Upstream: trailers support, u->conf->pass_trailers flag.

Basic trailer headers support allows one to access response trailers
via the $upstream_trailer_* variables.

Additionally, the u->conf->pass_trailers flag was introduced.  When the
flag is set, trailer headers from the upstream response are passed to
the client.  Like normal headers, trailer headers will be hidden
if present in u->conf->hide_headers_hash.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index fd00ba5..6d91c7c 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -55,6 +55,8 @@
 static ngx_int_t ngx_http_upstream_test_connect(ngx_connection_t *c);
 static ngx_int_t ngx_http_upstream_process_headers(ngx_http_request_t *r,
     ngx_http_upstream_t *u);
+static ngx_int_t ngx_http_upstream_process_trailers(ngx_http_request_t *r,
+    ngx_http_upstream_t *u);
 static void ngx_http_upstream_send_response(ngx_http_request_t *r,
     ngx_http_upstream_t *u);
 static void ngx_http_upstream_upgrade(ngx_http_request_t *r,
@@ -164,6 +166,8 @@
     ngx_http_request_t *r, ngx_http_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_http_upstream_header_variable(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data);
+static ngx_int_t ngx_http_upstream_trailer_variable(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_http_upstream_cookie_variable(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data);
 
@@ -423,6 +427,9 @@
     { ngx_string("upstream_http_"), NULL, ngx_http_upstream_header_variable,
       0, NGX_HTTP_VAR_NOCACHEABLE|NGX_HTTP_VAR_PREFIX, 0 },
 
+    { ngx_string("upstream_trailer_"), NULL, ngx_http_upstream_trailer_variable,
+      0, NGX_HTTP_VAR_NOCACHEABLE|NGX_HTTP_VAR_PREFIX, 0 },
+
     { ngx_string("upstream_cookie_"), NULL, ngx_http_upstream_cookie_variable,
       0, NGX_HTTP_VAR_NOCACHEABLE|NGX_HTTP_VAR_PREFIX, 0 },
 
@@ -1046,6 +1053,13 @@
         return NGX_ERROR;
     }
 
+    if (ngx_list_init(&u->headers_in.trailers, r->pool, 2,
+                      sizeof(ngx_table_elt_t))
+        != NGX_OK)
+    {
+        return NGX_ERROR;
+    }
+
     rc = u->process_header(r);
 
     if (rc == NGX_OK) {
@@ -1883,6 +1897,13 @@
         return NGX_ERROR;
     }
 
+    if (ngx_list_init(&u->headers_in.trailers, r->pool, 2,
+                      sizeof(ngx_table_elt_t))
+        != NGX_OK)
+    {
+        return NGX_ERROR;
+    }
+
     /* reinit the request chain */
 
     file_pos = 0;
@@ -2237,6 +2258,15 @@
             return;
         }
 
+        if (ngx_list_init(&u->headers_in.trailers, r->pool, 2,
+                          sizeof(ngx_table_elt_t))
+            != NGX_OK)
+        {
+            ngx_http_upstream_finalize_request(r, u,
+                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
+            return;
+        }
+
 #if (NGX_HTTP_CACHE)
 
         if (r->cache) {
@@ -2735,6 +2765,51 @@
 }
 
 
+static ngx_int_t
+ngx_http_upstream_process_trailers(ngx_http_request_t *r,
+    ngx_http_upstream_t *u)
+{
+    ngx_uint_t        i;
+    ngx_list_part_t  *part;
+    ngx_table_elt_t  *h, *ho;
+
+    if (!u->conf->pass_trailers) {
+        return NGX_OK;
+    }
+
+    part = &u->headers_in.trailers.part;
+    h = part->elts;
+
+    for (i = 0; /* void */; i++) {
+
+        if (i >= part->nelts) {
+            if (part->next == NULL) {
+                break;
+            }
+
+            part = part->next;
+            h = part->elts;
+            i = 0;
+        }
+
+        if (ngx_hash_find(&u->conf->hide_headers_hash, h[i].hash,
+                          h[i].lowcase_key, h[i].key.len))
+        {
+            continue;
+        }
+
+        ho = ngx_list_push(&r->headers_out.trailers);
+        if (ho == NULL) {
+            return NGX_ERROR;
+        }
+
+        *ho = h[i];
+    }
+
+    return NGX_OK;
+}
+
+
 static void
 ngx_http_upstream_send_response(ngx_http_request_t *r, ngx_http_upstream_t *u)
 {
@@ -4272,6 +4347,12 @@
     }
 
     if (rc == 0) {
+
+        if (ngx_http_upstream_process_trailers(r, u) != NGX_OK) {
+            ngx_http_finalize_request(r, NGX_ERROR);
+            return;
+        }
+
         rc = ngx_http_send_special(r, NGX_HTTP_LAST);
 
     } else if (flush) {
@@ -5382,6 +5463,21 @@
 
 
 static ngx_int_t
+ngx_http_upstream_trailer_variable(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data)
+{
+    if (r->upstream == NULL) {
+        v->not_found = 1;
+        return NGX_OK;
+    }
+
+    return ngx_http_variable_unknown_header(v, (ngx_str_t *) data,
+                                        &r->upstream->headers_in.trailers.part,
+                                        sizeof("upstream_trailer_") - 1);
+}
+
+
+static ngx_int_t
 ngx_http_upstream_cookie_variable(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data)
 {

src/http/ngx_http_upstream.h

diff --git a/src/http/ngx_http_upstream.h b/src/http/ngx_http_upstream.h
index 56b54d1..a56238d 100644
--- a/src/http/ngx_http_upstream.h
+++ b/src/http/ngx_http_upstream.h
@@ -222,6 +222,7 @@
     signed                           store:2;
     unsigned                         intercept_404:1;
     unsigned                         change_buffering:1;
+    unsigned                         pass_trailers:1;
 
 #if (NGX_HTTP_SSL || NGX_COMPAT)
     ngx_ssl_t                       *ssl;
@@ -251,6 +252,7 @@
 
 typedef struct {
     ngx_list_t                       headers;
+    ngx_list_t                       trailers;
 
     ngx_uint_t                       status_n;
     ngx_str_t                        status_line;