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@google.com>

3 files changed, 16 insertions, 1 deletions

diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index 772561640..d99dd8342 100644
--- 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 --git a/src/http/v2/ngx_http_v2.h b/src/http/v2/ngx_http_v2.h
index 4804658eb..42e0eb132 100644
--- 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 --git a/src/http/v2/ngx_http_v2_filter_module.c b/src/http/v2/ngx_http_v2_filter_module.c
index 8621e7a4d..74aac6ef0 100644
--- a/src/http/v2/ngx_http_v2_filter_module.c
+++ b/src/http/v2/ngx_http_v2_filter_module.c
@@ -139,6 +139,7 @@ ngx_http_v2_header_filter(ngx_http_request_t *r)
     ngx_connection_t          *fc;
     ngx_http_cleanup_t        *cln;
     ngx_http_v2_out_frame_t   *frame;
+    ngx_http_v2_connection_t  *h2c;
     ngx_http_core_loc_conf_t  *clcf;
     ngx_http_core_srv_conf_t  *cscf;
     u_char                     addr[NGX_SOCKADDR_STRLEN];
@@ -235,7 +236,11 @@ ngx_http_v2_header_filter(ngx_http_request_t *r)
         }
     }
 
-    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_request_t *r)
 
     start = pos;
 
+    if (h2c->table_update) {
+        ngx_log_debug0(NGX_LOG_DEBUG_HTTP, fc->log, 0,
+                       "http2 table size update: 0");
+        *pos++ = (1 << 5) | 0;
+        h2c->table_update = 0;
+    }
+
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, fc->log, 0,
                    "http2 output header: \":status: %03ui\"",
                    r->headers_out.status);