[PATCH] ignore ipv6=off resolver option when no ipv6 support

[Thibault Charbonnier]

Hello,

Please let me know how you feel about this relatively simple patch which
aims at making the "resolver" directive more robust with regards to the
"ipv6=" option when Nginx is built without IPv6 support.

Thanks!

# HG changeset patch
# User Thibault Charbonnier <thibaultcha at fastmail.com>
# Date 1481847421 28800
#      Thu Dec 15 16:17:01 2016 -0800
# Node ID 8bf038fe006fd8ae253d6b41fc6cf109a8912d3e
# Parent  a3dc657f4e9530623683e6b85bd7492662e4dc47
Resolver: ignore ipv6=off resolver option when no ipv6 support

Makes the resolver directive more robust: we only error out when ipv6
resolution is desired but not supported (ipv6=on).

use case 1: some configurations are sometimes re-used between builds 
with and
without ipv6 support. This patch avoids the need to remove the 
"ipv6=off" flag.

use case 2: currently, some tools rely on the --with-ipv6 configure 
option from
"nginx -V" to determine if ipv6 resolution should be disabled in some cases.
With this option disappearing in Nginx 1.11.5, this patch would allow 
such tools
to assume "ipv6=off" to be safe regardless of ipv6 support in the current
build.

diff -r a3dc657f4e95 -r 8bf038fe006f src/core/ngx_resolver.c
--- a/src/core/ngx_resolver.c    Thu Dec 15 21:44:34 2016 +0300
+++ b/src/core/ngx_resolver.c    Thu Dec 15 16:17:01 2016 -0800
@@ -224,14 +224,22 @@
                continue;
            }

-#if (NGX_HAVE_INET6)
            if (ngx_strncmp(names[i].data, "ipv6=", 5) == 0) {

                if (ngx_strcmp(&names[i].data[5], "on") == 0) {
+#if (NGX_HAVE_INET6)
                    r->ipv6 = 1;
+#else
+                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
+                                   "no ipv6 support but \"%V\" in 
resolver",
+                                   &names[i]);
+                return NULL;
+#endif

                } else if (ngx_strcmp(&names[i].data[5], "off") == 0) {
+#if (NGX_HAVE_INET6)
                    r->ipv6 = 0;
+#endif

                } else {
                    ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
@@ -241,7 +249,6 @@

                continue;
            }
-#endif

            ngx_memzero(&u, sizeof(ngx_url_t));