h3: add support for additional HEADERS

HTTP/3 request and response message exchanges allow multiple HEADERS
frames to be sent, which allows for trailer sections or non-final
responses. Previously, quiche has only supported generating a single
HEADERS frame per request or response, and didn't support receiving
them.

This change adds support for sending and receiving multiple HEADERS
frames on a request stream. It enforces the sequencing rules defined in
https://www.rfc-editor.org/rfc/rfc9114.html#section-4.1.
Sending additional HEADERS is done using the new
`send_additional_headers()` method. On the receive side, rather than add
a new Event enum variant for these additional uses of HEADERS, this
change reuses Event::Headers. Previosuly, this event included a
`has_body` boolean, which has been renamed to `more_frames` to better
align with how HTTP/3 streams work.

nginx/nginx-1.16.patch

@@ -2569,7 +2569,7 @@ index 000000000..d7c4fbf1d
 +        return;
 +    }
 +
-+    stream->in_closed = !quiche_h3_event_headers_has_body(ev);
++    stream->in_closed = !quiche_h3_event_headers_has_more_frames(ev);
 +
 +    ngx_http_v3_run_request(stream->request);
 +}

quiche/include/quiche.h

@@ -1060,8 +1060,8 @@ int quiche_h3_for_each_setting(quiche_h3_conn *conn,
                                          uint64_t value, void *argp),
                                void *argp);
 
-// Check whether data will follow the headers on the stream.
-bool quiche_h3_event_headers_has_body(quiche_h3_event *ev);
+// Check whether more frames will follow the headers on the stream.
+bool quiche_h3_event_headers_has_more_frames(quiche_h3_event *ev);
 
 // Check whether or not extended connection is enabled by the peer
 bool quiche_h3_extended_connect_enabled_by_peer(quiche_h3_conn *conn);
@@ -1099,6 +1099,12 @@ int quiche_h3_send_response_with_priority(quiche_h3_conn *conn,
                             const quiche_h3_header *headers, size_t headers_len,
                             quiche_h3_priority *priority, bool fin);
 
+// Sends additional HTTP/3 headers on the specified stream.
+int quiche_h3_send_additional_headers(quiche_h3_conn *conn,
+                            quiche_conn *quic_conn, uint64_t stream_id,
+                            quiche_h3_header *headers, size_t headers_len,
+                            bool is_trailer_section, bool fin);
+
 // Sends an HTTP/3 body chunk on the given stream.
 ssize_t quiche_h3_send_body(quiche_h3_conn *conn, quiche_conn *quic_conn,
                             uint64_t stream_id, const uint8_t *body, size_t body_len,

quiche/src/h3/ffi.rs

@@ -188,9 +188,9 @@ pub extern fn quiche_h3_event_for_each_header(
 }
 
 #[no_mangle]
-pub extern fn quiche_h3_event_headers_has_body(ev: &h3::Event) -> bool {
+pub extern fn quiche_h3_event_headers_has_more_frames(ev: &h3::Event) -> bool {
     match ev {
-        h3::Event::Headers { has_body, .. } => *has_body,
+        h3::Event::Headers { more_frames, .. } => *more_frames,
 
         _ => unreachable!(),
     }
@@ -265,6 +265,27 @@ pub extern fn quiche_h3_send_response_with_priority(
     }
 }
 
+#[no_mangle]
+pub extern fn quiche_h3_send_additional_headers(
+    conn: &mut h3::Connection, quic_conn: &mut Connection, stream_id: u64,
+    headers: *const Header, headers_len: size_t, is_trailer_section: bool,
+    fin: bool,
+) -> c_int {
+    let headers = headers_from_ptr(headers, headers_len);
+
+    match conn.send_additional_headers(
+        quic_conn,
+        stream_id,
+        &headers,
+        is_trailer_section,
+        fin,
+    ) {
+        Ok(_) => 0,
+
+        Err(e) => e.to_c() as c_int,
+    }
+}
+
 #[no_mangle]
 pub extern fn quiche_h3_send_body(
     conn: &mut h3::Connection, quic_conn: &mut Connection, stream_id: u64,