Caddy is not throwing an error on MacOS if address is already in use

Caddy starts up without issues on MacOS even if there is already another process listening on the same port.

Example:

# test1/Caddyfile
:80 {
  respond "hello world server1"
}

# test2/Caddyfile
:80 {
  respond "hello world server2"
}

Starting up server 1:

$ caddy run --config test1/Caddyfile
2024/08/12 11:58:43.773 INFO    using config from file  {"file": "test1/Caddyfile"}
2024/08/12 11:58:43.773 INFO    adapted config to JSON  {"adapter": "caddyfile"}
2024/08/12 11:58:43.775 INFO    admin   admin endpoint started  {"address": "localhost:2019", "enforce_origin": false, "origins": ["//[::1]:2019", "//127.0.0.1:2019", "//localhost:2019"]}
2024/08/12 11:58:43.775 WARN    http.auto_https server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server {"server_name": "srv0", "http_port": 80}
2024/08/12 11:58:43.775 INFO    tls.cache.maintenance   started background certificate maintenance  {"cache": "0xc000676280"}
2024/08/12 11:58:43.775 INFO    http.log    server running  {"name": "srv0", "protocols": ["h1", "h2", "h3"]}
2024/08/12 11:58:43.775 INFO    autosaved config (load with --resume flag)  {"file": "/Users/work/Library/Application Support/Caddy/autosave.json"}
2024/08/12 11:58:43.775 INFO    serving initial configuration
2024/08/12 11:58:43.799 INFO    tls storage cleaning happened too recently; skipping for now    {"storage": "FileStorage:/Users/work/Library/Application Support/Caddy", "instance": "394620b5-a9e0-4018-b6c0-60875ef7ed91", "try_again": "2024/08/13 11:58:43.799", "try_again_in": 86399.999999331}
2024/08/12 11:58:43.800 INFO    tls finished cleaning storage units

Starting up server 2:

$ caddy run --config test2/Caddyfile
2024/08/12 11:59:44.663 INFO    using config from file  {"file": "test2/Caddyfile"}
2024/08/12 11:59:44.664 INFO    adapted config to JSON  {"adapter": "caddyfile"}
2024/08/12 11:59:44.665 INFO    admin   admin endpoint started  {"address": "localhost:2019", "enforce_origin": false, "origins": ["//localhost:2019", "//[::1]:2019", "//127.0.0.1:2019"]}
2024/08/12 11:59:44.665 WARN    http.auto_https server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server {"server_name": "srv0", "http_port": 80}
2024/08/12 11:59:44.665 INFO    tls.cache.maintenance   started background certificate maintenance  {"cache": "0xc00068b780"}
2024/08/12 11:59:44.665 INFO    http.log    server running  {"name": "srv0", "protocols": ["h1", "h2", "h3"]}
2024/08/12 11:59:44.665 INFO    autosaved config (load with --resume flag)  {"file": "/Users/work/Library/Application Support/Caddy/autosave.json"}
2024/08/12 11:59:44.665 INFO    serving initial configuration
2024/08/12 11:59:44.687 INFO    tls storage cleaning happened too recently; skipping for now    {"storage": "FileStorage:/Users/work/Library/Application Support/Caddy", "instance": "394620b5-a9e0-4018-b6c0-60875ef7ed91", "try_again": "2024/08/13 11:59:44.687", "try_again_in": 86399.999999446}
2024/08/12 11:59:44.687 INFO    tls finished cleaning storage units

Sending a request is answered by server 1:

$ curl http://localhost
hello world server1%

Expected behaviour: Server 2 should not start up but throw an error, such as it does on Linux systems:

$ caddy run --config test2/Caddyfile
2024/08/12 11:56:25.354 INFO    using config from file  {"file": "test1/Caddyfile"}
2024/08/12 11:56:25.355 INFO    adapted config to JSON  {"adapter": "caddyfile"}
2024/08/12 11:56:25.356 INFO    admin   admin endpoint started  {"address": ":2020", "enforce_origin": false, "origins": ["//:2020"]}
2024/08/12 11:56:25.356 WARN    admin   admin endpoint on open interface; host checking disabled    {"address": ":2020"}
2024/08/12 11:56:25.356 WARN    http.auto_https server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server {"server_name": "srv0", "http_port": 80}
2024/08/12 11:56:25.356 INFO    tls.cache.maintenance   started background certificate maintenance  {"cache": "0xc00072be80"}
2024/08/12 11:56:25.356 INFO    tls.cache.maintenance   stopped background certificate maintenance  {"cache": "0xc00072be80"}
Error: loading initial config: loading new config: http app module: start: listening on :80: listen tcp :80: bind: address already in use

Versions:

$  caddy --version
v2.8.4 h1:q3pe0wpBj1OcHFZ3n/1nl4V4bxBrYoSoab7rL9BMYNk=

MacOS: 14.6.1

caddyserver / caddy

Caddy is not throwing an error on MacOS if address is already in use #6515