[nginx] Cache: keep c->body_start when Vary changes (ticket #2029).

[Sergey Kandaurov]

details:   https://hg.nginx.org/nginx/rev/3781de64e747
branches:  
changeset: 7705:3781de64e747
user:      Sergey Kandaurov <pluknet at nginx.com>
date:      Wed Sep 09 19:26:27 2020 +0300
description:
Cache: keep c->body_start when Vary changes (ticket #2029).

If the variant hash doesn't match one we used as a secondary cache key,
we switch back to the original key.  In this case, c->body_start was kept
updated from an existing cache node overwriting the new response value.
After file cache update, it led to discrepancy between a cache node and
cache file seen as critical errors "file cache .. has too long header".

diffstat:

 src/http/ngx_http_cache.h      |  1 +
 src/http/ngx_http_file_cache.c |  3 ++-
 2 files changed, 3 insertions(+), 1 deletions(-)

diffs (31 lines):

diff -r 847fd35f94de -r 3781de64e747 src/http/ngx_http_cache.h
--- a/src/http/ngx_http_cache.h	Fri Aug 04 19:37:37 2017 +0300
+++ b/src/http/ngx_http_cache.h	Wed Sep 09 19:26:27 2020 +0300
@@ -117,6 +117,7 @@ struct ngx_http_cache_s {
     unsigned                         purged:1;
     unsigned                         reading:1;
     unsigned                         secondary:1;
+    unsigned                         update_variant:1;
     unsigned                         background:1;
 
     unsigned                         stale_updating:1;
diff -r 847fd35f94de -r 3781de64e747 src/http/ngx_http_file_cache.c
--- a/src/http/ngx_http_file_cache.c	Fri Aug 04 19:37:37 2017 +0300
+++ b/src/http/ngx_http_file_cache.c	Wed Sep 09 19:26:27 2020 +0300
@@ -854,7 +854,7 @@ ngx_http_file_cache_exists(ngx_http_file
         if (fcn->exists || fcn->uses >= c->min_uses) {
 
             c->exists = fcn->exists;
-            if (fcn->body_start) {
+            if (fcn->body_start && !c->update_variant) {
                 c->body_start = fcn->body_start;
             }
 
@@ -1339,6 +1339,7 @@ ngx_http_file_cache_update_variant(ngx_h
     ngx_shmtx_unlock(&cache->shpool->mutex);
 
     c->file.name.len = 0;
+    c->update_variant = 1;
 
     ngx_memcpy(c->key, c->main, NGX_HTTP_CACHE_KEY_LEN);