Revert "Kernel: Make DoubleBuffer use a KBuffer instead of kmalloc()ing"

This reverts commit 1cca5142afbd76833deedfdb238230ac53424855.

This appears to be causing intermittent triple-faults and I don't know
why yet, so I'll just revert it to keep the tree in decent shape.

Kernel/DoubleBuffer.cpp

@@ -1,32 +1,20 @@
 #include <Kernel/DoubleBuffer.h>
 
-inline void DoubleBuffer::compute_lockfree_metadata()
+inline void DoubleBuffer::compute_emptiness()
 {
-    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;
-}
-
-DoubleBuffer::DoubleBuffer(size_t capacity)
-    : m_write_buffer(&m_buffer1)
-    , m_read_buffer(&m_buffer2)
-    , m_storage(KBuffer::create_with_size(capacity * 2))
-    , 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;
+    m_empty = m_read_buffer_index >= m_read_buffer->size() && m_write_buffer->is_empty();
 }
 
 void DoubleBuffer::flip()
 {
-    ASSERT(m_read_buffer_index == m_read_buffer->size);
+    ASSERT(m_read_buffer_index == m_read_buffer->size());
     swap(m_read_buffer, m_write_buffer);
-    m_write_buffer->size = 0;
+    if (m_write_buffer->capacity() < 32)
+        m_write_buffer->clear_with_capacity();
+    else
+        m_write_buffer->clear();
     m_read_buffer_index = 0;
-    compute_lockfree_metadata();
+    compute_emptiness();
 }
 
 ssize_t DoubleBuffer::write(const u8* data, ssize_t size)
@@ -34,11 +22,8 @@ ssize_t DoubleBuffer::write(const u8* data, ssize_t size)
     if (!size)
         return 0;
     LOCKER(m_lock);
-    ASSERT(size <= (ssize_t)space_for_writing());
-    u8* write_ptr = m_write_buffer->data + m_write_buffer->size;
-    m_write_buffer->size += size;
-    compute_lockfree_metadata();
-    memcpy(write_ptr, data, size);
+    m_write_buffer->append(data, size);
+    compute_emptiness();
     return size;
 }
 
@@ -47,13 +32,13 @@ ssize_t DoubleBuffer::read(u8* data, ssize_t size)
     if (!size)
         return 0;
     LOCKER(m_lock);
-    if (m_read_buffer_index >= m_read_buffer->size && m_write_buffer->size != 0)
+    if (m_read_buffer_index >= m_read_buffer->size() && !m_write_buffer->is_empty())
         flip();
-    if (m_read_buffer_index >= m_read_buffer->size)
+    if (m_read_buffer_index >= m_read_buffer->size())
         return 0;
-    ssize_t nread = min((ssize_t)m_read_buffer->size - (ssize_t)m_read_buffer_index, size);
-    memcpy(data, m_read_buffer->data + m_read_buffer_index, nread);
+    ssize_t nread = min((ssize_t)m_read_buffer->size() - m_read_buffer_index, size);
+    memcpy(data, m_read_buffer->data() + m_read_buffer_index, nread);
     m_read_buffer_index += nread;
-    compute_lockfree_metadata();
+    compute_emptiness();
     return nread;
 }

Kernel/DoubleBuffer.h

@@ -1,38 +1,34 @@
 #pragma once
 
 #include <AK/Types.h>
-#include <Kernel/KBuffer.h>
+#include <AK/Vector.h>
 #include <Kernel/Lock.h>
 
 class DoubleBuffer {
 public:
-    explicit DoubleBuffer(size_t capacity = 65536);
+    DoubleBuffer()
+        : m_write_buffer(&m_buffer1)
+        , m_read_buffer(&m_buffer2)
+    {
+    }
 
     ssize_t write(const u8*, ssize_t);
     ssize_t read(u8*, ssize_t);
 
     bool is_empty() const { return m_empty; }
 
-    size_t space_for_writing() const { return m_space_for_writing; }
+    // FIXME: Isn't this racy? What if we get interrupted between getting the buffer pointer and dereferencing it?
+    ssize_t bytes_in_write_buffer() const { return (ssize_t)m_write_buffer->size(); }
 
 private:
     void flip();
-    void compute_lockfree_metadata();
+    void compute_emptiness();
 
-    struct InnerBuffer {
-        u8* data { nullptr };
-        size_t size;
-    };
-
-    InnerBuffer* m_write_buffer { nullptr };
-    InnerBuffer* m_read_buffer { nullptr };
-    InnerBuffer m_buffer1;
-    InnerBuffer m_buffer2;
-
-    KBuffer m_storage;
-    size_t m_capacity { 0 };
-    size_t m_read_buffer_index { 0 };
-    size_t m_space_for_writing { 0 };
+    Vector<u8>* m_write_buffer { nullptr };
+    Vector<u8>* m_read_buffer { nullptr };
+    Vector<u8> m_buffer1;
+    Vector<u8> m_buffer2;
+    ssize_t m_read_buffer_index { 0 };
     bool m_empty { true };
-    mutable Lock m_lock { "DoubleBuffer" };
+    Lock m_lock { "DoubleBuffer" };
 };

Kernel/FileSystem/FIFO.cpp

@@ -90,7 +90,7 @@ bool FIFO::can_read(FileDescription&) const
 
 bool FIFO::can_write(FileDescription&) const
 {
-    return m_buffer.space_for_writing() || !m_readers;
+    return m_buffer.bytes_in_write_buffer() < 4096 || !m_readers;
 }
 
 ssize_t FIFO::read(FileDescription&, u8* buffer, ssize_t size)

Kernel/Net/LocalSocket.cpp

@@ -210,9 +210,9 @@ bool LocalSocket::can_write(FileDescription& description) const
 {
     auto role = this->role(description);
     if (role == Role::Accepted)
-        return !has_attached_peer(description) || m_for_client.space_for_writing();
+        return !has_attached_peer(description) || m_for_client.bytes_in_write_buffer() < 16384;
     if (role == Role::Connected)
-        return !has_attached_peer(description) || m_for_server.space_for_writing();
+        return !has_attached_peer(description) || m_for_server.bytes_in_write_buffer() < 16384;
     ASSERT_NOT_REACHED();
 }
 

Kernel/TTY/MasterPTY.cpp

@@ -81,7 +81,7 @@ bool MasterPTY::can_write_from_slave() const
 {
     if (m_closed)
         return true;
-    return m_buffer.space_for_writing();
+    return m_buffer.bytes_in_write_buffer() < 4096;
 }
 
 void MasterPTY::close()