new file mode 100644
--- /dev/null
+++ b/proxy_cache_error.t
@@ -0,0 +1,87 @@
+#!/usr/bin/perl
+
+# (C) Maxim Dounin
+
+# Tests for http proxy cache, "header already sent" alerts on backend errors,
+# http://mailman.nginx.org/pipermail/nginx-devel/2018-January/010737.html.
+
+###############################################################################
+
+use warnings;
+use strict;
+
+use Test::More;
+
+BEGIN { use FindBin; chdir($FindBin::Bin); }
+
+use lib 'lib';
+use Test::Nginx;
+
+###############################################################################
+
+select STDERR; $| = 1;
+select STDOUT; $| = 1;
+
+my $t = Test::Nginx->new()->has(qw/http proxy cache/)->plan(1)
+	->write_file_expand('nginx.conf', <<'EOF');
+
+%%TEST_GLOBALS%%
+
+daemon off;
+
+events {
+}
+
+http {
+    %%TEST_GLOBALS_HTTP%%
+
+    proxy_cache_path   %%TESTDIR%%/cache  levels=1:2
+                       keys_zone=NAME:1m;
+
+    server {
+        listen       127.0.0.1:8080;
+        server_name  localhost;
+
+        location / {
+            proxy_pass    http://127.0.0.1:8081;
+            proxy_cache   NAME;
+
+            proxy_read_timeout 100ms;
+        }
+    }
+
+    server {
+        listen       127.0.0.1:8081;
+        server_name  localhost;
+
+        location / {
+            limit_rate 512;
+            expires 1m;
+        }
+    }
+}
+
+EOF
+
+$t->write_file('big.html', 'x' x 1024);
+
+$t->run();
+
+###############################################################################
+
+# make a HEAD request; as cache is enabled, nginx convert HEAD to GET
+# and will set u->pipe->downstream_error to suppress sending the response
+# body to the client
+
+like(http_head('/big.html'), qr/200 OK/, 'head request');
+
+# once proxy_read_timeout expires, nginx will call
+# ngx_http_finalize_upstream_request() with u->pipe->downstream_error set
+# and rc = NGX_HTTP_GATEWAY_TIMEOUT; after revision ad3f342f14ba046c this
+# will result in ngx_http_finalize_request(NGX_HTTP_GATEWAY_TIMEOUT),
+# leading to an attempt to return additional error response and
+# the "header already sent" alert
+
+$t->todo_alerts();
+
+###############################################################################