[PATCH] SSI: fixed incorrect or duplicate stub output

[Maxim Dounin]

# HG changeset patch
# User Maxim Dounin <mdounin at mdounin.ru>
# Date 1719362615 -10800
#      Wed Jun 26 03:43:35 2024 +0300
# Node ID d78ecfffb8cd813554716e8d9d29f2007bd94422
# Parent  2706b60dc2255a36677884be43a4367fef081c40
SSI: fixed incorrect or duplicate stub output.

Following 3518:eb3aaf8bd2a9 (0.8.37), r->request_output is only set
if there are data in the first buffer sent in the subrequest.  As a
result, following the change mentioned this flag cannot be used to
prevent duplicate ngx_http_ssi_stub_output() calls, since it is not
set if there was already some output, but the first buffer was empty.

Still, when there are multiple subrequests, even an empty subrequest
response might be delayed by the postpone filter, leading to a second
call of ngx_http_ssi_stub_output() during finalization from
ngx_http_writer() the subreqest buffers are released by the postpone
filter.  Since r->request_output is not set after the first call, this
resulted in duplicate stub output.

Additionally, checking only the first buffer might be wrong in some
unusual cases.  For example, the first buffer might be empty if
$r->flush() is called before printing any data in the embedded Perl
module.

Depending on the postpone_output value and corresponding sizes, this
issue can result in either duplicate or unexpected stub output, or
"zero size buf in writer" alerts.

Following 8124:f5515e727656 (1.23.4), it became slightly easier to
reproduce the issue, as empty static files and empty cache items now
result in a response with an empty buffer.  Before the change, an empty
proxied response can be used to reproduce the issue.

Fix is check all buffers and set r->request_output if any non-empty
buffers are sent.  This ensures that all unusual cases of non-empty
responses are covered, and also that r->request_output will be set
after the first stub output, preventing duplicate output.

Reported by Jan Gassen.

diff --git a/src/http/ngx_http_copy_filter_module.c b/src/http/ngx_http_copy_filter_module.c
--- a/src/http/ngx_http_copy_filter_module.c
+++ b/src/http/ngx_http_copy_filter_module.c
@@ -83,6 +83,7 @@ static ngx_int_t
 ngx_http_copy_filter(ngx_http_request_t *r, ngx_chain_t *in)
 {
     ngx_int_t                     rc;
+    ngx_chain_t                  *cl;
     ngx_connection_t             *c;
     ngx_output_chain_ctx_t       *ctx;
     ngx_http_core_loc_conf_t     *clcf;
@@ -132,9 +133,14 @@ ngx_http_copy_filter(ngx_http_request_t 
             ctx->thread_handler = ngx_http_copy_thread_handler;
         }
 #endif
+    }
 
-        if (in && in->buf && ngx_buf_size(in->buf)) {
-            r->request_output = 1;
+    if (!r->request_output) {
+        for (cl = in; cl; cl = cl->next) {
+            if (ngx_buf_size(cl->buf)) {
+                r->request_output = 1;
+                break;
+            }
         }
     }