Add a new mDNS backend which uses mDNSResponder

This backend uses mDNSResponder, like the external-dns-sd one. However,
this one uses the dns-sd library instead of relying on an external
program.

mdns_dns_sd.c

@@ -0,0 +1,98 @@
+/*
+ * Embedded dns-sd client. This file is part of Shairport.
+ * Copyright (c) Paul Lietar 2013
+ * All rights reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <dns_sd.h>
+#include <string.h>
+#include "mdns.h"
+#include "common.h"
+
+static DNSServiceRef service;
+
+static int mdns_dns_sd_register(char *apname, int port) {
+    const char *record[] = { MDNS_RECORD, NULL };
+    uint16_t length = 0;
+
+    // Concatenate string contained i record into buf.
+
+    for (const char **field = record; *field; field ++)
+    {
+        length += strlen(*field) + 1; // One byte for length each time
+    } 
+
+    char *buf = malloc(length * sizeof(char));
+    if (buf == NULL)
+    {
+        warn("dns_sd: buffer record allocation failed");
+        return -1;
+    }
+
+    char *p = buf;
+
+    for (const char **field = record; *field; field ++)
+    {
+        char * newp = stpcpy(p + 1, *field);
+        *p = newp - p - 1;
+        p = newp;
+    }
+
+    DNSServiceErrorType error;
+    error = DNSServiceRegister(&service,
+                               0,
+                               kDNSServiceInterfaceIndexAny,
+                               apname,
+                               "_raop._tcp",
+                               "",
+                               NULL,
+                               htons((uint16_t)port),
+                               length,
+                               buf,
+                               NULL,
+                               NULL);
+
+    free(buf);
+
+    if (error == kDNSServiceErr_NoError)
+        return 0;
+    else
+    {
+        warn("dns-sd: DNSServiceRegister error %d", error);
+        return -1;
+    }
+}
+
+static void mdns_dns_sd_unregister(void) {
+    if (service)
+    {
+        DNSServiceRefDeallocate(service);
+        service = NULL;
+    }
+}
+
+mdns_backend mdns_dns_sd = {
+    .name = "dns-sd",
+    .mdns_register = mdns_dns_sd_register,
+    .mdns_unregister = mdns_dns_sd_unregister
+};