ngx.redirect 307 makes srcache_fetch fail

Hi,

using nginx 1.18 (+ openssl callback patch), http-lua 0.10.19, srcache 0.32, luajit-2.1.20201027 on a debian box,
when i use ngx.redirect(url) in access_by_lua on a working srcache config, everything's all right.
However if i use ngx.redirect(url, 307) i get

fetching key "test2.localhost.localdomain" in shared dict "myDomains"
shmtx lock
shmtx unlock
lua redirect to "https://test3.localhost.localdomain:17443/mypath" with code 307
lua resume returned 1
lua thread yielded
lua thread aborting request with status 307
lua request cleanup: forcible=0
http lua finalize threads
lua deleting light thread 00007FAC45F13F00 (ref 2)
lua caching unused lua thread 00007FAC45F13F00 (ref 2)
http finalize request: 307, "/mypath?" a:1, c:1
http special response: 307, "/mypath?"
http script var: "0"
srcache_store decides to store the response
srcache store header HTTP/1.1 307
Content-Type: text/html
Location: https://test3.localhost.localdomain:17443/mypath

srcache_fetch: buffer error when serializing the response header: -17 left, client: 127.0.0.1, server: _, request: "GET /mypath HTTP/2.0", host: "test2.localhost.localdomain:17443"
http finalize request: -1, "/mypath?" a:1, c:1
http terminate request count:1
http terminate cleanup count:1 blk:0
http posted request: "/mypath?"
http terminate handler count:1
http request count:1 blk:0
http2 close stream 1, queued 0, processing 1, pushing 0
http2 send RST_STREAM frame sid:1, status:2
http close request

and the client dies with

curl: (92) HTTP/2 stream 0 was not closed cleanly: INTERNAL_ERROR (err 2)

For comparison here's the log with a "standard" redirection:

fetching key "test2.localhost.localdomain" in shared dict "myDomains"
shmtx lock
shmtx unlock
lua redirect to "https://test3.localhost.localdomain:17443/mypath" with code 307
lua resume returned 1
lua thread yielded
lua thread aborting request with status 307
lua request cleanup: forcible=0
http lua finalize threads
lua deleting light thread 00007F9E5E1BADB0 (ref 2)
lua caching unused lua thread 00007F9E5E1BADB0 (ref 2)
http finalize request: 307, "/mypath?" a:1, c:1
http special response: 307, "/mypath?"
http script var: "0"
srcache_store bypassed because of unmatched status code 307 with srcache_store_statuses
headers more header filter, uri "/mypath"
lua header filter for user lua code, uri "/mypath"
http cleanup add: 000055A7B2533998
code cache lookup (key='header_filter_by_lua_nhli_46c354449c5f02150cba6fbbeb9a2288', ref=-1)
code cache miss (key='header_filter_by_lua_nhli_46c354449c5f02150cba6fbbeb9a2288', ref=-1)
fetching key "test2.localhost.localdomain" in shared dict "upcacheLocks"
shmtx lock
shmtx unlock
[lua] upcache.lua:50: response(): response key 'nil', client: 127.0.0.1, server: _, request: "GET /mypath HTTP/2.0", host: "test2.localhost.localdomain:17443"
lua capture header filter, uri "/mypath"
charset: "" > "utf-8"
http2 header filter
posix_memalign: 000055A7B2535AC0:4096 @16
http2 output header: ":status: 307"
http2 output header: "server: nginx"
http2 output header: "date: Fri, 13 Nov 2020 13:11:18 GMT"
http2 output header: "content-type: text/html; charset=utf-8"
http2 output header: "content-length: 164"
http2 output header: "location: https://test3.localhost.localdomain:17443/mypath"
http2 output header: "strict-transport-security: max-age=63072000; includeSubDomains; preload"
http2:1 create HEADERS frame 000055A7B2535BF8: len:166 fin:0
http cleanup add: 000055A7B2535D00
http2 frame out: 000055A7B2535BF8 sid:1 bl:1 len:166
SSL buf copy: 9
SSL buf copy: 166
http2:1 HEADERS frame 000055A7B2535BF8 was sent
http2 frame sent: 000055A7B2535BF8 sid:1 bl:1 len:166
http output filter "/mypath?"
http copy filter: "/mypath?"
lua capture body filter, uri "/mypath"
http postpone filter "/mypath?" 000055A7B2535E40

Start at the srcache_fetch response-header serialization path and reproduce the 307 redirect with the listed nginx, http-lua, srcache, and LuaJIT versions. Compare it with the standard redirection log; done means the 307 response no longer produces the buffer serialization error or causes the HTTP/2 client stream to fail.