Make default min frame payload size to 16K

Previously we use 16K - 9 bytes (frame header) as frame payload size
so that whole frame fits in 1 TLS record size (16K).  But it turns out
that in proxy use case, we will receive 16K payload from backend and
we have to split it into 2 odd looking frames (16K - 9 and 9), and
latter is highly inefficient.  To avoid this situation, we decided to
use min frame payload size to 16K.  Since we operates on TLS as stream
of data, we are not so much restricted in its record size.

lib/nghttp2_frame.h

@@ -55,10 +55,8 @@
 /* Number of inbound buffer */
 #define NGHTTP2_FRAMEBUF_MAX_NUM 5
 
-/* The default length of DATA frame payload. This should be small enough
- * for the data payload and the header to fit into 1 TLS record */
-#define NGHTTP2_DATA_PAYLOADLEN                                                \
-  ((NGHTTP2_MAX_FRAME_SIZE_MIN) - (NGHTTP2_FRAME_HDLEN))
+/* The default length of DATA frame payload. */
+#define NGHTTP2_DATA_PAYLOADLEN NGHTTP2_MAX_FRAME_SIZE_MIN
 
 /* Maximum headers payload length, calculated in compressed form.
    This applies to transmission only. */

tests/nghttp2_session_test.c

@@ -4485,6 +4485,7 @@ void test_nghttp2_session_defer_data(void) {
   my_user_data ud;
   nghttp2_data_provider data_prd;
   nghttp2_outbound_item *item;
+  nghttp2_stream *stream;
 
   memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
   callbacks.on_frame_send_callback = on_frame_send_callback;
@@ -4495,8 +4496,13 @@ void test_nghttp2_session_defer_data(void) {
   ud.data_source_length = NGHTTP2_DATA_PAYLOADLEN * 4;
 
   nghttp2_session_server_new(&session, &callbacks, &ud);
-  nghttp2_session_open_stream(session, 1, NGHTTP2_STREAM_FLAG_NONE,
-                              &pri_spec_default, NGHTTP2_STREAM_OPENING, NULL);
+  stream = nghttp2_session_open_stream(session, 1, NGHTTP2_STREAM_FLAG_NONE,
+                                       &pri_spec_default,
+                                       NGHTTP2_STREAM_OPENING, NULL);
+
+  session->remote_window_size = 1 << 20;
+  stream->remote_window_size = 1 << 20;
+
   nghttp2_submit_response(session, 1, NULL, 0, &data_prd);
 
   ud.block_count = 1;
@@ -4524,7 +4530,7 @@ void test_nghttp2_session_defer_data(void) {
 
   /* Deferred again */
   item->aux_data.data.data_prd.read_callback = defer_data_source_read_callback;
-  /* This is needed since 4KiB block is already read and waiting to be
+  /* This is needed since 16KiB block is already read and waiting to be
      sent. No read_callback invocation. */
   ud.block_count = 1;
   CU_ASSERT(0 == nghttp2_session_send(session));
@@ -4536,7 +4542,7 @@ void test_nghttp2_session_defer_data(void) {
   item->aux_data.data.data_prd.read_callback =
       fixed_length_data_source_read_callback;
   ud.block_count = 1;
-  /* Reads 2 4KiB blocks */
+  /* Reads 2 16KiB blocks */
   CU_ASSERT(0 == nghttp2_session_send(session));
   CU_ASSERT(ud.data_source_length == 0);
 
@@ -5049,9 +5055,15 @@ void test_nghttp2_session_data_backoff_by_high_pri_frame(void) {
   nghttp2_session_client_new(&session, &callbacks, &ud);
   nghttp2_submit_request(session, NULL, NULL, 0, &data_prd, NULL);
 
+  session->remote_window_size = 1 << 20;
+
   ud.block_count = 2;
   /* Sends request HEADERS + DATA[0] */
   CU_ASSERT(0 == nghttp2_session_send(session));
+
+  stream = nghttp2_session_get_stream(session, 1);
+  stream->remote_window_size = 1 << 20;
+
   CU_ASSERT(NGHTTP2_DATA == ud.sent_frame_type);
   /* data for DATA[1] is read from data_prd but it is not sent */
   CU_ASSERT(ud.data_source_length == NGHTTP2_DATA_PAYLOADLEN * 2);
@@ -5068,7 +5080,6 @@ void test_nghttp2_session_data_backoff_by_high_pri_frame(void) {
   /* Sends DATA[2..3] */
   CU_ASSERT(0 == nghttp2_session_send(session));
 
-  stream = nghttp2_session_get_stream(session, 1);
   CU_ASSERT(stream->shut_flags & NGHTTP2_SHUT_WR);
 
   nghttp2_session_del(session);