http2: Support trailer fields

This commit adds trailer support in HTTP/2.  In HTTP/1.1, chunked
encoding must be used to send trialer fields.  HTTP/2 deprecated any
trandfer-encoding, including chunked.  But trailer fields are now
always available.

Since trailer fields are relatively rare these days (gRPC uses them
extensively though), allocating buffer for trailer fields is done when
we detect that HEADERS frame containing trailer fields is started.  We
use Curl_add_buffer_* functions to buffer all trailers, just like we
do for regular header fields.  And then deliver them when stream is
closed.  We have to be careful here so that all data are delivered to
upper layer before sending trailers to the application.

We can deliver trailer field one by one using NGHTTP2_ERR_PAUSE
mechanism, but current method is far more simple.

Another possibility is use chunked encoding internally for HTTP/2
traffic.  I have not tested it, but it could add another overhead.

Closes #564

3 files changed, 71 insertions, 13 deletions

diff --git a/lib/http.c b/lib/http.c
index b77003fe7..f2a644fb5 100644
--- a/lib/http.c
+++ b/lib/http.c
@@ -1478,6 +1478,8 @@ CURLcode Curl_http_done(struct connectdata *conn,
     DEBUGF(infof(data, "free header_recvbuf!!\n"));
     Curl_add_buffer_free(http->header_recvbuf);
     http->header_recvbuf = NULL; /* clear the pointer */
+    Curl_add_buffer_free(http->trailer_recvbuf);
+    http->trailer_recvbuf = NULL; /* clear the pointer */
     if(http->push_headers) {
       /* if they weren't used and then freed before */
       for(; http->push_headers_used > 0; --http->push_headers_used) {
diff --git a/lib/http.h b/lib/http.h
index fe4f39bc6..a35a0af23 100644
--- a/lib/http.h
+++ b/lib/http.h
@@ -163,6 +163,7 @@ struct HTTP {
   Curl_send_buffer *header_recvbuf;
   size_t nread_header_recvbuf; /* number of bytes in header_recvbuf fed into
                                   upper layer */
+  Curl_send_buffer *trailer_recvbuf;
   int status_code; /* HTTP status code */
   const uint8_t *pausedata; /* pointer to data received in on_data_chunk */
   size_t pauselen; /* the number of bytes left in data */
diff --git a/lib/http2.c b/lib/http2.c
index dae7f4a6f..eb54d89b0 100644
--- a/lib/http2.c
+++ b/lib/http2.c
@@ -111,6 +111,8 @@ static CURLcode http2_disconnect(struct connectdata *conn,
   if(http) {
     Curl_add_buffer_free(http->header_recvbuf);
     http->header_recvbuf = NULL; /* clear the pointer */
+    Curl_add_buffer_free(http->trailer_recvbuf);
+    http->trailer_recvbuf = NULL; /* clear the pointer */
     for(; http->push_headers_used > 0; --http->push_headers_used) {
       free(http->push_headers[http->push_headers_used - 1]);
     }
@@ -446,13 +448,9 @@ static int on_frame_recv(nghttp2_session *session, const nghttp2_frame *frame,
     }
     break;
   case NGHTTP2_HEADERS:
-    if(frame->headers.cat == NGHTTP2_HCAT_REQUEST)
-      break;
-
     if(stream->bodystarted) {
       /* Only valid HEADERS after body started is trailer HEADERS.  We
-         ignores trailer HEADERS for now.  nghttp2 guarantees that it
-         has END_STREAM flag set. */
+         buffer them in on_header callback. */
       break;
     }
 
@@ -685,13 +683,36 @@ static int on_stream_close(nghttp2_session *session, int32_t stream_id,
 static int on_begin_headers(nghttp2_session *session,
                             const nghttp2_frame *frame, void *userp)
 {
+  struct HTTP *stream;
   struct SessionHandle *data_s = NULL;
   (void)userp;
 
   data_s = nghttp2_session_get_stream_user_data(session, frame->hd.stream_id);
-  if(data_s) {
-    DEBUGF(infof(data_s, "on_begin_headers() was called\n"));
+  if(!data_s) {
+    return 0;
+  }
+
+  DEBUGF(infof(data_s, "on_begin_headers() was called\n"));
+
+  if(frame->hd.type != NGHTTP2_HEADERS) {
+    return 0;
+  }
+
+  stream = data_s->req.protop;
+  if(!stream || !stream->bodystarted) {
+    return 0;
+  }
+
+  /* This is trailer HEADERS started.  Allocate buffer for them. */
+  DEBUGF(infof(data_s, "trailer field started\n"));
+
+  assert(stream->trailer_recvbuf == NULL);
+
+  stream->trailer_recvbuf = Curl_add_buffer_init();
+  if(!stream->trailer_recvbuf) {
+    return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
   }
+
   return 0;
 }
 
@@ -750,10 +771,22 @@ static int on_header(nghttp2_session *session, const nghttp2_frame *frame,
     return NGHTTP2_ERR_CALLBACK_FAILURE;
   }
 
-  if(stream->bodystarted)
-    /* Ignore trailer or HEADERS not mapped to HTTP semantics.  The
-       consequence is handled in on_frame_recv(). */
+  if(stream->bodystarted) {
+    /* This is trailer fields. */
+    /* 3 is for ":" and "\r\n". */
+    uint32_t n = (uint32_t)(namelen + valuelen + 3);
+
+    DEBUGF(infof(data_s, "h2 trailer: %.*s: %.*s\n", namelen, name, valuelen,
+                 value));
+
+    Curl_add_buffer(stream->trailer_recvbuf, &n, sizeof(n));
+    Curl_add_buffer(stream->trailer_recvbuf, name, namelen);
+    Curl_add_buffer(stream->trailer_recvbuf, ":", 1);
+    Curl_add_buffer(stream->trailer_recvbuf, value, valuelen);
+    Curl_add_buffer(stream->trailer_recvbuf, "\r\n\0", 3);
+
     return 0;
+  }
 
   /* Store received PUSH_PROMISE headers to be used when the subsequent
      PUSH_PROMISE callback comes */
@@ -990,9 +1023,13 @@ CURLcode Curl_http2_request_upgrade(Curl_send_buffer *req,
   return result;
 }
 
-static ssize_t http2_handle_stream_close(struct http_conn *httpc,
+static ssize_t http2_handle_stream_close(struct connectdata *conn,
                                          struct SessionHandle *data,
                                          struct HTTP *stream, CURLcode *err) {
+  char *trailer_pos, *trailer_end;
+  CURLcode result;
+  struct http_conn *httpc = &conn->proto.httpc;
+
   if(httpc->pause_stream_id == stream->stream_id) {
     httpc->pause_stream_id = 0;
   }
@@ -1005,6 +1042,24 @@ static ssize_t http2_handle_stream_close(struct http_conn *httpc,
     *err = CURLE_HTTP2;
     return -1;
   }
+
+  trailer_pos = stream->trailer_recvbuf->buffer;
+  trailer_end = trailer_pos + stream->trailer_recvbuf->size_used;
+
+  for(; trailer_pos < trailer_end;) {
+    uint32_t n;
+    memcpy(&n, trailer_pos, sizeof(n));
+    trailer_pos += sizeof(n);
+
+    result = Curl_client_write(conn, CLIENTWRITE_HEADER, trailer_pos, n);
+    if(result) {
+      *err = result;
+      return -1;
+    }
+
+    trailer_pos += n + 1;
+  }
+
   DEBUGF(infof(data, "http2_recv returns 0, http2_handle_stream_close\n"));
   return 0;
 }
@@ -1078,7 +1133,7 @@ static ssize_t http2_recv(struct connectdata *conn, int sockindex,
      otherwise, we may be going to read from underlying connection,
      and gets EAGAIN, and we will get stuck there. */
   if(stream->memlen == 0 && stream->closed) {
-    return http2_handle_stream_close(httpc, data, stream, err);
+    return http2_handle_stream_close(conn, data, stream, err);
   }
 
   /* Nullify here because we call nghttp2_session_send() and they
@@ -1246,7 +1301,7 @@ static ssize_t http2_recv(struct connectdata *conn, int sockindex,
   /* If stream is closed, return 0 to signal the http routine to close
      the connection */
   if(stream->closed) {
-    return http2_handle_stream_close(httpc, data, stream, err);
+    return http2_handle_stream_close(conn, data, stream, err);
   }
   *err = CURLE_AGAIN;
   DEBUGF(infof(data, "http2_recv returns AGAIN for stream %u\n",