[srv.init] Serve a service locator protocol from init

The init process now serves as a service locator for its children,
passing all children a mailbox handle on which it is serving the service
locator protocol.

src/user/srv.init/service_locator.cpp

@@ -0,0 +1,97 @@
+#include <unordered_map>
+
+#include <j6/errors.h>
+#include <j6/flags.h>
+#include <j6/types.h>
+#include <j6/protocols/service_locator.h>
+#include <j6/syscalls.h>
+#include <util/map.h>
+
+#include "service_locator.h"
+
+void
+service_locator_start(j6_handle_t mb)
+{
+    // TODO: This should be a multimap
+    util::map<uint64_t, j6_handle_t> services;
+
+    uint64_t tag;
+    uint16_t reply_tag;
+
+    uint8_t data[100];
+    size_t data_len = sizeof(data);
+
+    j6_handle_t handles[10];
+    size_t handles_count = sizeof(handles)/sizeof(j6_handle_t);
+
+    j6_status_t s = j6_mailbox_receive(mb, &tag,
+            data, &data_len,
+            handles, &handles_count,
+            &reply_tag, nullptr,
+            j6_mailbox_block);
+
+    uint64_t proto_id;
+    uint64_t *data_words = reinterpret_cast<uint64_t*>(data);
+
+    while (true) {
+        j6_handle_t *found = nullptr;
+
+        switch (tag) {
+        case j6_proto_base_get_proto_id:
+            tag = j6_proto_base_proto_id;
+            *data_words = j6_proto_sl_id;
+            data_len = sizeof(j6_status_t);
+            handles_count = 0;
+            break;
+
+        case j6_proto_sl_register:
+            proto_id = *data_words;
+            if (handles_count != 1) {
+                tag = j6_proto_base_status;
+                *data_words = j6_err_invalid_arg;
+                data_len = sizeof(j6_status_t);
+                handles_count = 0;
+                break;
+            }
+
+            services.insert( proto_id, handles[0] );
+            tag = j6_proto_base_status;
+            *data_words = j6_status_ok;
+            data_len = sizeof(j6_status_t);
+            handles_count = 0;
+            break;
+
+        case j6_proto_sl_find:
+            proto_id = *data_words;
+            tag = j6_proto_sl_result;
+            data_len = 0;
+
+            found = services.find(proto_id);
+            if (found) {
+                handles_count = 1;
+                handles[0] = *found;
+            } else {
+                handles_count = 0;
+            }
+            break;
+
+        default:
+            tag = j6_proto_base_status;
+            *data_words = j6_err_invalid_arg;
+            data_len = sizeof(j6_status_t);
+            handles_count = 0;
+            break;
+        }
+
+        size_t data_in = data_len;
+        size_t handles_in = handles_count;
+        data_len = sizeof(data);
+        handles_count = sizeof(handles)/sizeof(j6_handle_t);
+
+        s = j6_mailbox_respond_receive(mb, &tag,
+                data, &data_len, data_in,
+                handles, &handles_count, handles_in,
+                &reply_tag, nullptr,
+                j6_mailbox_block);
+    }
+}