Added "keepalive_min_timeout" directive.

The directive sets a timeout during which a keepalive connection will
not be closed by nginx for connection reuse or graceful shutdown.

The change allows clients that send multiple requests over the same
connection without delay or with a small delay between them, to avoid
receiving a TCP RST in response to one of them.  This excludes network
issues and non-graceful shutdown.  As a side-effect, it also addresses
the TCP reset problem described in RFC 9112, Section 9.6, when the last
sent HTTP response could be damaged by a followup TCP RST.  It is important
for non-idempotent requests, which cannot be retried by client.

It is not recommended to set keepalive_min_timeout to large values as
this can introduce an additional delay during graceful shutdown and may
restrict nginx from effective connection reuse.

src/http/ngx_http_core_module.c

@@ -509,6 +509,13 @@ static ngx_command_t  ngx_http_core_commands[] = {
       0,
       NULL },
 
+    { ngx_string("keepalive_min_timeout"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_msec_slot,
+      NGX_HTTP_LOC_CONF_OFFSET,
+      offsetof(ngx_http_core_loc_conf_t, keepalive_min_timeout),
+      NULL },
+
     { ngx_string("keepalive_requests"),
       NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
       ngx_conf_set_num_slot,
@@ -3606,6 +3613,7 @@ ngx_http_core_create_loc_conf(ngx_conf_t *cf)
     clcf->keepalive_time = NGX_CONF_UNSET_MSEC;
     clcf->keepalive_timeout = NGX_CONF_UNSET_MSEC;
     clcf->keepalive_header = NGX_CONF_UNSET;
+    clcf->keepalive_min_timeout = NGX_CONF_UNSET_MSEC;
     clcf->keepalive_requests = NGX_CONF_UNSET_UINT;
     clcf->lingering_close = NGX_CONF_UNSET_UINT;
     clcf->lingering_time = NGX_CONF_UNSET_MSEC;
@@ -3844,6 +3852,8 @@ ngx_http_core_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
                               prev->keepalive_timeout, 75000);
     ngx_conf_merge_sec_value(conf->keepalive_header,
                               prev->keepalive_header, 0);
+    ngx_conf_merge_msec_value(conf->keepalive_min_timeout,
+                              prev->keepalive_min_timeout, 0);
     ngx_conf_merge_uint_value(conf->keepalive_requests,
                               prev->keepalive_requests, 1000);
     ngx_conf_merge_uint_value(conf->lingering_close,

src/http/ngx_http_core_module.h

@@ -370,6 +370,7 @@ struct ngx_http_core_loc_conf_s {
     ngx_msec_t    send_timeout;            /* send_timeout */
     ngx_msec_t    keepalive_time;          /* keepalive_time */
     ngx_msec_t    keepalive_timeout;       /* keepalive_timeout */
+    ngx_msec_t    keepalive_min_timeout;   /* keepalive_min_timeout */
     ngx_msec_t    lingering_time;          /* lingering_time */
     ngx_msec_t    lingering_timeout;       /* lingering_timeout */
     ngx_msec_t    resolver_timeout;        /* resolver_timeout */

src/http/ngx_http_request.c

@@ -2799,6 +2799,13 @@ ngx_http_finalize_connection(ngx_http_request_t *r)
         r->lingering_close = 1;
     }
 
+    if (r->keepalive
+        && clcf->keepalive_min_timeout > 0)
+    {
+        ngx_http_set_keepalive(r);
+        return;
+    }
+
     if (!ngx_terminate
          && !ngx_exiting
          && r->keepalive
@@ -3301,10 +3308,22 @@ ngx_http_set_keepalive(ngx_http_request_t *r)
     r->http_state = NGX_HTTP_KEEPALIVE_STATE;
 #endif
 
-    c->idle = 1;
-    ngx_reusable_connection(c, 1);
+    if (clcf->keepalive_min_timeout == 0) {
+        c->idle = 1;
+        ngx_reusable_connection(c, 1);
+    }
 
-    ngx_add_timer(rev, clcf->keepalive_timeout);
+    if (clcf->keepalive_min_timeout > 0
+        && clcf->keepalive_timeout > clcf->keepalive_min_timeout)
+    {
+        hc->keepalive_timeout = clcf->keepalive_timeout
+                                - clcf->keepalive_min_timeout;
+
+    } else {
+        hc->keepalive_timeout = 0;
+    }
+
+    ngx_add_timer(rev, clcf->keepalive_timeout - hc->keepalive_timeout);
 
     if (rev->ready) {
         ngx_post_event(rev, &ngx_posted_events);
@@ -3315,15 +3334,32 @@ ngx_http_set_keepalive(ngx_http_request_t *r)
 static void
 ngx_http_keepalive_handler(ngx_event_t *rev)
 {
-    size_t             size;
-    ssize_t            n;
-    ngx_buf_t         *b;
-    ngx_connection_t  *c;
+    size_t                  size;
+    ssize_t                 n;
+    ngx_buf_t              *b;
+    ngx_connection_t       *c;
+    ngx_http_connection_t  *hc;
 
     c = rev->data;
+    hc = c->data;
 
     ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0, "http keepalive handler");
 
+    if (!ngx_terminate
+         && !ngx_exiting
+         && rev->timedout
+         && hc->keepalive_timeout > 0)
+    {
+        c->idle = 1;
+        ngx_reusable_connection(c, 1);
+
+        ngx_add_timer(rev, hc->keepalive_timeout);
+
+        hc->keepalive_timeout = 0;
+        rev->timedout = 0;
+        return;
+    }
+
     if (rev->timedout || c->close) {
         ngx_http_close_connection(c);
         return;

src/http/ngx_http_request.h

@@ -329,6 +329,8 @@ typedef struct {
 
     ngx_chain_t                      *free;
 
+    ngx_msec_t                        keepalive_timeout;
+
     unsigned                          ssl:1;
     unsigned                          proxy_protocol:1;
 } ngx_http_connection_t;