[PATCH] Fixed segfault when switching off master process during upgrade

[Maxim Dounin]

# HG changeset patch
# User Maxim Dounin <mdounin at mdounin.ru>
# Date 1667097733 -10800
#      Sun Oct 30 05:42:13 2022 +0300
# Node ID ef9c94be7fe4685f0eeee41f76b964ea252f519f
# Parent  b73d95226c84b93e51f23f7b35782d98d3b516b9
Fixed segfault when switching off master process during upgrade.

Binary upgrades are not supported without master process, but it is,
however, possible, that nginx running with master process is asked
to upgrade binary, and the configuration file as available on disk
at this time includes "master_process off;".

If this happens, listening sockets inherited from the previous binary
will have ls[i].previous set.  But the old cycle on initial process
startup, including startup after binary upgrade, is destroyed by
ngx_init_cycle() once configuration parsing is complete.  As a result,
an attempt to dereference ls[i].previous in ngx_event_process_init()
accesses already freed memory.

Fix is to avoid looking into ls[i].previous if the old cycle is already
freed.

diff --git a/src/event/ngx_event.c b/src/event/ngx_event.c
--- a/src/event/ngx_event.c
+++ b/src/event/ngx_event.c
@@ -813,7 +813,9 @@ ngx_event_process_init(ngx_cycle_t *cycl
         rev->deferred_accept = ls[i].deferred_accept;
 #endif
 
-        if (!(ngx_event_flags & NGX_USE_IOCP_EVENT)) {
+        if (!(ngx_event_flags & NGX_USE_IOCP_EVENT)
+            && cycle->old_cycle)
+        {
             if (ls[i].previous) {
 
                 /*