[kernel] Replace endpoint with new mailbox API

The new mailbox kernel object API offers asynchronous message-based IPC
for sending data and handles between threads, as opposed to endpoint's
synchronous model.

src/kernel/objects/mailbox.cpp

@@ -0,0 +1,123 @@
+#include <util/counted.h>
+
+#include "objects/handle.h"
+#include "objects/mailbox.h"
+#include "objects/thread.h"
+
+DEFINE_SLAB_ALLOCATOR(obj::mailbox::message);
+
+namespace obj {
+
+static_assert(mailbox::message::slab_size % sizeof(mailbox::message) == 0,
+        "mailbox message size does not fit cleanly into N pages.");
+
+constexpr uint64_t no_message = 1;
+
+mailbox::mailbox() :
+    kobject(kobject::type::mailbox),
+    m_closed {false},
+    m_next_reply_tag {0}
+{
+}
+
+mailbox::~mailbox()
+{
+    close();
+}
+
+void
+mailbox::close()
+{
+    util::scoped_lock lock {m_message_lock};
+
+    // If this was previously closed, we're done
+    if (closed()) return;
+    m_closed = true;
+
+    while (!m_messages.empty()) {
+        message *msg = m_messages.pop_front();
+        delete msg;
+    }
+
+    for (auto &p : m_pending) {
+        delete p.val.msg;
+        p.val.sender->wake(no_message);
+    }
+}
+
+void
+mailbox::send(message *msg)
+{
+    util::scoped_lock lock {m_message_lock};
+    m_messages.push_back(msg);
+
+    thread *t = m_queue.pop_next();
+    if (t) t->wake();
+}
+
+bool
+mailbox::call(message *msg)
+{
+    util::scoped_lock lock {m_message_lock};
+
+    if (!++m_next_reply_tag) ++m_next_reply_tag;
+    msg->reply_tag = m_next_reply_tag;
+
+    thread &current = thread::current();
+    m_pending.insert(m_next_reply_tag, {&current, msg});
+
+    m_messages.push_back(msg);
+
+    thread *t = m_queue.pop_next();
+    if (t) t->wake();
+
+    return (current.block() != no_message);
+}
+
+bool
+mailbox::receive(mailbox::message *&msg, bool block)
+{
+    util::scoped_lock lock {m_message_lock};
+
+    // This needs to be a loop because we're re-acquiring the lock
+    // after waking up, and may have missed the message that woke us
+    while (m_messages.empty()) {
+        if (!block) {
+            msg = nullptr;
+            return false;
+        }
+
+        thread &cur = thread::current();
+        m_queue.add_thread(&cur);
+
+        lock.release();
+        uint64_t result = cur.block();
+        if (result == no_message)
+            return false;
+        lock.reacquire();
+    }
+
+    msg = m_messages.pop_front();
+    return true;
+}
+
+mailbox::replyer
+mailbox::reply(uint16_t reply_tag)
+{
+    util::scoped_lock lock {m_message_lock};
+
+    pending *p = m_pending.find(reply_tag);
+    if (!p) return {};
+
+    thread *caller = p->sender;
+    message *msg = p->msg;
+    m_pending.erase(reply_tag);
+
+    return {msg, caller};
+}
+
+mailbox::replyer::~replyer()
+{
+}
+
+} // namespace obj