[PATCH]HTTP/2 connection not properly closing during graceful shutdown

[Kasei Wang]

Hello,

I found that there is a slight probability of HTTP/2 connections not
properly closing during graceful shutdown, leading to worker processes
in shutting down state remaining stuck for an extended period. After
investigation, the issue appears to stem from the following:

1. worker processes in shutting down state use
ngx_close_idle_connections to close all idle connections, including
HTTP/2 connections.
2. For HTTP/2 connections, c->idle is set to true in ngx_http_v2_init.
According to the explanation in
https://hg.nginx.org/nginx/rev/5e95b9fb33b7, GOAWAY should be sent to
all HTTP/2 connections.
3. There might be a time gap between ngx_event_accept and
ngx_http_v2_init. For TLS connections, ngx_http_v2_init will be
executed after ALPN received, and for plaintext http2 connections,
ngx_http_v2_init will be executed after parsing the http2 preface. If
ngx_close_idle_connections is executed between ngx_event_accept and
ngx_http_v2_init, there's a possibility that c->idle of some
connections is set to true AFTER ngx_close_idle_connections, causing
those connections to not enter the GOAWAY process and leading to the
aforementioned problem.

To verify this, I've written a simple HTTP/2 client. This program will
wait 15 seconds after TCP connection establishment before starting to
send data. The purpose of sleep is to to raise the probability of
encountering the issue. You can reproduce the problem by executing
"nginx -s reload" during this 15-second wait. If you're interested,
you can try my test program
(<https://github.com/kaseiwang/http2-grace-shutdown-test>) to
reproduce the issue.

I believe the simplest solution is to set c->close based on
ngx_exiting in ngx_http_v2_init. In this way, the connection will send
GOAWAY in ngx_http_v2_read_handler.

Please confirm if this issue exists, review my analysis and the patch
if possible. Thank you very much.

# HG changeset patch
# User Kasei Wang <kasei at kasei.im>
# Date 1713863474 -28800
# Tue Apr 23 17:11:14 2024 +0800
# Branch help
# Node ID e5ba8996973cdd1edd7591a28ee1c145af86c556
# Parent 8618e4d900cc71082fbe7dc72af087937d64faf5
HTTP/2: close http2 connections created during graceful shutdown.

diff -r 8618e4d900cc -r e5ba8996973c src/http/v2/ngx_http_v2.c
--- a/src/http/v2/ngx_http_v2.c Tue Apr 16 18:27:50 2024 +0400
+++ b/src/http/v2/ngx_http_v2.c Tue Apr 23 17:11:14 2024 +0800
@@ -304,6 +304,10 @@
c->idle = 1;
ngx_reusable_connection(c, 0);

+ if (ngx_exiting) {
+ c->close = 1;
+ }
+
if (c->buffer) {
p = c->buffer->pos;
end = c->buffer->last;