Let an empty IRC_LISTEN_ADDR disable the IRC listener (closes #124)

Viper treats an environment variable set to the empty string as unset,
so an empty IRC_LISTEN_ADDR fell back to the :6667 default and the IRC
listener could not be disabled from the environment. The config now
reads IRC_LISTEN_ADDR with os.LookupEnv as well, so an empty value
disables the listener and an unset one keeps :6667. Viper's own switch
for this applies to every setting, so it is not used. New config tests
cover both cases, and README.md says how to disable the listener.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -2583,7 +2583,9 @@ is also loaded automatically via [godotenv](https://github.com/joho/godotenv),
 and Viper also reads a YAML file `neoirc.yaml` from `/etc/neoirc/` or
 `$HOME/.config/neoirc/` if one exists, with the same names as keys; an
 environment variable wins over the file. An environment variable set to the
-empty string counts as unset, so the file's value or the default applies.
+empty string counts as unset, so the file's value or the default applies; the
+one exception is `IRC_LISTEN_ADDR`, where the empty string disables the IRC
+listener.
 
 | Variable               | Type   | Default                                                                                | Description                                                                                                                                                                                                                                                                             |
 | ---------------------- | ------ | -------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
@@ -2605,7 +2607,7 @@ empty string counts as unset, so the file's value or the default applies.
 | `NEOIRC_OPER_PASSWORD` | string | `""`                                                                                   | Server operator (o-line) password. Both name and password must be set to enable OPER.                                                                                                                                                                                                   |
 | `LOGIN_RATE_LIMIT`     | float  | `1`                                                                                    | Allowed login attempts per second per IP address.                                                                                                                                                                                                                                       |
 | `LOGIN_RATE_BURST`     | int    | `5`                                                                                    | Maximum burst of login attempts per IP before rate limiting kicks in.                                                                                                                                                                                                                   |
-| `IRC_LISTEN_ADDR`      | string | `:6667`                                                                                | TCP address for the traditional IRC protocol listener. Only an empty value in `neoirc.yaml` disables it; an empty environment variable does not.                                                                                                                                        |
+| `IRC_LISTEN_ADDR`      | string | `:6667`                                                                                | TCP address for the traditional IRC protocol listener. Set it to the empty string, in the environment or in `neoirc.yaml`, to disable the listener.                                                                                                                                     |
 | `MAINTENANCE_MODE`     | bool   | `false`                                                                                | Maintenance mode flag (reserved)                                                                                                                                                                                                                                                        |
 
 ### Example `.env` file
@@ -2632,9 +2634,14 @@ and others.
 ### Configuration
 
 The IRC listener is **enabled by default** on `:6667`. To disable it, set
-`IRC_LISTEN_ADDR` to an empty string in `neoirc.yaml` (see
+`IRC_LISTEN_ADDR` to an empty string (see [Configuration](#configuration)),
-[Configuration](#configuration)); an empty environment variable counts as unset
+either in the environment:
-and leaves it on `:6667`:
+
+```bash
+IRC_LISTEN_ADDR=
+```
+
+or in `neoirc.yaml`:
 
 ```yaml
 IRC_LISTEN_ADDR: ""

internal/config/config.go

@@ -4,6 +4,7 @@ package config
 import (
 	"errors"
 	"log/slog"
+	"os"
 
 	"github.com/spf13/viper"
 	"go.uber.org/fx"
@@ -129,6 +130,13 @@ func New(
 		params:             &params,
 	}
 
+	// Viper counts an empty environment variable as unset, but an
+	// empty IRC_LISTEN_ADDR must turn the IRC listener off.
+	ircListenAddr, set := os.LookupEnv("IRC_LISTEN_ADDR")
+	if set {
+		cfg.IRCListenAddr = ircListenAddr
+	}
+
 	if cfg.Debug {
 		params.Logger.EnableDebugLogging()
 		cfg.log = params.Logger.Get()

internal/config/config_test.go

@@ -0,0 +1,61 @@
+package config_test
+
+import (
+	"os"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/neoirc/internal/config"
+	"sneak.berlin/go/neoirc/internal/globals"
+	"sneak.berlin/go/neoirc/internal/logger"
+)
+
+func newConfig(t *testing.T) *config.Config {
+	t.Helper()
+
+	globs := &globals.Globals{
+		Appname:   "neoirc-test",
+		Version:   "test",
+		StartTime: time.Now(),
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: globs})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := config.New(
+		nil, config.Params{Globals: globs, Logger: log},
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return cfg
+}
+
+func TestIRCListenAddrUnsetKeepsDefault(t *testing.T) {
+	// t.Setenv puts back whatever value was there after the test.
+	t.Setenv("IRC_LISTEN_ADDR", "")
+
+	err := os.Unsetenv("IRC_LISTEN_ADDR")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	const want = ":6667"
+
+	cfg := newConfig(t)
+	if cfg.IRCListenAddr != want {
+		t.Errorf("IRCListenAddr = %q, want %q", cfg.IRCListenAddr, want)
+	}
+}
+
+func TestIRCListenAddrEmptyTurnsListenerOff(t *testing.T) {
+	t.Setenv("IRC_LISTEN_ADDR", "")
+
+	cfg := newConfig(t)
+	if cfg.IRCListenAddr != "" {
+		t.Errorf("IRCListenAddr = %q, want empty", cfg.IRCListenAddr)
+	}
+}