LibWeb: Implement FileReaderSync interface

This interface allows the user to read File or Blob objects
synchronously from inside workers.

Libraries/LibWeb/FileAPI/FileReaderSync.cpp

@@ -0,0 +1,106 @@
+/*
+ * Copyright (c) 2025, Tim Ledbetter <tim.ledbetter@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/ArrayBuffer.h>
+#include <LibJS/Runtime/Realm.h>
+#include <LibJS/Runtime/TypedArray.h>
+#include <LibWeb/Bindings/FileReaderSyncPrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/FileAPI/Blob.h>
+#include <LibWeb/FileAPI/FileReaderSync.h>
+#include <LibWeb/HTML/EventLoop/EventLoop.h>
+#include <LibWeb/Streams/AbstractOperations.h>
+#include <LibWeb/Streams/ReadableStreamDefaultReader.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::FileAPI {
+
+GC_DEFINE_ALLOCATOR(FileReaderSync);
+
+FileReaderSync::~FileReaderSync() = default;
+
+FileReaderSync::FileReaderSync(JS::Realm& realm)
+    : PlatformObject(realm)
+{
+}
+
+void FileReaderSync::initialize(JS::Realm& realm)
+{
+    Base::initialize(realm);
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(FileReaderSync);
+}
+
+GC::Ref<FileReaderSync> FileReaderSync::create(JS::Realm& realm)
+{
+    return realm.create<FileReaderSync>(realm);
+}
+
+GC::Ref<FileReaderSync> FileReaderSync::construct_impl(JS::Realm& realm)
+{
+    return FileReaderSync::create(realm);
+}
+
+// https://w3c.github.io/FileAPI/#dfn-readAsArrayBufferSync
+WebIDL::ExceptionOr<GC::Root<JS::ArrayBuffer>> FileReaderSync::read_as_array_buffer(Blob& blob)
+{
+    return read_as<GC::Root<JS::ArrayBuffer>>(blob, FileReader::Type::ArrayBuffer);
+}
+
+// https://w3c.github.io/FileAPI/#dfn-readAsBinaryStringSync
+WebIDL::ExceptionOr<String> FileReaderSync::read_as_binary_string(Blob& blob)
+{
+    return read_as<String>(blob, FileReader::Type::BinaryString);
+}
+
+// https://w3c.github.io/FileAPI/#dfn-readAsTextSync
+WebIDL::ExceptionOr<String> FileReaderSync::read_as_text(Blob& blob, Optional<String> const& encoding)
+{
+    return read_as<String>(blob, FileReader::Type::Text, encoding);
+}
+
+// https://w3c.github.io/FileAPI/#dfn-readAsDataURLSync
+WebIDL::ExceptionOr<String> FileReaderSync::read_as_data_url(Blob& blob)
+{
+    return read_as<String>(blob, FileReader::Type::DataURL);
+}
+
+template<typename Result>
+WebIDL::ExceptionOr<Result> FileReaderSync::read_as(Blob& blob, FileReader::Type type, Optional<String> const& encoding)
+{
+    // 1. Let stream be the result of calling get stream on blob.
+    auto stream = blob.get_stream();
+
+    // 2. Let reader be the result of getting a reader from stream.
+    auto reader = TRY(stream->get_a_reader());
+
+    // 3. Let promise be the result of reading all bytes from stream with reader.
+    auto promise_capability = reader->read_all_bytes_deprecated();
+
+    // FIXME: Try harder to not reach into promise's [[Promise]] slot
+    auto promise = GC::Ref { as<JS::Promise>(*promise_capability->promise()) };
+
+    // 4. Wait for promise to be fulfilled or rejected.
+    // FIXME: Create spec issue to use WebIDL react to promise steps here instead of this custom logic
+    HTML::main_thread_event_loop().spin_until(GC::create_function(heap(), [promise]() {
+        return promise->state() == JS::Promise::State::Fulfilled || promise->state() == JS::Promise::State::Rejected;
+    }));
+
+    // 5. If promise fulfilled with a byte sequence bytes:
+    auto result = promise->result();
+    auto* array_buffer = result.extract_pointer<JS::ArrayBuffer>();
+    if (promise->state() == JS::Promise::State::Fulfilled && array_buffer) {
+        // AD-HOC: This diverges from the spec as wrritten, where the type argument is specified explicitly for each caller.
+        // 1. Return the result of package data given bytes, type, blob’s type, and encoding.
+        auto result = TRY(FileReader::blob_package_data(realm(), array_buffer->buffer(), type, blob.type(), encoding));
+        return result.get<Result>();
+    }
+
+    // 6. Throw promise’s rejection reason.
+    VERIFY(promise->state() == JS::Promise::State::Rejected);
+    return JS::throw_completion(result);
+}
+
+}