nm-daemon-helper: add "service" argument

Introduce a new argument to specify a comma-separated list of NSS
services to use for the "resolve-address" command. For now only accept
"dns" and "files"; the latter can be used to do a lookup into
/etc/hosts.

Note that previously the command failed in presence of extra
arguments. Therefore, when downgrading NetworkManager without
restarting the service, the previously-installed version of the daemon
(newer) would spawn the helper with the extra argument, and the
newly-installed version of the helper (older) would fail. This issue
only impacts hostname resolution and can be fixed by just restarting
the daemon.

In the upgrade path everything works as before, with the only
difference that the helper will use by default both "dns" and "files"
services.

Don't strictly check for the absence of extra arguments, so that in
the future we can introduce more arguments without necessarily break
the downgrade path.

src/nm-daemon-helper/nm-daemon-helper.c

@@ -55,26 +55,31 @@ cmd_version(void)
 static int
 cmd_resolve_address(void)
 {
-    nm_auto_free char *address = NULL;
+    nm_auto_free char *address  = NULL;
+    nm_auto_free char *services = NULL;
     union {
         struct sockaddr_in  in;
         struct sockaddr_in6 in6;
     } sockaddr;
     socklen_t sockaddr_size;
     char      name[NI_MAXHOST];
+    char     *saveptr = NULL;
+    char     *service;
+    char     *str;
     int       ret;
 
     address = read_arg();
     if (!address)
         return RETURN_INVALID_ARGS;
 
-    if (more_args())
+    services = read_arg();
-        return RETURN_INVALID_ARGS;
+    if (!services) {
+        /* Called by an old NM version which doesn't support the 'services'
+         * argument. Use both services. */
+        services = strdup("dns,files");
+    }
 
     memset(&sockaddr, 0, sizeof(sockaddr));
-#if defined(__GLIBC__)
-    __nss_configure_lookup("hosts", "dns");
-#endif
 
     if (inet_pton(AF_INET, address, &sockaddr.in.sin_addr) == 1) {
         sockaddr.in.sin_family = AF_INET;
@@ -85,33 +90,51 @@ cmd_resolve_address(void)
     } else
         return RETURN_INVALID_ARGS;
 
-    ret = getnameinfo((struct sockaddr *) &sockaddr,
+    for (str = services; (service = strtok_r(str, ",", &saveptr)); str = NULL) {
-                      sockaddr_size,
+        if (!NM_IN_STRSET(service, "dns", "files")) {
-                      name,
+            fprintf(stderr, "Unsupported resolver service '%s'\n", service);
-                      sizeof(name),
+            continue;
-                      NULL,
+        }
-                      0,
+
-                      NI_NAMEREQD);
+#if defined(__GLIBC__)
-    if (ret != 0) {
+        __nss_configure_lookup("hosts", service);
-        if (ret == EAI_SYSTEM) {
+#endif
-            int  errsv = errno;
+
+        ret = getnameinfo((struct sockaddr *) &sockaddr,
+                          sockaddr_size,
+                          name,
+                          sizeof(name),
+                          NULL,
+                          0,
+                          NI_NAMEREQD);
+
+        if (ret == 0) {
+            printf("%s", name);
+            return RETURN_SUCCESS;
+        } else if (ret == EAI_SYSTEM) {
             char buf[1024];
+            int  errsv = errno;
 
             fprintf(stderr,
-                    "getnameinfo() failed: %d (%s), system error: %d (%s)\n",
+                    "getnameinfo() via service '%s' failed: %d (%s), system error: %d (%s)\n",
+                    service,
                     ret,
                     gai_strerror(ret),
                     errsv,
                     _nm_strerror_r(errsv, buf, sizeof(buf)));
         } else {
-            fprintf(stderr, "getnameinfo() failed: %d (%s)\n", ret, gai_strerror(ret));
+            fprintf(stderr,
+                    "getnameinfo() via service '%s' failed: %d (%s)\n",
+                    service,
+                    ret,
+                    gai_strerror(ret));
         }
-        return RETURN_ERROR;
+#if !defined(__GLIBC__)
+        break;
+#endif
     }
 
-    printf("%s", name);
+    return RETURN_ERROR;
-
-    return RETURN_SUCCESS;
 }
 
 int