Stream: the $upstream_status variable.

Keeps status code of upstream server. The status code reflects only
the status of the upstream, not the code in the upstream response.
Status codes of several responses are separated by commas like addresses
in the $upstream_addr variable.
If a server cannot be selected, the variable keeps the 502 status code.

src/stream/ngx_stream_proxy_module.c

@@ -2198,6 +2198,8 @@ ngx_stream_proxy_next_upstream(ngx_stream_session_t *s)
 
     pscf = ngx_stream_get_module_srv_conf(s, ngx_stream_proxy_module);
 
+    u->state->status = NGX_STREAM_BAD_GATEWAY;
+
     timeout = pscf->next_upstream_timeout;
 
     if (u->peer.tries == 0
@@ -2260,6 +2262,10 @@ ngx_stream_proxy_finalize(ngx_stream_session_t *s, ngx_uint_t rc)
             u->state->response_time = ngx_current_msec - u->start_time;
         }
 
+        if (u->state->status == 0) {
+            u->state->status = rc;
+        }
+
         if (pc) {
             u->state->bytes_received = u->received;
             u->state->bytes_sent = pc->sent;

src/stream/ngx_stream_upstream.c

@@ -13,6 +13,8 @@
 static ngx_int_t ngx_stream_upstream_add_variables(ngx_conf_t *cf);
 static ngx_int_t ngx_stream_upstream_addr_variable(ngx_stream_session_t *s,
     ngx_stream_variable_value_t *v, uintptr_t data);
+static ngx_int_t ngx_stream_upstream_status_variable(ngx_stream_session_t *s,
+    ngx_stream_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_stream_upstream_response_time_variable(
     ngx_stream_session_t *s, ngx_stream_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_stream_upstream_bytes_variable(ngx_stream_session_t *s,
@@ -103,6 +105,10 @@ static ngx_stream_variable_t  ngx_stream_upstream_vars[] = {
       ngx_stream_upstream_addr_variable, 0,
       NGX_STREAM_VAR_NOCACHEABLE, 0 },
 
+    { ngx_string("upstream_status"), NULL,
+      ngx_stream_upstream_status_variable, 0,
+      NGX_STREAM_VAR_NOCACHEABLE, 0 },
+
     { ngx_string("upstream_bytes_sent"), NULL,
       ngx_stream_upstream_bytes_variable, 0,
       NGX_STREAM_VAR_NOCACHEABLE, 0 },
@@ -203,6 +209,58 @@ ngx_stream_upstream_addr_variable(ngx_stream_session_t *s,
 }
 
 
+static ngx_int_t
+ngx_stream_upstream_status_variable(ngx_stream_session_t *s,
+    ngx_stream_variable_value_t *v, uintptr_t data)
+{
+    u_char                       *p;
+    size_t                        len;
+    ngx_uint_t                    i;
+    ngx_stream_upstream_state_t  *state;
+
+    v->valid = 1;
+    v->no_cacheable = 0;
+    v->not_found = 0;
+
+    if (s->upstream_states == NULL || s->upstream_states->nelts == 0) {
+        v->not_found = 1;
+        return NGX_OK;
+    }
+
+    len = s->upstream_states->nelts * (3 + 2);
+
+    p = ngx_pnalloc(s->connection->pool, len);
+    if (p == NULL) {
+        return NGX_ERROR;
+    }
+
+    v->data = p;
+
+    i = 0;
+    state = s->upstream_states->elts;
+
+    for ( ;; ) {
+        if (state[i].status) {
+            p = ngx_sprintf(p, "%ui", state[i].status);
+
+        } else {
+            *p++ = '-';
+        }
+
+        if (++i == s->upstream_states->nelts) {
+            break;
+        }
+
+        *p++ = ',';
+        *p++ = ' ';
+    }
+
+    v->len = p - v->data;
+
+    return NGX_OK;
+}
+
+
 static ngx_int_t
 ngx_stream_upstream_bytes_variable(ngx_stream_session_t *s,
     ngx_stream_variable_value_t *v, uintptr_t data)

src/stream/ngx_stream_upstream.h

@@ -93,6 +93,7 @@ struct ngx_stream_upstream_srv_conf_s {
 
 
 typedef struct {
+    ngx_uint_t                         status;
     ngx_msec_t                         response_time;
     ngx_msec_t                         connect_time;
     ngx_msec_t                         first_byte_time;