Configuring remote admin without identity management triggers panic in CertMagic

[yroc92]

As discussed with @mholt: given a config like the one below, Caddy incorrectly calls CertMagic and triggers a panic. See stack trace below.

Config

{
  "logging": {
    "logs": {
      "default": {
        "level": "DEBUG"
      }
    }
  },
  "admin": {
    "remote": {
      "listen": ":2099",
      "access_control": [{
	"public_keys": [<redacted>]
      }]
    }
  }
}

Stack trace

panic: a certificate cache is required

goroutine 1 [running]:
github.com/caddyserver/certmagic.New(...)
        github.com/caddyserver/[email protected]/config.go:169
github.com/caddyserver/caddy/v2.(*IdentityConfig).certmagicConfig(0x0, 0x140002f2cc0, 0x0)
        github.com/caddyserver/caddy/[email protected]/admin.go:536 +0x248
github.com/caddyserver/caddy/v2.replaceRemoteAdminServer({{0x1037fd2d0, 0x1400003a480}, 0x14000879a70, 0x14000052050, {0x0, 0x0, 0x0}}, 0x14000052050)
        github.com/caddyserver/caddy/[email protected]/admin.go:470 +0x328
github.com/caddyserver/caddy/v2.finishSettingUp({{0x1037fd2d0, 0x1400003a480}, 0x14000879a70, 0x14000052050, {0x0, 0x0, 0x0}}, 0x14000052050)
        github.com/caddyserver/caddy/[email protected]/caddy.go:473 +0xd8
github.com/caddyserver/caddy/v2.run(0x14000052050, 0x1)
        github.com/caddyserver/caddy/[email protected]/caddy.go:458 +0x390
github.com/caddyserver/caddy/v2.unsyncedDecodeAndRun({0x1400083b500, 0x658, 0x700}, 0x1)
        github.com/caddyserver/caddy/[email protected]/caddy.go:277 +0x140
github.com/caddyserver/caddy/v2.changeConfig({0x103207bc8, 0x4}, {0x103212800, 0x7}, {0x1400083a700, 0x6be, 0x6bf}, 0x1)
        github.com/caddyserver/caddy/[email protected]/caddy.go:168 +0x4d0
github.com/caddyserver/caddy/v2.Load({0x1400083a700, 0x6be, 0x6bf}, 0x1)
        github.com/caddyserver/caddy/[email protected]/caddy.go:114 +0x88
github.com/caddyserver/caddy/v2/cmd.cmdRun({0x140001b35c0})
        github.com/caddyserver/caddy/[email protected]/cmd/commandfuncs.go:213 +0x7bc
github.com/caddyserver/caddy/v2/cmd.Main()
        github.com/caddyserver/caddy/[email protected]/cmd/main.go:85 +0x4a4
main.main()
        caddy/main.go:11 +0x20

[mholt]

Thanks Cory.

@francislavoie Just FYI I was on the phone with Cory earlier and this is a bug in how Caddy is calling CertMagic. It's a user error, but we should handle it better and improve documentation too.