Perl: avoid returning 500 if header was already sent.

Returning NGX_HTTP_INTERNAL_SERVER_ERROR if a perl code died after
sending header will lead to a "header already sent" alert.  To avoid
it, we now check if header was already sent, and return NGX_ERROR
instead if it was.

src/http/modules/perl/nginx.xs

@@ -164,6 +164,8 @@ send_http_header(r, ...)
         }
     }
 
+    ctx->header_sent = 1;
+
     r->disable_not_modified = 1;
 
     rc = ngx_http_send_header(r);

src/http/modules/perl/ngx_http_perl_module.c

@@ -780,6 +780,10 @@ ngx_http_perl_call_handler(pTHX_ ngx_http_request_t *r,
 
         ctx->redirect_uri.len = 0;
 
+        if (ctx->header_sent) {
+            return NGX_ERROR;
+        }
+
         return NGX_HTTP_INTERNAL_SERVER_ERROR;
     }
 

src/http/modules/perl/ngx_http_perl_module.h

@@ -34,6 +34,7 @@ typedef struct {
     unsigned                  done:1;
     unsigned                  error:1;
     unsigned                  variable:1;
+    unsigned                  header_sent:1;
 
     ngx_array_t              *variables;  /* array of ngx_http_perl_var_t */