Kernel: Convert MasterPTY creation to use DoubleBuffer factory

In order to remove the public DoubleBuffer constructor, we need to
convert the callers to the factory instead, allowing the caller to
observe OOM.

Kernel/TTY/MasterPTY.cpp

@@ -16,17 +16,36 @@
 
 namespace Kernel {
 
-MasterPTY::MasterPTY(unsigned index)
+RefPtr<MasterPTY> MasterPTY::try_create(unsigned int index)
+{
+    auto buffer = DoubleBuffer::try_create();
+    if (!buffer)
+        return {};
+
+    auto master_pty = adopt_ref_if_nonnull(new (nothrow) MasterPTY(index, buffer.release_nonnull()));
+    if (!master_pty)
+        return {};
+
+    auto slave_pty = adopt_ref_if_nonnull(new (nothrow) SlavePTY(*master_pty, index));
+    if (!slave_pty)
+        return {};
+
+    master_pty->m_slave = slave_pty;
+
+    return master_pty;
+}
+
+MasterPTY::MasterPTY(unsigned index, NonnullOwnPtr<DoubleBuffer> buffer)
     : CharacterDevice(200, index)
-    , m_slave(adopt_ref(*new SlavePTY(*this, index)))
     , m_index(index)
+    , m_buffer(move(buffer))
 {
     m_pts_name = String::formatted("/dev/pts/{}", m_index);
     auto process = Process::current();
     set_uid(process->uid());
     set_gid(process->gid());
 
-    m_buffer.set_unblock_callback([this]() {
+    m_buffer->set_unblock_callback([this]() {
         if (m_slave)
             evaluate_block_conditions();
     });
@@ -45,9 +64,9 @@ String MasterPTY::pts_name() const
 
 KResultOr<size_t> MasterPTY::read(FileDescription&, u64, UserOrKernelBuffer& buffer, size_t size)
 {
-    if (!m_slave && m_buffer.is_empty())
+    if (!m_slave && m_buffer->is_empty())
         return 0;
-    return m_buffer.read(buffer, size);
+    return m_buffer->read(buffer, size);
 }
 
 KResultOr<size_t> MasterPTY::write(FileDescription&, u64, const UserOrKernelBuffer& buffer, size_t size)
@@ -62,7 +81,7 @@ bool MasterPTY::can_read(const FileDescription&, size_t) const
 {
     if (!m_slave)
         return true;
-    return !m_buffer.is_empty();
+    return !m_buffer->is_empty();
 }
 
 bool MasterPTY::can_write(const FileDescription&, size_t) const
@@ -83,14 +102,14 @@ KResultOr<size_t> MasterPTY::on_slave_write(const UserOrKernelBuffer& data, size
 {
     if (m_closed)
         return EIO;
-    return m_buffer.write(data, size);
+    return m_buffer->write(data, size);
 }
 
 bool MasterPTY::can_write_from_slave() const
 {
     if (m_closed)
         return true;
-    return m_buffer.space_for_writing();
+    return m_buffer->space_for_writing();
 }
 
 KResult MasterPTY::close()

Kernel/TTY/MasterPTY.h

@@ -16,7 +16,7 @@ class SlavePTY;
 
 class MasterPTY final : public CharacterDevice {
 public:
-    explicit MasterPTY(unsigned index);
+    [[nodiscard]] static RefPtr<MasterPTY> try_create(unsigned index);
     virtual ~MasterPTY() override;
 
     unsigned index() const { return m_index; }
@@ -33,6 +33,7 @@ public:
     virtual String device_name() const override;
 
 private:
+    explicit MasterPTY(unsigned index, NonnullOwnPtr<DoubleBuffer> buffer);
     // ^CharacterDevice
     virtual KResultOr<size_t> read(FileDescription&, u64, UserOrKernelBuffer&, size_t) override;
     virtual KResultOr<size_t> write(FileDescription&, u64, const UserOrKernelBuffer&, size_t) override;
@@ -46,7 +47,7 @@ private:
     RefPtr<SlavePTY> m_slave;
     unsigned m_index;
     bool m_closed { false };
-    DoubleBuffer m_buffer;
+    NonnullOwnPtr<DoubleBuffer> m_buffer;
     String m_pts_name;
 };
 

Kernel/TTY/PTYMultiplexer.cpp

@@ -40,7 +40,7 @@ KResultOr<NonnullRefPtr<FileDescription>> PTYMultiplexer::open(int options)
     if (m_freelist.is_empty())
         return EBUSY;
     auto master_index = m_freelist.take_last();
-    auto master = try_create<MasterPTY>(master_index);
+    auto master = MasterPTY::try_create(master_index);
     if (!master)
         return ENOMEM;
     dbgln_if(PTMX_DEBUG, "PTYMultiplexer::open: Vending master {}", master->index());