Resolver: added the "async" flag to resolver context.

The flag indicates that the resolve handler is called asynchronously after the
resolve function ngx_resolve_name()/ngx_resolve_addr() exited.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index a18fd1f..91f8a5e 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -746,6 +746,7 @@
             last->next = rn->waiting;
             rn->waiting = ctx;
             ctx->state = NGX_AGAIN;
+            ctx->async = 1;
 
             do {
                 ctx->node = rn;
@@ -892,6 +893,7 @@
     rn->waiting = ctx;
 
     ctx->state = NGX_AGAIN;
+    ctx->async = 1;
 
     do {
         ctx->node = rn;
@@ -1023,6 +1025,7 @@
             ctx->next = rn->waiting;
             rn->waiting = ctx;
             ctx->state = NGX_AGAIN;
+            ctx->async = 1;
             ctx->node = rn;
 
             /* unlock addr mutex */
@@ -1119,6 +1122,7 @@
     /* unlock addr mutex */
 
     ctx->state = NGX_AGAIN;
+    ctx->async = 1;
     ctx->node = rn;
 
     return NGX_OK;
@@ -3019,6 +3023,7 @@
     srv = cctx->srvs;
 
     ctx->count--;
+    ctx->async |= cctx->async;
 
     srv->ctx = NULL;
     srv->state = cctx->state;

src/core/ngx_resolver.h

diff --git a/src/core/ngx_resolver.h b/src/core/ngx_resolver.h
index a0d6fc3..6f099b7 100644
--- a/src/core/ngx_resolver.h
+++ b/src/core/ngx_resolver.h
@@ -218,7 +218,8 @@
     void                     *data;
     ngx_msec_t                timeout;
 
-    ngx_uint_t                quick;  /* unsigned  quick:1; */
+    unsigned                  quick:1;
+    unsigned                  async:1;
     ngx_uint_t                recursion;
     ngx_event_t              *event;
 };