LibGfx: Fix handling of partially corrupt GIFs

GIF loader was completely failing when encountering errors with
frame descriptors or individual frames, even when some frames were
successfully loaded. Now we attempt to decode at least some frames
and fail only when no frames can be decoded at all.

Libraries/LibGfx/ImageFormats/GIFLoader.cpp

@@ -468,29 +468,31 @@ size_t GIFImageDecoderPlugin::first_animated_frame_index()
 
 ErrorOr<ImageFrameDescriptor> GIFImageDecoderPlugin::frame(size_t index, Optional<IntSize>)
 {
-    if (m_context->error_state >= GIFLoadingContext::ErrorState::FailedToDecodeAnyFrame) {
+    if (m_context->error_state == GIFLoadingContext::ErrorState::FailedToDecodeAnyFrame) {
         return Error::from_string_literal("GIFImageDecoderPlugin: Decoding failed");
     }
 
     if (m_context->state < GIFLoadingContext::State::FrameDescriptorsLoaded) {
         if (auto result = load_gif_frame_descriptors(*m_context); result.is_error()) {
             m_context->error_state = GIFLoadingContext::ErrorState::FailedToLoadFrameDescriptors;
-            return result.release_error();
+
+            // If we failed to load frame descriptors but we have some images, we can still try to decode them.
+            if (m_context->images.is_empty()) {
+                return result.release_error();
+            }
         }
     }
 
-    if (m_context->error_state == GIFLoadingContext::ErrorState::NoError) {
-        if (auto result = decode_frame(*m_context, index); result.is_error()) {
-            if (m_context->state < GIFLoadingContext::State::FrameComplete) {
-                m_context->error_state = GIFLoadingContext::ErrorState::FailedToDecodeAnyFrame;
-                return result.release_error();
-            }
-            if (auto result = decode_frame(*m_context, 0); result.is_error()) {
-                m_context->error_state = GIFLoadingContext::ErrorState::FailedToDecodeAnyFrame;
-                return result.release_error();
-            }
-            m_context->error_state = GIFLoadingContext::ErrorState::FailedToDecodeAllFrames;
+    if (auto result = decode_frame(*m_context, index); result.is_error()) {
+        if (m_context->state < GIFLoadingContext::State::FrameComplete) {
+            m_context->error_state = GIFLoadingContext::ErrorState::FailedToDecodeAnyFrame;
+            return result.release_error();
         }
+        if (auto result = decode_frame(*m_context, 0); result.is_error()) {
+            m_context->error_state = GIFLoadingContext::ErrorState::FailedToDecodeAnyFrame;
+            return result.release_error();
+        }
+        m_context->error_state = GIFLoadingContext::ErrorState::FailedToDecodeAllFrames;
     }
 
     ImageFrameDescriptor frame {};

Tests/LibGfx/TestImageDecoder.cpp

@@ -93,7 +93,7 @@ TEST_CASE(test_ico_malformed_frame)
 
 TEST_CASE(test_gif)
 {
-    auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("download-animation.gif"sv)));
+    auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("gif/download-animation.gif"sv)));
     EXPECT(Gfx::GIFImageDecoderPlugin::sniff(file->bytes()));
     auto plugin_decoder = TRY_OR_FAIL(Gfx::GIFImageDecoderPlugin::create(file->bytes()));
 
@@ -105,6 +105,16 @@ TEST_CASE(test_gif)
     EXPECT(frame.duration == 400);
 }
 
+TEST_CASE(test_corrupted_gif)
+{
+    auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("gif/corrupted.gif"sv)));
+    EXPECT(Gfx::GIFImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = TRY_OR_FAIL(Gfx::GIFImageDecoderPlugin::create(file->bytes()));
+
+    auto frame = TRY_OR_FAIL(plugin_decoder->frame(0));
+    EXPECT_EQ(plugin_decoder->frame_count(), 1u);
+}
+
 TEST_CASE(test_gif_without_global_color_table)
 {
     Array<u8, 35> gif_data {