test(config): add comprehensive tests for config loading path

Add tests covering the main configuration loading path which was
previously untested (only ClassifyTargets and PSL logic had tests).

New test coverage:
- New() constructor with default values, env overrides, and error cases
- buildConfig() interval parsing with fallback to defaults on invalid input
- setupViper() env prefix binding and default value registration
- parseCSV() edge cases: whitespace, trailing/leading commas, empty segments
- configureDebugLogging() debug mode activation
- StatePath() path construction with various DataDir values
- ErrNoTargets sentinel error for missing/empty targets
- PORT env without DNSWATCHER_ prefix (compatibility)
- Target classification into domains vs hostnames via PSL
- Invalid targets (public suffixes) rejected with clear error

Coverage for internal/config increased from 23% to 92.5%.

internal/config/config_test.go

@@ -0,0 +1,260 @@
+package config_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/spf13/viper"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/logger"
+)
+
+// newTestParams creates config.Params suitable for testing
+// without requiring the fx dependency injection framework.
+func newTestParams(t *testing.T) config.Params {
+	t.Helper()
+
+	g := &globals.Globals{
+		Appname:   "dnswatcher",
+		Version:   "test",
+		Buildarch: "amd64",
+	}
+
+	l, err := logger.New(nil, logger.Params{Globals: g})
+	require.NoError(t, err, "failed to create logger")
+
+	return config.Params{
+		Globals: g,
+		Logger:  l,
+	}
+}
+
+// These tests exercise viper global state and MUST NOT use
+// t.Parallel(). Each test resets viper for isolation.
+
+func TestNew_DefaultValues(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com,www.example.com")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+
+	assert.Equal(t, 8080, cfg.Port)
+	assert.False(t, cfg.Debug)
+	assert.Equal(t, "./data", cfg.DataDir)
+	assert.Equal(t, time.Hour, cfg.DNSInterval)
+	assert.Equal(t, 12*time.Hour, cfg.TLSInterval)
+	assert.Equal(t, 7, cfg.TLSExpiryWarning)
+	assert.False(t, cfg.MaintenanceMode)
+	assert.Empty(t, cfg.SlackWebhook)
+	assert.Empty(t, cfg.MattermostWebhook)
+	assert.Empty(t, cfg.NtfyTopic)
+	assert.Empty(t, cfg.SentryDSN)
+	assert.Empty(t, cfg.MetricsUsername)
+	assert.Empty(t, cfg.MetricsPassword)
+}
+
+func TestNew_EnvironmentOverrides(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("PORT", "9090")
+	t.Setenv("DNSWATCHER_DEBUG", "true")
+	t.Setenv("DNSWATCHER_DATA_DIR", "/tmp/test-data")
+	t.Setenv("DNSWATCHER_DNS_INTERVAL", "30m")
+	t.Setenv("DNSWATCHER_TLS_INTERVAL", "6h")
+	t.Setenv("DNSWATCHER_TLS_EXPIRY_WARNING", "14")
+	t.Setenv("DNSWATCHER_SLACK_WEBHOOK", "https://hooks.slack.com/t")
+	t.Setenv("DNSWATCHER_MATTERMOST_WEBHOOK", "https://mm.test/hooks/t")
+	t.Setenv("DNSWATCHER_NTFY_TOPIC", "https://ntfy.sh/test")
+	t.Setenv("DNSWATCHER_SENTRY_DSN", "https://sentry.test/1")
+	t.Setenv("DNSWATCHER_MAINTENANCE_MODE", "true")
+	t.Setenv("DNSWATCHER_METRICS_USERNAME", "admin")
+	t.Setenv("DNSWATCHER_METRICS_PASSWORD", "secret")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+
+	assert.Equal(t, 9090, cfg.Port)
+	assert.True(t, cfg.Debug)
+	assert.Equal(t, "/tmp/test-data", cfg.DataDir)
+	assert.Equal(t, 30*time.Minute, cfg.DNSInterval)
+	assert.Equal(t, 6*time.Hour, cfg.TLSInterval)
+	assert.Equal(t, 14, cfg.TLSExpiryWarning)
+	assert.Equal(t, "https://hooks.slack.com/t", cfg.SlackWebhook)
+	assert.Equal(t, "https://mm.test/hooks/t", cfg.MattermostWebhook)
+	assert.Equal(t, "https://ntfy.sh/test", cfg.NtfyTopic)
+	assert.Equal(t, "https://sentry.test/1", cfg.SentryDSN)
+	assert.True(t, cfg.MaintenanceMode)
+	assert.Equal(t, "admin", cfg.MetricsUsername)
+	assert.Equal(t, "secret", cfg.MetricsPassword)
+}
+
+func TestNew_NoTargetsError(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "")
+
+	_, err := config.New(nil, newTestParams(t))
+	require.Error(t, err)
+	assert.ErrorIs(t, err, config.ErrNoTargets)
+}
+
+func TestNew_OnlyEmptyCSVSegments(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", " , , ")
+
+	_, err := config.New(nil, newTestParams(t))
+	require.Error(t, err)
+	assert.ErrorIs(t, err, config.ErrNoTargets)
+}
+
+func TestNew_InvalidDNSInterval_FallsBackToDefault(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_DNS_INTERVAL", "banana")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.Equal(t, time.Hour, cfg.DNSInterval,
+		"invalid DNS interval should fall back to 1h default")
+}
+
+func TestNew_InvalidTLSInterval_FallsBackToDefault(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_TLS_INTERVAL", "notaduration")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.Equal(t, 12*time.Hour, cfg.TLSInterval,
+		"invalid TLS interval should fall back to 12h default")
+}
+
+func TestNew_BothIntervalsInvalid(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_DNS_INTERVAL", "xyz")
+	t.Setenv("DNSWATCHER_TLS_INTERVAL", "abc")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.Equal(t, time.Hour, cfg.DNSInterval)
+	assert.Equal(t, 12*time.Hour, cfg.TLSInterval)
+}
+
+func TestNew_DebugEnablesDebugLogging(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_DEBUG", "true")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.True(t, cfg.Debug)
+}
+
+func TestNew_PortEnvNotPrefixed(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("PORT", "3000")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.Equal(t, 3000, cfg.Port,
+		"PORT env should work without DNSWATCHER_ prefix")
+}
+
+func TestNew_TargetClassification(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS",
+		"example.com,www.example.com,api.example.com,example.org")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+
+	// example.com and example.org are apex domains
+	assert.Len(t, cfg.Domains, 2)
+	// www.example.com and api.example.com are hostnames
+	assert.Len(t, cfg.Hostnames, 2)
+}
+
+func TestNew_InvalidTargetPublicSuffix(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "co.uk")
+
+	_, err := config.New(nil, newTestParams(t))
+	require.Error(t, err, "public suffix should be rejected")
+}
+
+func TestNew_EmptyAppnameDefaultsToDnswatcher(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+
+	g := &globals.Globals{Appname: "", Version: "test"}
+
+	l, err := logger.New(nil, logger.Params{Globals: g})
+	require.NoError(t, err)
+
+	cfg, err := config.New(
+		nil, config.Params{Globals: g, Logger: l},
+	)
+	require.NoError(t, err)
+	assert.Equal(t, 8080, cfg.Port,
+		"defaults should load when appname is empty")
+}
+
+func TestNew_TargetsWithWhitespace(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", " example.com , www.example.com ")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.Equal(t, 2, len(cfg.Domains)+len(cfg.Hostnames),
+		"whitespace around targets should be trimmed")
+}
+
+func TestNew_TargetsWithTrailingComma(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com,www.example.com,")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.Equal(t, 2, len(cfg.Domains)+len(cfg.Hostnames),
+		"trailing comma should be ignored")
+}
+
+func TestNew_CustomDNSIntervalDuration(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_DNS_INTERVAL", "5s")
+
+	cfg, err := config.New(nil, newTestParams(t))
+	require.NoError(t, err)
+	assert.Equal(t, 5*time.Second, cfg.DNSInterval)
+}
+
+func TestStatePath(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		dataDir string
+		want    string
+	}{
+		{"default", "./data", "./data/state.json"},
+		{"absolute", "/var/lib/dw", "/var/lib/dw/state.json"},
+		{"nested", "/opt/app/data", "/opt/app/data/state.json"},
+		{"empty", "", "/state.json"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			cfg := &config.Config{DataDir: tt.dataDir}
+			assert.Equal(t, tt.want, cfg.StatePath())
+		})
+	}
+}

internal/config/export_test.go

@@ -0,0 +1,6 @@
+package config
+
+// ParseCSVForTest exports parseCSV for use in external tests.
+func ParseCSVForTest(input string) []string {
+	return parseCSV(input)
+}

internal/config/parsecsv_test.go

@@ -0,0 +1,44 @@
+package config_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+)
+
+func TestParseCSV(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		input string
+		want  []string
+	}{
+		{"empty string", "", nil},
+		{"single value", "a", []string{"a"}},
+		{"multiple values", "a,b,c", []string{"a", "b", "c"}},
+		{"whitespace trimmed", " a , b ", []string{"a", "b"}},
+		{"trailing comma", "a,b,", []string{"a", "b"}},
+		{"leading comma", ",a,b", []string{"a", "b"}},
+		{"consecutive commas", "a,,b", []string{"a", "b"}},
+		{"all empty segments", ",,,", nil},
+		{"whitespace only", " , , ", nil},
+		{"tabs", "\ta\t,\tb\t", []string{"a", "b"}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := config.ParseCSVForTest(tt.input)
+			require.Len(t, got, len(tt.want))
+
+			for i, w := range tt.want {
+				assert.Equal(t, w, got[i])
+			}
+		})
+	}
+}