AK: Move ConstrainedStream from LibWasm and limit discarding

AK/ConstrainedStream.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ * Copyright (c) 2023, Tim Schumacher <timschumi@gmx.de>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/ConstrainedStream.h>
+
+namespace AK {
+
+ConstrainedStream::ConstrainedStream(MaybeOwned<Stream> stream, u64 limit)
+    : m_stream(move(stream))
+    , m_limit(limit)
+{
+}
+
+ErrorOr<Bytes> ConstrainedStream::read_some(Bytes bytes)
+{
+    if (bytes.size() >= m_limit)
+        bytes = bytes.trim(m_limit);
+
+    auto result = TRY(m_stream->read_some(bytes));
+
+    m_limit -= result.size();
+
+    return result;
+}
+
+ErrorOr<void> ConstrainedStream::discard(size_t discarded_bytes)
+{
+    if (discarded_bytes >= m_limit)
+        return Error::from_string_literal("Trying to discard more bytes than allowed");
+
+    // Note: This will remove more bytes from the limit than intended if a failing discard yields partial results.
+    //       However, stopping early is most likely better than running over the end of the stream.
+    m_limit -= discarded_bytes;
+    TRY(m_stream->discard(discarded_bytes));
+
+    return {};
+}
+
+ErrorOr<size_t> ConstrainedStream::write_some(ReadonlyBytes)
+{
+    return Error::from_errno(EBADF);
+}
+
+bool ConstrainedStream::is_eof() const
+{
+    return m_limit == 0 || m_stream->is_eof();
+}
+
+bool ConstrainedStream::is_open() const
+{
+    return m_stream->is_open();
+}
+
+void ConstrainedStream::close()
+{
+}
+
+}