[PATCH] HTTP/2: signal 0-byte HPACK's dynamic table size

[Piotr Sikora]

# HG changeset patch
# User Piotr Sikora <piotrsikora at google.com>
# Date 1504129931 25200
#      Wed Aug 30 14:52:11 2017 -0700
# Node ID 7e1e91f9ca063563cb293136e7b1cede36e63dc6
# Parent  c7d4017c8876af6d8570e400320537d7d39e9578
HTTP/2: signal 0-byte HPACK's dynamic table size.

This change lets NGINX talk to clients with SETTINGS_HEADER_TABLE_SIZE
smaller than the default 4KB. Previously, NGINX would ACK the SETTINGS
frame with a small dynamic table size, but it would never send dynamic
table size update, leading to a connection-level COMPRESSION_ERROR.

Also, it allows clients to release 4KB of memory per connection, since
NGINX doesn't use HPACK's dynamic table when encoding headers, however
clients had to maintain it, since NGINX never signaled that it doesn't
use it.

Signed-off-by: Piotr Sikora <piotrsikora at google.com>

diff -r c7d4017c8876 -r 7e1e91f9ca06 src/http/v2/ngx_http_v2.c
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -245,6 +245,8 @@ ngx_http_v2_init(ngx_event_t *rev)
 
     h2c->frame_size = NGX_HTTP_V2_DEFAULT_FRAME_SIZE;
 
+    h2c->table_update = 1;
+
     h2scf = ngx_http_get_module_srv_conf(hc->conf_ctx, ngx_http_v2_module);
 
     h2c->pool = ngx_create_pool(h2scf->pool_size, h2c->connection->log);
diff -r c7d4017c8876 -r 7e1e91f9ca06 src/http/v2/ngx_http_v2.h
--- a/src/http/v2/ngx_http_v2.h
+++ b/src/http/v2/ngx_http_v2.h
@@ -144,6 +144,7 @@ struct ngx_http_v2_connection_s {
 
     unsigned                         closed_nodes:8;
     unsigned                         settings_ack:1;
+    unsigned                         table_update:1;
     unsigned                         blocked:1;
     unsigned                         goaway:1;
 };
diff -r c7d4017c8876 -r 7e1e91f9ca06 src/http/v2/ngx_http_v2_filter_module.c
--- a/src/http/v2/ngx_http_v2_filter_module.c
+++ b/src/http/v2/ngx_http_v2_filter_module.c
@@ -141,6 +141,7 @@ ngx_http_v2_header_filter(ngx_http_reque
     ngx_http_v2_out_frame_t   *frame;
     ngx_http_core_loc_conf_t  *clcf;
     ngx_http_core_srv_conf_t  *cscf;
+    ngx_http_v2_connection_t  *h2c;
     u_char                     addr[NGX_SOCKADDR_STRLEN];
 
     static const u_char nginx[5] = "\x84\xaa\x63\x55\xe7";
@@ -235,7 +236,11 @@ ngx_http_v2_header_filter(ngx_http_reque
         }
     }
 
-    len = status ? 1 : 1 + ngx_http_v2_literal_size("418");
+    h2c = r->stream->connection;
+
+    len = h2c->table_update ? 1 : 0;
+
+    len += status ? 1 : 1 + ngx_http_v2_literal_size("418");
 
     clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
 
@@ -423,6 +428,13 @@ ngx_http_v2_header_filter(ngx_http_reque
 
     start = pos;
 
+    if (h2c->table_update) {
+        ngx_log_debug0(NGX_LOG_DEBUG_HTTP, fc->log, 0,
+                       "http2 sending dynamic table size update: 0");
+        *pos++ = 32;
+        h2c->table_update = 0;
+    }
+
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, fc->log, 0,
                    "http2 output header: \":status: %03ui\"",
                    r->headers_out.status);