Everywhere: Move global Kernel pattern code to Kernel/Library directory

This has KString, KBuffer, DoubleBuffer, KBufferBuilder, IOWindow,
UserOrKernelBuffer and ScopedCritical classes being moved to the
Kernel/Library subdirectory.

Also, move the panic and assertions handling code to that directory.

Kernel/Library/DoubleBuffer.cpp

@@ -0,0 +1,93 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringView.h>
+#include <Kernel/InterruptDisabler.h>
+#include <Kernel/Library/DoubleBuffer.h>
+
+namespace Kernel {
+
+inline void DoubleBuffer::compute_lockfree_metadata()
+{
+    InterruptDisabler disabler;
+    m_empty = m_read_buffer_index >= m_read_buffer->size && m_write_buffer->size == 0;
+    m_space_for_writing = m_capacity - m_write_buffer->size;
+}
+
+ErrorOr<NonnullOwnPtr<DoubleBuffer>> DoubleBuffer::try_create(StringView name, size_t capacity)
+{
+    auto storage = TRY(KBuffer::try_create_with_size(name, capacity * 2, Memory::Region::Access::ReadWrite));
+    return adopt_nonnull_own_or_enomem(new (nothrow) DoubleBuffer(capacity, move(storage)));
+}
+
+DoubleBuffer::DoubleBuffer(size_t capacity, NonnullOwnPtr<KBuffer> storage)
+    : m_write_buffer(&m_buffer1)
+    , m_read_buffer(&m_buffer2)
+    , m_storage(move(storage))
+    , m_capacity(capacity)
+{
+    m_buffer1.data = m_storage->data();
+    m_buffer1.size = 0;
+    m_buffer2.data = m_storage->data() + capacity;
+    m_buffer2.size = 0;
+    m_space_for_writing = capacity;
+}
+
+void DoubleBuffer::flip()
+{
+    VERIFY(m_read_buffer_index == m_read_buffer->size);
+    swap(m_read_buffer, m_write_buffer);
+    m_write_buffer->size = 0;
+    m_read_buffer_index = 0;
+    compute_lockfree_metadata();
+}
+
+ErrorOr<size_t> DoubleBuffer::write(UserOrKernelBuffer const& data, size_t size)
+{
+    if (!size)
+        return 0;
+    MutexLocker locker(m_lock);
+    size_t bytes_to_write = min(size, m_space_for_writing);
+    u8* write_ptr = m_write_buffer->data + m_write_buffer->size;
+    TRY(data.read(write_ptr, bytes_to_write));
+    m_write_buffer->size += bytes_to_write;
+    compute_lockfree_metadata();
+    if (m_unblock_callback && !m_empty)
+        m_unblock_callback();
+    return bytes_to_write;
+}
+
+ErrorOr<size_t> DoubleBuffer::read_impl(UserOrKernelBuffer& data, size_t size, MutexLocker&, bool advance_buffer_index)
+{
+    if (size == 0)
+        return 0;
+    if (m_read_buffer_index >= m_read_buffer->size && m_write_buffer->size != 0)
+        flip();
+    if (m_read_buffer_index >= m_read_buffer->size)
+        return 0;
+    size_t nread = min(m_read_buffer->size - m_read_buffer_index, size);
+    TRY(data.write(m_read_buffer->data + m_read_buffer_index, nread));
+    if (advance_buffer_index)
+        m_read_buffer_index += nread;
+    compute_lockfree_metadata();
+    if (m_unblock_callback && m_space_for_writing > 0)
+        m_unblock_callback();
+    return nread;
+}
+
+ErrorOr<size_t> DoubleBuffer::read(UserOrKernelBuffer& data, size_t size)
+{
+    MutexLocker locker(m_lock);
+    return read_impl(data, size, locker, true);
+}
+
+ErrorOr<size_t> DoubleBuffer::peek(UserOrKernelBuffer& data, size_t size)
+{
+    MutexLocker locker(m_lock);
+    return read_impl(data, size, locker, false);
+}
+
+}