fix a bug that in the function ngx_http_subrequest, it will make the headers_in.headers incorrect，and it will cause many problems

[fishgege@…]

sr->headers_in = r->headers_in;
This line of code copy the headers ,but it maybe make the struct mistake that sr->headers_in.headers.last mismatch &sr->headers_in.headers.part, when only one part be copied, the mismatch happen. if somewhere use sr->headers_in.headers.last ，it will cause assert or some other problems。fx：in the function ngx_http_headers_more_assert
so only when there is only one part, the mismatch happen , and I update for this.

src/http/ngx_http_core_module.c

@@ -2274,6 +2274,9 @@ ngx_http_subrequest(ngx_http_request_t *r,

sr->pool = r->pool;

sr->headers_in = r->headers_in;

+ if (sr->headers_in.headers.part.next == NULL) {
+ sr->headers_in.headers.last = &sr->headers_in.headers.part;
+ }

ngx_http_clear_content_length(sr);
ngx_http_clear_accept_ranges(sr);

[fishgege@…]

src/http/ngx_http_core_module.c
@@ -2274,6 +2274,9 @@ ngx_http_subrequest(ngx_http_request_t *r,
sr->pool = r->pool;

sr->headers_in = r->headers_in;
+ if (sr->headers_in.headers.part.next == NULL) {
+ sr->headers_in.headers.last = &sr->headers_in.headers.part;
+ }

ngx_http_clear_content_length(sr);
ngx_http_clear_accept_ranges(sr);

[Maxim Dounin]

The ngx_http_subrequest() function provides good enough copy of r->headers_in to read the header list using defined procedures. Trying to do anything beyond that would be a bug in the code which does it. In particular, trying to modify r->header_in (where r->headers_in->last is used) would be a bug in the module which does it, as no code is expected to modify r->headers_in, with the only exception of nginx http core where reading of the http headers happens.