Add best-effort geolocation to the IP lookup from geojs and DB-IP City Lite (closes #239)

Each result gains `geo`. One request to geojs carries every address, 1 second in all. On a timeout, an error or an answer with no country, city or coordinates, the lookup reads DB-IP City Lite, downloaded monthly into the state directory. geojs answers stay in memory (250,000, least recently used dropped) and in a new `cache.db` table for 7 days.

Judgement call: a reserved address goes to neither source, and its `geo` is null.
Judgement call: `updated` for DB-IP is when the copy was downloaded.
Deviation: the cache is SQLite, by sneak's words.

Model: opus-5-5

internal/server/handlers_internal_test.go

@@ -15,6 +15,7 @@ import (
 	"slices"
 	"strconv"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"testing/synctest"
@@ -22,6 +23,7 @@ import (
 
 	"git.eeqj.de/sneak/routewatch/internal/config"
 	"git.eeqj.de/sneak/routewatch/internal/database"
+	"git.eeqj.de/sneak/routewatch/internal/geo"
 	"git.eeqj.de/sneak/routewatch/internal/iplist"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
 	"git.eeqj.de/sneak/routewatch/internal/metrics"
@@ -1613,6 +1615,131 @@ func TestIPLookupFlags(t *testing.T) {
 	}
 }
 
+// fakeGeoLocator answers every address with location, and keeps the addresses
+// of each call. It is safe for the concurrent subtests that share it.
+type fakeGeoLocator struct {
+	location *geo.Location
+
+	mu    sync.Mutex
+	calls [][]netip.Addr
+}
+
+func (f *fakeGeoLocator) Lookup(
+	_ context.Context, addrs []netip.Addr,
+) []*geo.Location {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	f.calls = append(f.calls, addrs)
+
+	answers := make([]*geo.Location, len(addrs))
+	for i := range addrs {
+		answers[i] = f.location
+	}
+
+	return answers
+}
+
+// TestIPLookupGivesTheGeoObject looks up two public addresses. They go to the
+// locator in one call, and each result has the geo object under the keys the
+// API documents. These addresses are public, because a reserved one is not
+// looked up.
+func TestIPLookupGivesTheGeoObject(t *testing.T) {
+	t.Parallel()
+
+	s := newServerWithTestRoutes(t)
+	useFakeResolver(s, &fakeResolver{})
+
+	locator := &fakeGeoLocator{location: &geo.Location{
+		CountryCode: new("DE"),
+		City:        new("Berlin"),
+		Latitude:    new(52.5),
+		Source:      "geojs",
+		Updated:     testRouteTime(0),
+	}}
+	s.SetGeoLocator(locator)
+
+	path := "/api/v1/ip/1.1.1.1,8.8.8.8"
+
+	results := resultFields(t, get(t, s, path).Body.Bytes())
+	if len(results) != 2 {
+		t.Fatalf("%s: %d results, want 2", path, len(results))
+	}
+
+	if len(locator.calls) != 1 || len(locator.calls[0]) != 2 {
+		t.Errorf("locator calls %v, want one with both addresses", locator.calls)
+	}
+
+	var got map[string]json.RawMessage
+
+	err := json.Unmarshal(results[0]["geo"], &got)
+	if err != nil {
+		t.Fatalf("decoding geo %s: %v", results[0]["geo"], err)
+	}
+
+	wantKeys := []string{
+		"accuracy_radius_km", "city", "continent_code", "country",
+		"country_code", "latitude", "longitude", "region", "source",
+		"time_zone", "updated",
+	}
+	if keys := slices.Sorted(maps.Keys(got)); !slices.Equal(keys, wantKeys) {
+		t.Errorf("geo keys %v, want %v", keys, wantKeys)
+	}
+
+	if string(got["city"]) != `"Berlin"` || string(got["latitude"]) != "52.5" ||
+		string(got["region"]) != jsonNull {
+		t.Errorf("wrong geo %s", results[0]["geo"])
+	}
+}
+
+// TestIPLookupOfAReservedAddressHasNoGeo looks up a reserved address alone and
+// beside a public one. The reserved one never reaches the locator, and its geo
+// is null.
+func TestIPLookupOfAReservedAddressHasNoGeo(t *testing.T) {
+	t.Parallel()
+
+	s := newServerWithTestRoutes(t)
+	useFakeResolver(s, &fakeResolver{})
+
+	locator := &fakeGeoLocator{location: &geo.Location{Source: "geojs"}}
+	s.SetGeoLocator(locator)
+
+	results := resultFields(t, get(t, s, "/api/v1/ip/"+addrInTestPrefixV4).Body.Bytes())
+	if len(results) != 1 || string(results[0]["geo"]) != jsonNull {
+		t.Fatalf("results %v, want one with geo null", results)
+	}
+
+	if len(locator.calls) != 0 {
+		t.Errorf("locator calls %v, want none", locator.calls)
+	}
+
+	results = resultFields(t,
+		get(t, s, "/api/v1/ip/"+addrInTestPrefixV4+",1.1.1.1").Body.Bytes())
+	if len(results) != 2 || string(results[0]["geo"]) != jsonNull ||
+		string(results[1]["geo"]) == jsonNull {
+		t.Errorf("results %v, want geo only for the public address", results)
+	}
+
+	if len(locator.calls) != 1 || len(locator.calls[0]) != 1 {
+		t.Errorf("locator calls %v, want one with one address", locator.calls)
+	}
+}
+
+// TestIPLookupGeoIsNullWhenNoSourceAnswers checks an address the locator has
+// no answer for.
+func TestIPLookupGeoIsNullWhenNoSourceAnswers(t *testing.T) {
+	t.Parallel()
+
+	s := newServerWithTestRoutes(t)
+	useFakeResolver(s, &fakeResolver{})
+	s.SetGeoLocator(&fakeGeoLocator{})
+
+	results := resultFields(t, get(t, s, "/api/v1/ip/1.1.1.1").Body.Bytes())
+	if len(results) != 1 || string(results[0]["geo"]) != jsonNull {
+		t.Errorf("results %v, want one with geo null", results)
+	}
+}
+
 // TestIPLookupReportsVPNProviders looks up addresses on no list, on a list
 // that is not a VPN list, on one VPN list and on two. is_vpn says whether any
 // VPN list holds the address, and vpn_providers names only those lists. An