Ignore routes shorter than /8 or /16 in the IP lookup (closes #57)

The IP lookup tried every mask length down to /0, so a default route or
a very short route such as 2000::/3 was reported as the owner of any
address it covered until a more specific route was seen. Such a route
only says where a peer sends traffic it has no better route for.

The lookup now stops at /8 for IPv4 and /16 for IPv6; an address covered
only by shorter routes gets "no route found". The live tables still
store those routes. README states the limit.

Model: opus-5-5

internal/database/database_test.go

@@ -133,9 +133,10 @@ func TestGetIPInfoFindsMostSpecificLiveRoute(t *testing.T) {
 	lookup("2001:db8:1:2::3", "2001:db8:1::/48")
 }
 
-// TestGetIPInfoIgnoresShortRoutes checks that a default route or a route
-// shorter than the lookup's minimum mask length is not reported as the route
-// for an address, and that a longer route covering the address still is.
+// TestGetIPInfoIgnoresShortRoutes checks that the IP lookup does not report a
+// default route, or a route shorter than /8 for IPv4 or /16 for IPv6, as the
+// route for an address, and that a route of exactly /8 or /16, or longer, still
+// is. The prefixes are written out so that a wrong limit fails the test.
 func TestGetIPInfoIgnoresShortRoutes(t *testing.T) {
 	cfg := &config.Config{StateDir: t.TempDir()}
 
@@ -165,8 +166,26 @@ func TestGetIPInfoIgnoresShortRoutes(t *testing.T) {
 		}
 	}
 
+	// lookup checks that ip resolves to the live prefix netblock from asn.
+	lookup := func(ip, netblock string, asn int) {
+		t.Helper()
+
+		info, err := db.GetIPInfo(ip)
+		if err != nil {
+			t.Errorf("GetIPInfo(%s): %v", ip, err)
+
+			return
+		}
+		if info.Netblock != netblock || info.ASN != asn {
+			t.Errorf("GetIPInfo(%s) = %s AS%d, want %s AS%d", ip, info.Netblock, info.ASN, netblock, asn)
+		}
+	}
+
+	// Default and very short routes, and routes one bit shorter than each limit.
 	announce("2000::/3", 64500)
 	announce("0.0.0.0/0", 64500)
+	announce("2000::/15", 64500)
+	announce("192.0.0.0/7", 64500)
 
 	for _, ip := range []string{"2001:db8::1", "192.0.2.1"} {
 		if info, err := db.GetIPInfo(ip); !errors.Is(err, ErrNoRoute) {
@@ -174,15 +193,16 @@ func TestGetIPInfoIgnoresShortRoutes(t *testing.T) {
 		}
 	}
 
-	announce("2001:db8::/32", 64501)
+	// Routes of exactly the minimum length are used.
+	announce("2001::/16", 64501)
+	announce("192.0.0.0/8", 64501)
 
-	info, err := db.GetIPInfo("2001:db8::1")
-	if err != nil {
-		t.Fatalf("GetIPInfo(2001:db8::1) after a /32 is added: %v", err)
-	}
-	if info.Netblock != "2001:db8::/32" || info.ASN != 64501 {
-		t.Errorf("GetIPInfo(2001:db8::1) = %s AS%d, want 2001:db8::/32 AS64501", info.Netblock, info.ASN)
-	}
+	lookup("2001:db8::1", "2001::/16", 64501)
+	lookup("192.0.2.1", "192.0.0.0/8", 64501)
+
+	announce("2001:db8::/32", 64502)
+
+	lookup("2001:db8::1", "2001:db8::/32", 64502)
 }
 
 // TestConnectionPoolPragmas holds several pooled connections open at once and