Debug DNS dig on Mac m1
===
想要了解 dig 是如何運作的，所以下載了 [dig source code](https://github.com/isc-projects/bind9)
查看了一下目前使用的版本

$ dig -v
DiG 9.18.25

把下載後的版本設為 9.18.25

$ cd bind9
$ git checkout v9.18.25

接著根據文件 `doc/arm/build.inc.rst`開始編譯，為了要 debug 需要 `-g`

$ autoreconf -fi
$ CFLAGS="-g" ./configure
$ make

編譯完設定 `launch.json`
一開始設定 `program` 路徑是 `${workspaceFolder}/bin/dig/dig`
後來發現那是一個 script
需要改成 `${workspaceFolder}/bin/dig/.libs/dig`
並根據 script 的提示，加上 `DYLD_LIBRARY_PATH`，否則找不到 lib

        {
            "type": "lldb",
            "request": "launch",
            "name": "dig",
            "program": "${workspaceFolder}/bin/dig/.libs/dig",
            "args": [
                "www.google.com"
            ],
            "cwd": "${workspaceFolder}",
            "env": {
                "DYLD_LIBRARY_PATH": "/Users/dennishuang/workspace/code/bind9/lib/isc/.libs:/Users/dennishuang/workspace/code/bind9/lib/dns/.libs:/Users/dennishuang/workspace/code/bind9/lib/isccfg/.libs:/Users/dennishuang/workspace/code/bind9/lib/irs/.libs:/Users/dennishuang/workspace/code/bind9/lib/bind9/.libs:/Users/dennishuang/workspace/code/bind9/lib/ns/.libs:$DYLD_LIBRARY_PATH"
            }
        }

做完上述設定，在 vscode 執行 debug，執行到 `isc_app_ctxrun` 會觸發 `UNREACHABLE()`，不過已經有找到答案

; <<>> DiG 9.18.25 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58484
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.google.com.                        IN      A

;; ANSWER SECTION:
www.google.com.         149     IN      A       172.217.160.68

;; Query time: 3 msec
;; SERVER: fe80::f01f:c7ff:fe60:d564%27#53(fe80::f01f:c7ff:fe60:d564%27%27) (UDP)
;; WHEN: Wed May 01 17:27:41 CST 2024
;; MSG SIZE  rcvd: 59

app.c:236: INSIST(unreachable) failed, back trace
0   libisc-9.18.25.dylib                0x000000010058b724 default_callback + 72
1   libisc-9.18.25.dylib                0x000000010058b690 isc_assertion_failed + 56
2   libisc-9.18.25.dylib                0x000000010058a5dc isc_app_ctxrun + 1560
3   libisc-9.18.25.dylib                0x000000010058a938 isc_app_run + 176
4   dig                                 0x0000000100007ae8 dig_startup + 88
5   dig                                 0x0000000100007bf0 main + 72
6   dyld                                0x0000000199e820e0 start + 2360

執行的 function `ns__client_request`
使用 epoll 模型

# Ref
http://www.cppblog.com/zhangyq/archive/2010/12/20/136419.html