+            if (this->current_request().state() == ImageRequest::State::CompletelyAvailable) {
+                // 2.2.4.1 FIXME: Decode the image.
+                // 2.2.4.2 FIXME: If decoding does not need to be performed for this image (for example because it is a vector graphic), resolve promise with undefined.
+                // 2.2.4.3 FIXME: If decoding fails (for example due to invalid image data), reject promise with an "EncodingError" DOMException.
+                // 2.2.4.4 FIXME: If the decoding process completes successfully, resolve promise with undefined.
+                // 2.2.4.5 FIXME: User agents should ensure that the decoded media data stays readily available until at least the end of the next successful update
+                // the rendering step in the event loop. This is an important part of the API contract, and should not be broken if at all possible.
+                // (Typically, this would only be violated in low-memory situations that require evicting decoded image data, or when the image is too large
+                // to keep in decoded form for this period of time.)
+
+                HTML::TemporaryExecutionContext execution_context { Bindings::host_defined_environment_settings_object(realm) };
+                WebIDL::resolve_promise(realm, promise, JS::js_undefined());
+            }
+        });
+    }));
+
+    return JS::NonnullGCPtr { verify_cast<JS::Promise>(*promise->promise()) };
+}
+
 Optional<ARIA::Role> HTMLImageElement::default_role() const
 {
     // https://www.w3.org/TR/html-aria/#el-img

Userland/Libraries/LibWeb/HTML/HTMLImageElement.h

@@ -60,6 +60,9 @@ public:
     // https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-currentsrc
     String current_src() const;
 
+    // https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-decode
+    [[nodiscard]] WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::Promise>> decode() const;
+
     virtual Optional<ARIA::Role> default_role() const override;
 
     // https://html.spec.whatwg.org/multipage/images.html#img-environment-changes

Userland/Libraries/LibWeb/HTML/HTMLImageElement.idl

@@ -26,7 +26,7 @@ interface HTMLImageElement : HTMLElement {
     [CEReactions, Enumerated=LazyLoadingAttribute, Reflect] attribute DOMString loading;
     [CEReactions, Enumerated=FetchPriorityAttribute, Reflect=fetchpriority] attribute DOMString fetchPriority;
 
-    [FIXME] Promise<undefined> decode();
+    Promise<undefined> decode();
 
     // Obsolete
     [CEReactions, Reflect] attribute DOMString name;