Cannot remove headers set with add_header and variable value from map in http block

[kkmuffme]

openresty/1.25.3.2

http {
    map $upstream_http_cache_control $custom_cache_control {
        "" "private, no-cache, no-store, must-revalidate, max-age=0";
        default "";
    }
    add_header Cache-Control "$custom_cache_control" always;
    
    server {
        ...
        location / {
            // **

            content_by_lua_file '/my.lua';
        }
    }
}

my.lua

ngx.header["Expires"] = nil
local now = os.time( os.date( "!*t" ) )
ngx.header["Expires"] = os.date( "%a, %d %b %Y %H:%M:%S ", now + 3600 ) .. "GMT"
return ngx.exit( ngx.HTTP_OK )

At the place marked with // ** I also tried:

more_clear_headers 'Expires';

=> this doesn't work either - it will only remove the new Expires header I set with the .lua but not the one from http block

What works is using this at // ** :

add_header Remove "" always;

Then I will only get my Expires header I expected (however all other headers set with add_header are lost/I need to include/set them all there too, which I'd rather avoid)

EDIT: adding ngx.log( ngx.ALERT, ngx.header["Expires"] ) BEFORE ngx.header["Expires"] = nil results in nil, which means the header isn't set yet, so obviously I can't remove it there.
Which goes against what the docs for both ngx.header and more_clear_headers say though?

[kkmuffme]

I checked with debug http://nginx.org/en/docs/debugging_log.html but that wasn't really useful, since the issue is already obvious (and that's also what I see in the debug log that those headers aren't set yet)
(fyi there are some other scripts running that are irrelevant for this, it's included in the output. I highlighted relevant things below though)

[debug] 2268815#2268815: *418 http cl:-1 max:104857600
[debug] 2268815#2268815: *418 rewrite phase: 4
[debug] 2268815#2268815: *418 rewrite phase: 5
[debug] 2268815#2268815: *418 rewrite phase: 6
[debug] 2268815#2268815: *418 lua rewrite handler, uri:"/" c:1
[debug] 2268815#2268815: *418 code cache lookup (key='rewrite_by_lua_nhli_6206f9203148bf74ea0eb32c0c536c67', ref=2)
[debug] 2268815#2268815: *418 code cache hit (key='rewrite_by_lua_nhli_6206f9203148bf74ea0eb32c0c536c67', ref=2)
[debug] 2268815#2268815: *418 lua creating new thread
[debug] 2268815#2268815: *418 lua reusing cached lua thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 lua reset ctx
[debug] 2268815#2268815: *418 http lua finalize threads
[debug] 2268815#2268815: *418 lua run thread, top:0 c:1
[debug] 2268815#2268815: *418 add cleanup: 0000000001C29398
[debug] 2268815#2268815: *418 lua resume returned 0
[debug] 2268815#2268815: *418 lua light thread ended normally
[debug] 2268815#2268815: *418 lua deleting light thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 lua caching unused lua thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 post rewrite phase: 7
[debug] 2268815#2268815: *418 generic phase: 8
[debug] 2268815#2268815: *418 generic phase: 9
[debug] 2268815#2268815: *418 generic phase: 10
[debug] 2268815#2268815: *418 access phase: 11
[debug] 2268815#2268815: *418 access phase: 12
[debug] 2268815#2268815: *418 access phase: 13
[debug] 2268815#2268815: *418 lua access handler, uri:"/" c:1
[debug] 2268815#2268815: *418 code cache lookup (key='nhlf_f26d53dac9d31d2ad545f24404e10fe7', ref=3)
[debug] 2268815#2268815: *418 code cache hit (key='nhlf_f26d53dac9d31d2ad545f24404e10fe7', ref=3)
[debug] 2268815#2268815: *418 lua creating new thread
[debug] 2268815#2268815: *418 lua reusing cached lua thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 lua reset ctx
[debug] 2268815#2268815: *418 http lua finalize threads
[debug] 2268815#2268815: *418 lua run thread, top:0 c:1
[debug] 2268815#2268815: *418 lua resume returned 0
[debug] 2268815#2268815: *418 lua light thread ended normally
[debug] 2268815#2268815: *418 lua deleting light thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 lua caching unused lua thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 post access phase: 14
[debug] 2268815#2268815: *418 generic phase: 15
[debug] 2268815#2268815: *418 generic phase: 16
[debug] 2268815#2268815: *418 lua content handler, uri:"/" c:1
[debug] 2268815#2268815: *418 code cache lookup (key='nhlf_916db558d68aa4dac3b6a064618d1323', ref=4)
[debug] 2268815#2268815: *418 code cache hit (key='nhlf_916db558d68aa4dac3b6a064618d1323', ref=4)
[debug] 2268815#2268815: *418 lua reset ctx
[debug] 2268815#2268815: *418 http lua finalize threads
[debug] 2268815#2268815: *418 lua creating new thread
[debug] 2268815#2268815: *418 lua reusing cached lua thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 lua run thread, top:0 c:1
[debug] 2268815#2268815: *418 lua tcp socket pool get keepalive peer
[debug] 2268815#2268815: *418 lua tcp socket get keepalive peer: using connection 00007F2DD52838C0, fd:105
[debug] 2268815#2268815: *418 event timer del: 105: 33240101260
[debug] 2268815#2268815: *418 http cleanup add: 0000000001C29440
[debug] 2268815#2268815: *418 lua tcp socket send timeout: 5000
[debug] 2268815#2268815: *418 lua allocate new chainlink and new buf of size 99, cl:0000000001C29458
[debug] 2268815#2268815: *418 lua tcp socket send data
[debug] 2268815#2268815: *418 send: fd:105 71 of 71
[debug] 2268815#2268815: *418 lua tcp socket sent all the data
[debug] 2268815#2268815: *418 lua tcp socket calling receive() method
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua allocate new chainlink and new buf of size 4096, cl:0000000001C29468
[debug] 2268815#2268815: *418 malloc: 0000000002A90620:4096
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:0
[debug] 2268815#2268815: *418 event timer add: 105: 5000:33239884283
[debug] 2268815#2268815: *418 lua resume returned 1
[debug] 2268815#2268815: *418 lua thread yielded
[debug] 2268815#2268815: *418 http finalize request: -4, "/?" a:1, c:2
[debug] 2268815#2268815: *418 http request count:2 blk:0
[debug] 2268815#2268815: *418 lua tcp socket handler for "/?", wev 1
[debug] 2268815#2268815: *418 lua tcp socket dummy handler
[debug] 2268815#2268815: *418 lua tcp socket handler for "/?", wev 0
[debug] 2268815#2268815: *418 lua tcp socket read handler
[debug] 2268815#2268815: *418 event timer del: 105: 33239884283
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:1
[debug] 2268815#2268815: *418 lua tcp socket try to recv data 4096: "/?"
[debug] 2268815#2268815: *418 recv: eof:0, avail:-1
[debug] 2268815#2268815: *418 recv: fd:105 4096 of 4096
[debug] 2268815#2268815: *418 recv: avail:21541
[debug] 2268815#2268815: *418 lua tcp socket recv returned 4096: "/?"
[debug] 2268815#2268815: *418 lua tcp socket read line
[debug] 2268815#2268815: *418 lua read the final line part: "*2
[debug] 2268815#2268815: *418 lua tcp socket receive done: wait:1, eof:0, uri:"/?"
[debug] 2268815#2268815: *418 lua tcp socket waking up the current request (read)
[debug] 2268815#2268815: *418 lua tcp operation done, resuming lua thread
[debug] 2268815#2268815: *418 lua tcp socket calling prepare retvals handler 0000000000549A00, u:00007F2DD465A870
[debug] 2268815#2268815: *418 lua tcp socket receive return value handler
[debug] 2268815#2268815: *418 lua run thread, top:0 c:1
[debug] 2268815#2268815: *418 lua tcp socket calling receive() method
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:0
[debug] 2268815#2268815: *418 lua tcp socket read line
[debug] 2268815#2268815: *418 lua read the final line part: "$24557
[debug] 2268815#2268815: *418 lua tcp socket receive done: wait:0, eof:0, uri:"/?"
[debug] 2268815#2268815: *418 lua tcp socket receive done in a single run
[debug] 2268815#2268815: *418 lua tcp socket receive return value handler
[debug] 2268815#2268815: *418 lua tcp socket calling receive() method
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:0
[debug] 2268815#2268815: *418 lua tcp socket read chunk 4084
[debug] 2268815#2268815: *418 lua allocate new chainlink and new buf of size 4096, cl:0000000001C29488
[debug] 2268815#2268815: *418 malloc: 0000000002A9FF40:4096
[debug] 2268815#2268815: *418 lua tcp socket try to recv data 4096: "/?"
[debug] 2268815#2268815: *418 recv: eof:0, avail:21541
[debug] 2268815#2268815: *418 recv: fd:105 4096 of 4096
[debug] 2268815#2268815: *418 recv: avail:17445
[debug] 2268815#2268815: *418 lua tcp socket recv returned 4096: "/?"
[debug] 2268815#2268815: *418 lua tcp socket read chunk 4096
[debug] 2268815#2268815: *418 lua allocate new chainlink and new buf of size 4096, cl:0000000002A8FC18
[debug] 2268815#2268815: *418 malloc: 0000000002AA0F50:4096
[debug] 2268815#2268815: *418 lua tcp socket try to recv data 4096: "/?"
[debug] 2268815#2268815: *418 recv: eof:0, avail:17445
[debug] 2268815#2268815: *418 recv: fd:105 4096 of 4096
[debug] 2268815#2268815: *418 recv: avail:13349
[debug] 2268815#2268815: *418 lua tcp socket recv returned 4096: "/?"
[debug] 2268815#2268815: *418 lua tcp socket read chunk 4096
[debug] 2268815#2268815: *418 lua allocate new chainlink and new buf of size 4096, cl:0000000002A8FC88
[debug] 2268815#2268815: *418 malloc: 0000000002AA1F60:4096
[debug] 2268815#2268815: *418 lua tcp socket try to recv data 4096: "/?"
[debug] 2268815#2268815: *418 recv: eof:0, avail:13349
[debug] 2268815#2268815: *418 recv: fd:105 4096 of 4096
[debug] 2268815#2268815: *418 recv: avail:9253
[debug] 2268815#2268815: *418 lua tcp socket recv returned 4096: "/?"
[debug] 2268815#2268815: *418 lua tcp socket read chunk 4096
[debug] 2268815#2268815: *418 lua allocate new chainlink and new buf of size 4096, cl:0000000002A8FCF8
[debug] 2268815#2268815: *418 malloc: 0000000002AA2F70:4096
[debug] 2268815#2268815: *418 lua tcp socket try to recv data 4096: "/?"
[debug] 2268815#2268815: *418 recv: eof:0, avail:9253
[debug] 2268815#2268815: *418 recv: fd:105 4096 of 4096
[debug] 2268815#2268815: *418 recv: avail:5157
[debug] 2268815#2268815: *418 lua tcp socket recv returned 4096: "/?"
[debug] 2268815#2268815: *418 lua tcp socket read chunk 4096
[debug] 2268815#2268815: *418 lua allocate new chainlink and new buf of size 4096, cl:0000000002A8FD68
[debug] 2268815#2268815: *418 malloc: 0000000002AA3F80:4096
[debug] 2268815#2268815: *418 lua tcp socket try to recv data 4096: "/?"
[debug] 2268815#2268815: *418 recv: eof:0, avail:5157
[debug] 2268815#2268815: *418 recv: fd:105 4096 of 4096
[debug] 2268815#2268815: *418 recv: avail:1061
[debug] 2268815#2268815: *418 lua tcp socket recv returned 4096: "/?"
[debug] 2268815#2268815: *418 lua tcp socket read chunk 4096
[debug] 2268815#2268815: *418 lua tcp socket receive done: wait:0, eof:0, uri:"/?"
[debug] 2268815#2268815: *418 lua tcp socket receive done in a single run
[debug] 2268815#2268815: *418 lua tcp socket receive return value handler
[debug] 2268815#2268815: *418 lua tcp socket calling receive() method
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:0
[debug] 2268815#2268815: *418 lua tcp socket read chunk 7
[debug] 2268815#2268815: *418 lua tcp socket receive done: wait:0, eof:0, uri:"/?"
[debug] 2268815#2268815: *418 lua tcp socket receive done in a single run
[debug] 2268815#2268815: *418 lua tcp socket receive return value handler
[debug] 2268815#2268815: *418 lua tcp socket calling receive() method
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:0
[debug] 2268815#2268815: *418 lua tcp socket read line
[debug] 2268815#2268815: *418 lua read partial line data: $1057
[debug] 2268815#2268815: *418 lua reuse free buf memory 4096 >= 4096, cl:0000000001C29468, p:0000000002A90620
[debug] 2268815#2268815: *418 lua tcp socket try to recv data 4096: "/?"
[debug] 2268815#2268815: *418 recv: eof:0, avail:1061
[debug] 2268815#2268815: *418 recv: fd:105 1061 of 4096
[debug] 2268815#2268815: *418 recv: avail:0
[debug] 2268815#2268815: *418 lua tcp socket recv returned 1061: "/?"
[debug] 2268815#2268815: *418 lua tcp socket read line
"024/09/09 07:35:47 [debug] 2268815#2268815: *418 lua read the final line part: "
[debug] 2268815#2268815: *418 lua tcp socket receive done: wait:0, eof:0, uri:"/?"
[debug] 2268815#2268815: *418 lua tcp socket receive done in a single run
[debug] 2268815#2268815: *418 lua tcp socket receive return value handler
[debug] 2268815#2268815: *418 lua tcp socket calling receive() method
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:0
[debug] 2268815#2268815: *418 lua tcp socket read chunk 1059
[debug] 2268815#2268815: *418 lua tcp socket receive done: wait:0, eof:0, uri:"/?"
[debug] 2268815#2268815: *418 lua tcp socket receive done in a single run
[debug] 2268815#2268815: *418 lua tcp socket receive return value handler
[debug] 2268815#2268815: *418 lua tcp socket calling receive() method
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read timeout: 5000
[debug] 2268815#2268815: *418 lua tcp socket read data: wait:0
[debug] 2268815#2268815: *418 lua tcp socket read chunk 2
[debug] 2268815#2268815: *418 lua tcp socket receive done: wait:0, eof:0, uri:"/?"
[debug] 2268815#2268815: *418 lua tcp socket receive done in a single run
[debug] 2268815#2268815: *418 lua tcp socket receive return value handler
[debug] 2268815#2268815: *418 lua tcp socket set keepalive: saving connection 00007F2DD52838C0
[debug] 2268815#2268815: *418 lua tcp socket clear current socket connection
[debug] 2268815#2268815: *418 lua tcp socket keepalive timeout: 300000 ms
[debug] 2268815#2268815: *418 event timer add: 105: 300000:33240179283
[debug] 2268815#2268815: *418 lua finalize socket
[debug] 2268815#2268815: *418 lua http cleanup free: 0000000001C29440
[alert] 2268815#2268815: *418 [lua] my.lua:5: nil ----------------------------------------------------> here ngx.log ngx.header["Expires"]
[debug] 2268815#2268815: *418 lua reuse free buf chain, but reallocate memory because 24557 >= 99, cl:0000000001C29458, p:0000000002A8FB00
[debug] 2268815#2268815: *418 malloc: 0000000002AA4F90:24557
[debug] 2268815#2268815: *418 lua print response
[debug] 2268815#2268815: *418 headers more header filter, uri "/" **---> only here the headers from http block will get set **
[debug] 2268815#2268815: *418 lua capture header filter, uri "/"
[debug] 2268815#2268815: *418 http map started
[debug] 2268815#2268815: *418 http map: "" "private, no-cache, no-store, must-revalidate, max-age=0"
[debug] 2268815#2268815: *418 http script var: "private, no-cache, no-store, must-revalidate, max-age=0"
[debug] 2268815#2268815: *418 http map started
[debug] 2268815#2268815: *418 http map: "" "Wed, 11 Jan 1984 05:00:00 GMT"
[debug] 2268815#2268815: *418 http script var: "Wed, 11 Jan 1984 05:00:00 GMT"
[debug] 2268815#2268815: *418 http script copy: "1; mode=block; report=https://"
[debug] 2268815#2268815: *418 http script var: "example.com"
[debug] 2268815#2268815: *418 http script copy: "/some-file.php"
[debug] 2268815#2268815: *418 posix_memalign: 0000000002AAAF90:4096 @16
[debug] 2268815#2268815: *418 http script copy: "default-src ..."
[debug] 2268815#2268815: *418 http script var: "example.com"
[debug] 2268815#2268815: *418 http script copy: "/some-file.php; report-to some-endpoint"
[debug] 2268815#2268815: *418 http script copy: "{ ..."
[debug] 2268815#2268815: *418 http script var: "example.com"
[debug] 2268815#2268815: *418 http script copy: "/some-file.php" }"
[debug] 2268815#2268815: *418 http script var: "example.com"
[debug] 2268815#2268815: *418 http script copy: "/some-file.php" } ] }"
[debug] 2268815#2268815: *418 http2 header filter
[debug] 2268815#2268815: *418 posix_memalign: 0000000002AABFA0:4096 @16
[debug] 2268815#2268815: *418 http2 output header: ":status: 200"
[debug] 2268815#2268815: *418 http2 output header: "content-type: text/html; charset=UTF-8"
[debug] 2268815#2268815: *418 http2 output header: "date: Mon, 09 Sep 2024 07:35:47 GMT"
[debug] 2268815#2268815: *418 http2 output header: "expires: Mon, 09 Sep 2024 08:35:47 GMT"
[debug] 2268815#2268815: *418 http2 output header: "cache-control: private, no-cache, no-store, must-revalidate, max-age=0"
[debug] 2268815#2268815: *418 http2 output header: "expires: Wed, 11 Jan 1984 05:00:00 GMT"
...
[debug] 2268815#2268815: *418 http2:1 create HEADERS frame 0000000002A904C0: len:2576 fin:0
[debug] 2268815#2268815: *418 http cleanup add: 0000000002A905C8
[debug] 2268815#2268815: *418 http output filter "/?"
[debug] 2268815#2268815: *418 http copy filter: "/?"
[debug] 2268815#2268815: *418 lua capture body filter, uri "/"
[debug] 2268815#2268815: *418 http postpone filter "/?" 0000000001C29458
[debug] 2268815#2268815: *418 http brotli filter
[debug] 2268815#2268815: *418 write new buf t:1 f:0 0000000002AA4F90, pos 0000000002AA4F90, size: 24557 file: 0, size: 0
[debug] 2268815#2268815: *418 http write filter: l:0 f:0 s:24557
[debug] 2268815#2268815: *418 http write filter limit 5242880
[debug] 2268815#2268815: *418 http2 send chain: 0000000002A905E0
[debug] 2268815#2268815: *418 http2:1 create DATA frame 0000000002A904C0: len:8192 flags:0
[debug] 2268815#2268815: *418 http2:1 create DATA frame 0000000002AABB50: len:8192 flags:0
[debug] 2268815#2268815: *418 http2:1 create DATA frame 0000000002AABC48: len:8173 flags:0
[debug] 2268815#2268815: *418 http write filter 0000000000000000
[debug] 2268815#2268815: *418 http copy filter: 0 "/?"
[debug] 2268815#2268815: *418 lua print response has no busy bufs
[debug] 2268815#2268815: *418 lua exit with code 200
[debug] 2268815#2268815: *418 lua resume returned 1
[debug] 2268815#2268815: *418 lua thread yielded
[debug] 2268815#2268815: *418 lua thread aborting request with status 200
[debug] 2268815#2268815: *418 lua request cleanup: forcible=0
[debug] 2268815#2268815: *418 http lua finalize threads
[debug] 2268815#2268815: *418 lua deleting light thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 lua caching unused lua thread 00007F2DD9A068F0 (ref 1)
[debug] 2268815#2268815: *418 lua sending last buf of the response body
[debug] 2268815#2268815: *418 http output filter "/?"
[debug] 2268815#2268815: *418 http copy filter: "/?"
[debug] 2268815#2268815: *418 lua capture body filter, uri "/"
[debug] 2268815#2268815: *418 http postpone filter "/?" 00007FFED8658440
[debug] 2268815#2268815: *418 http brotli filter
[debug] 2268815#2268815: *418 write new buf t:0 f:0 0000000000000000, pos 0000000000000000, size: 0 file: 0, size: 0
[debug] 2268815#2268815: *418 http write filter: l:1 f:0 s:0
[debug] 2268815#2268815: *418 http write filter limit 5242880
[debug] 2268815#2268815: *418 http2 send chain: 0000000002A905E0
[debug] 2268815#2268815: *418 http2:1 create DATA frame 0000000002AABC48: len:0 flags:1
[debug] 2268815#2268815: *418 http write filter 0000000000000000
[debug] 2268815#2268815: *418 http copy filter: 0 "/?"
[debug] 2268815#2268815: *418 lua run thread returned 0
[debug] 2268815#2268815: *418 http finalize request: 0, "/?" a:1, c:1
[debug] 2268815#2268815: *418 http request count:1 blk:0
[debug] 2268815#2268815: *418 http close request
[debug] 2268815#2268815: *418 http log handler
[debug] 2268815#2268815: *418 run cleanup: 0000000001C29398
[debug] 2268815#2268815: *418 run cleanup: 0000000000D467D0
[debug] 2268815#2268815: *418 free: 0000000002AA4F90
[debug] 2268815#2268815: *418 free: 0000000002AA3F80
[debug] 2268815#2268815: *418 free: 0000000002AA2F70
[debug] 2268815#2268815: *418 free: 0000000002AA1F60
[debug] 2268815#2268815: *418 free: 0000000002AA0F50
[debug] 2268815#2268815: *418 free: 0000000002A9FF40
[debug] 2268815#2268815: *418 free: 0000000002A90620
[debug] 2268815#2268815: *418 free: 0000000001C294B0
[debug] 2268815#2268815: *418 free: 0000000000D45930, unused: 0
[debug] 2268815#2268815: *418 free: 0000000001C284A0, unused: 0
[debug] 2268815#2268815: *418 free: 0000000002A8F610, unused: 0
[debug] 2268815#2268815: *418 free: 0000000002AAAF90, unused: 576
[debug] 2268815#2268815: *418 free: 0000000002AABFA0, unused: 510

[HanadaLee]

The add_header directive takes effect after the headers_more and lua content and lua header_filter phase. If you want to use lua code or headers_more directive, please consider the following configuration

http {
    map $upstream_http_cache_control $custom_cache_control {
        "" "private, no-cache, no-store, must-revalidate, max-age=0";
        default "";
    }
# I think -a should not be used for Cache-Control header, it is just to restore the functionality of add_header
    more_set_headers -a "Cache-Control: $custom_cache_control";
    
    server {
        ...
        location / {
            // **

            # You created variables in http based on $upstream_http_cache_control through map, but proxy_pass and content_by_lua_block cannot be used at the same time, so I believe $upstream_http_cache_control will never exist. You may need to show your specific use case again.
            content_by_lua_file '/my.lua';
            # When you need to modify the response header, you should use header_filter_by_lua_file instead of content_by_lua_file
            header_filter_by_lua_file '/header_filter.lua';
        }
    }
}

According to the execution phase, it is probably as follows, you can verify it again:

1. content_by_lua_xxx
2. more_set_headers
3. header_filter_by_lua_xxx
4. add_header

Finally, there is a question: do you want to operate on the Cache-Control or Expires header, or both?