Kernel: Make pseudo terminals not overwrite old data

If pseudo terminal buffer was filled, old implementation would overwrite
old data. This is bad if producer is capable of producing more data than
consumer can handle.

kernel/kernel/Terminal/PseudoTerminal.cpp

@@ -85,16 +85,19 @@ namespace Kernel
 		return BAN::Error::from_errno(ENODEV);
 	}
 
-	void PseudoTerminalMaster::putchar(uint8_t ch)
+	bool PseudoTerminalMaster::putchar(uint8_t ch)
 	{
 		SpinLockGuard _(m_buffer_lock);
 
-		reinterpret_cast<uint8_t*>(m_buffer->vaddr())[(m_buffer_tail + m_buffer_size) % m_buffer->size()] = ch;
+		if (m_buffer_size >= m_buffer->size())
+			return false;
 
-		if (m_buffer_size < m_buffer->size())
-			m_buffer_size++;
-		else
-			m_buffer_tail = (m_buffer_tail + 1) % m_buffer->size();
+		reinterpret_cast<uint8_t*>(m_buffer->vaddr())[(m_buffer_tail + m_buffer_size) % m_buffer->size()] = ch;
+		m_buffer_size++;
+
+		m_buffer_blocker.unblock();
+
+		return true;
 	}
 
 	BAN::ErrorOr<size_t> PseudoTerminalMaster::read_impl(off_t, BAN::ByteSpan buffer)
@@ -166,10 +169,11 @@ namespace Kernel
 		(void)write_impl(0, BAN::ConstByteSpan::from(message));
 	}
 
-	void PseudoTerminalSlave::putchar_impl(uint8_t ch)
+	bool PseudoTerminalSlave::putchar_impl(uint8_t ch)
 	{
 		if (auto master = m_master.lock())
-			master->putchar(ch);
+			return master->putchar(ch);
+		return false;
 	}
 
 	BAN::ErrorOr<long> PseudoTerminalSlave::ioctl_impl(int request, void* argument)