Add primitive FIFO and hook it up to sys$pipe().

It's now possible to do this in bash:

cat kernel.map | fgrep List

This is very cool! :^)

Kernel/FIFO.cpp

@@ -0,0 +1,87 @@
+#include "FIFO.h"
+#include <AK/StdLib.h>
+
+//#define FIFO_DEBUG
+
+RetainPtr<FIFO> FIFO::create()
+{
+    return adopt(*new FIFO);
+}
+
+FIFO::FIFO()
+{
+}
+
+void FIFO::open(Direction direction)
+{
+    if (direction == Reader) {
+        ++m_readers;
+#ifdef FIFO_DEBUG
+        kprintf("open reader (%u)\n", m_readers);
+#endif
+    } else if (direction == Writer) {
+        ++m_writers;
+#ifdef FIFO_DEBUG
+        kprintf("open writer (%u)\n", m_writers);
+#endif
+    }
+}
+
+void FIFO::close(Direction direction)
+{
+    if (direction == Reader) {
+#ifdef FIFO_DEBUG
+        kprintf("close reader (%u - 1)\n", m_readers);
+#endif
+        ASSERT(m_readers);
+        --m_readers;
+    } else if (direction == Writer) {
+#ifdef FIFO_DEBUG
+        kprintf("close writer (%u - 1)\n", m_writers);
+#endif
+        ASSERT(m_writers);
+        --m_writers;
+    }
+}
+
+bool FIFO::can_read() const
+{
+    return !m_queue.isEmpty() || !m_writers;
+}
+
+bool FIFO::can_write() const
+{
+#ifdef FIFO_DEBUG
+    dbgprintf("can_write? size(%u) < capacity(%u) || !readers(%u)\n", m_queue.size(), m_queue.capacity(), m_readers);
+#endif
+    return m_queue.size() < m_queue.capacity() || !m_readers;
+}
+
+ssize_t FIFO::read(byte* buffer, size_t size)
+{
+    if (!m_writers && m_queue.isEmpty())
+        return 0;
+#ifdef FIFO_DEBUG
+    dbgprintf("fifo: read(%u)\n",size);
+#endif
+    size_t nread = min(size, m_queue.size());
+    for (size_t i = 0; i < nread; ++i)
+        buffer[i] = m_queue.dequeue();
+#ifdef FIFO_DEBUG
+    dbgprintf("   -> read (%c) %u\n", buffer[0], nread);
+#endif
+    return nread;
+}
+
+ssize_t FIFO::write(const byte* buffer, size_t size)
+{
+    if (!m_readers)
+        return 0;
+#ifdef FIFO_DEBUG
+    dbgprintf("fifo: write(%p, %u)\n", buffer, size);
+#endif
+    size_t nwritten = min(size, m_queue.capacity() - m_queue.size());
+    for (size_t i = 0; i < nwritten; ++i)
+        m_queue.enqueue(buffer[i]);
+    return nwritten;
+}