From c8b273fd997dd021d049105ed10f6bf054f239ba Mon Sep 17 00:00:00 2001
From: Vladimir Homutov <vl@nginx.com>
Date: Wed, 5 May 2021 18:11:55 +0300
Subject: [PATCH] QUIC: relaxed client id requirements.

Client IDs cannot be reused on different paths.  This change allows to reuse
client id previosly seen on the same path (but with different dcid) in case
when no unused client IDs are available.
---
 src/event/quic/ngx_event_quic_connid.c    | 29 +++++++++++++++++++++++
 src/event/quic/ngx_event_quic_connid.h    |  2 ++
 src/event/quic/ngx_event_quic_migration.c | 19 ++++++++++-----
 3 files changed, 44 insertions(+), 6 deletions(-)

diff --git a/src/event/quic/ngx_event_quic_connid.c b/src/event/quic/ngx_event_quic_connid.c
index a30e7ef35..4dbb2bbd6 100644
--- a/src/event/quic/ngx_event_quic_connid.c
+++ b/src/event/quic/ngx_event_quic_connid.c
@@ -325,6 +325,35 @@ ngx_quic_next_client_id(ngx_connection_t *c)
 }
 
 
+ngx_quic_client_id_t *
+ngx_quic_used_client_id(ngx_connection_t *c, ngx_quic_path_t *path)
+{
+    ngx_queue_t            *q;
+    ngx_quic_socket_t      *qsock;
+    ngx_quic_connection_t  *qc;
+
+    qc = ngx_quic_get_connection(c);
+
+    /* best guess: cid used by active path is good for us */
+    if (qc->socket->path == path) {
+        return qc->socket->cid;
+    }
+
+    for (q = ngx_queue_head(&qc->sockets);
+         q != ngx_queue_sentinel(&qc->sockets);
+         q = ngx_queue_next(q))
+    {
+        qsock = ngx_queue_data(q, ngx_quic_socket_t, queue);
+
+        if (qsock->path && qsock->path == path) {
+            return qsock->cid;
+        }
+    }
+
+    return NULL;
+}
+
+
 ngx_int_t
 ngx_quic_handle_retire_connection_id_frame(ngx_connection_t *c,
     ngx_quic_header_t *pkt, ngx_quic_retire_cid_frame_t *f)
diff --git a/src/event/quic/ngx_event_quic_connid.h b/src/event/quic/ngx_event_quic_connid.h
index fc7850a9c..a0552e9dd 100644
--- a/src/event/quic/ngx_event_quic_connid.h
+++ b/src/event/quic/ngx_event_quic_connid.h
@@ -23,6 +23,8 @@ ngx_int_t ngx_quic_create_server_id(ngx_connection_t *c, u_char *id);
 ngx_quic_client_id_t *ngx_quic_create_client_id(ngx_connection_t *c,
     ngx_str_t *id, uint64_t seqnum, u_char *token);
 ngx_quic_client_id_t *ngx_quic_next_client_id(ngx_connection_t *c);
+ngx_quic_client_id_t *ngx_quic_used_client_id(ngx_connection_t *c,
+    ngx_quic_path_t *path);
 void ngx_quic_unref_client_id(ngx_connection_t *c, ngx_quic_client_id_t *cid);
 
 #endif /* _NGX_EVENT_QUIC_CONNID_H_INCLUDED_ */
diff --git a/src/event/quic/ngx_event_quic_migration.c b/src/event/quic/ngx_event_quic_migration.c
index f1c923f83..74dc0113b 100644
--- a/src/event/quic/ngx_event_quic_migration.c
+++ b/src/event/quic/ngx_event_quic_migration.c
@@ -348,16 +348,23 @@ ngx_quic_update_paths(ngx_connection_t *c, ngx_quic_header_t *pkt)
         }
     }
 
+    /* prefer unused client IDs if available */
     cid = ngx_quic_next_client_id(c);
     if (cid == NULL) {
-        qc = ngx_quic_get_connection(c);
-        qc->error = NGX_QUIC_ERR_CONNECTION_ID_LIMIT_ERROR;
-        qc->error_reason = "no available client ids for new path";
 
-        ngx_log_error(NGX_LOG_ERR, c->log, 0,
-                      "no available client ids for new path");
+        /* try to reuse connection ID used on the same path */
+        cid = ngx_quic_used_client_id(c, path);
+        if (cid == NULL) {
 
-        return NGX_ERROR;
+            qc = ngx_quic_get_connection(c);
+            qc->error = NGX_QUIC_ERR_CONNECTION_ID_LIMIT_ERROR;
+            qc->error_reason = "no available client ids for new path";
+
+            ngx_log_error(NGX_LOG_ERR, c->log, 0,
+                          "no available client ids for new path");
+
+            return NGX_ERROR;
+        }
     }
 
     ngx_quic_connect(c, qsock, path, cid);