Reading ngx.var.http_host prevents updating the Host header

Not sure if this is a bug, but the behavior is very strange. When the Host header is read using ngx.var.http_host, setting the Host header later with ngx.req.set_header doesn't seem to be updating it.

Test for reproducing (thanks to @mikz):

use Test::Nginx::Socket::Lua 'no_plan';

run_tests();

__DATA__

=== TEST 1: passing test
--- config
location = /t {
  proxy_pass http://localhost:$TEST_NGINX_SERVER_PORT/upstream;
  proxy_set_header Host $http_host;
  access_by_lua_block {
    local headers = ngx.req.get_headers()

    ngx.log(ngx.STDERR, 'ngx.req.get_headers().host: ', headers.host)
    ngx.req.set_header('Host', 'foobar')
  }
}

location = /upstream {
  content_by_lua_block {
    ngx.say('Host: ', ngx.var.host)
  }
}

--- request
GET /t
--- more_headers
Host: original
--- error_code: 200
--- response_body
Host: foobar



=== TEST 2: failing test
--- config
location = /t {
  proxy_pass http://localhost:$TEST_NGINX_SERVER_PORT/upstream;
  proxy_set_header Host $http_host;
  access_by_lua_block {
    local headers = ngx.req.get_headers()

    local _ = ngx.var.http_host

    ngx.log(ngx.STDERR, 'ngx.req.get_headers().host: ', headers.host)
    ngx.req.set_header('Host', 'foobar')
  }
}

location = /upstream {
  content_by_lua_block {
    ngx.say('Host: ', ngx.var.host)
  }
}

--- request
GET /t
--- more_headers
Host: original
--- error_code: 200
--- response_body
Host: foobar

The only difference between the two is the local _ = ngx.var.http_host line in the failing test. In the failing test the subrequest receives the Host: original instead of Host: foobar.

OpenResty version:

$ openresty -V 
nginx version: openresty/1.13.6.1
built by clang 9.0.0 (clang-900.0.38)
built with OpenSSL 1.0.2l  25 May 2017 (running with OpenSSL 1.0.2o  27 Mar 2018)
TLS SNI support enabled
configure arguments: --prefix=/usr/local/openresty/nginx --with-cc-opt='-O2 -I/usr/local/opt/openssl/include/ -I/usr/local/opt/pcre/include/' --add-module=../ngx_devel_kit-0.3.0 --add-module=../iconv-nginx-module-0.14 --add-module=../echo-nginx-module-0.61 --add-module=../xss-nginx-module-0.05 --add-module=../ngx_coolkit-0.2rc3 --add-module=../set-misc-nginx-module-0.31 --add-module=../form-input-nginx-module-0.12 --add-module=../encrypted-session-nginx-module-0.07 --add-module=../srcache-nginx-module-0.31 --add-module=../ngx_lua-0.10.11 --add-module=../ngx_lua_upstream-0.07 --add-module=../headers-more-nginx-module-0.33 --add-module=../array-var-nginx-module-0.05 --add-module=../memc-nginx-module-0.18 --add-module=../redis2-nginx-module-0.14 --add-module=../redis-nginx-module-0.3.7 --add-module=../rds-json-nginx-module-0.15 --add-module=../rds-csv-nginx-module-0.08 --add-module=../ngx_stream_lua-0.0.3 --with-ld-opt='-Wl,-rpath,/usr/local/openresty/luajit/lib -L/usr/local/opt/openssl/lib/ -L/usr/local/opt/pcre/lib/' --with-ipv6 --with-http_ssl_module --with-http_realip_module --with-http_gunzip_module --with-http_stub_status_module --with-stream --with-stream_ssl_module

Please let me know if there is any additional information I can provide.

Start by running the inline Test::Nginx::Socket::Lua reproduction and compare TEST 1 with TEST 2. Trace the ngx.var.http_host access and ngx.req.set_header entry points to determine why the later Host update is not forwarded, then make both cases report Host: foobar.