Kea: reservations in memory when there is no hosts database

Found against Kea 3.0.5 in the lab: reservation-add answers "Host
database not available" on a server that keeps its configuration in a
file. Then reservations go into the running configuration
(operation-target memory) and config-write saves them; with a hosts
database nothing changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/rollcall/dhcpres_kea.go

@@ -2,8 +2,10 @@ package main
 
 // Kea: the control API (Kea 3.0's own HTTP control socket, or the Control
 // Agent before it) with the host_cmds hook. Subnet ids come from
-// config-get; reservations carry user-context {"rollcall": <machine>};
+// config-get; reservations carry user-context {"rollcall": <machine>}.
-// config-write keeps reservations made in memory (no hosts database).
+// A server with a hosts database keeps them there; one without answers
+// "Host database not available", and then they go into its running
+// configuration (operation-target memory), which config-write saves.
 
 import (
 	"bytes"
@@ -22,6 +24,7 @@ type keaProvider struct {
 	hc              *http.Client
 	persist         bool
 	service         *bool // send "service": ["dhcp4"] (a Control Agent wants it; Kea 3.0's own socket refuses it)
+	memory          bool  // no hosts database: reservations go into the running configuration
 }
 
 func (k *keaProvider) Name() string { return "kea" }
@@ -193,7 +196,21 @@ func (k *keaProvider) Put(ctx context.Context, r dhcpRes) error {
 	if r.Hostname != "" {
 		res["hostname"] = r.Hostname
 	}
-	_, err = k.ok(ctx, "reservation-add", map[string]any{"reservation": res})
+	return k.change(ctx, "reservation-add", map[string]any{"reservation": res})
+}
+
+// change runs reservation-add/-del against the hosts database, or the
+// configuration in memory when the server has no hosts database.
+func (k *keaProvider) change(ctx context.Context, command string, args map[string]any) error {
+	if k.memory {
+		args["operation-target"] = "memory"
+	}
+	_, err := k.ok(ctx, command, args)
+	if err != nil && !k.memory && strings.Contains(err.Error(), "database not available") {
+		k.memory = true
+		args["operation-target"] = "memory"
+		_, err = k.ok(ctx, command, args)
+	}
 	return err
 }
 
@@ -202,12 +219,11 @@ func (k *keaProvider) Delete(ctx context.Context, r dhcpRes) error {
 	if err != nil {
 		return err
 	}
-	_, err = k.ok(ctx, "reservation-del", map[string]any{"subnet-id": id, "identifier-type": "hw-address", "identifier": r.MAC})
+	return k.change(ctx, "reservation-del", map[string]any{"subnet-id": id, "identifier-type": "hw-address", "identifier": r.MAC})
-	return err
 }
 
 func (k *keaProvider) Done(ctx context.Context) error {
-	if !k.persist {
+	if !k.persist || !k.memory { // a hosts database keeps them by itself
 		return nil
 	}
 	if _, err := k.ok(ctx, "config-write", map[string]any{}); err != nil {

cmd/rollcall/dhcpres_test.go

@@ -43,6 +43,7 @@ type fakeKea struct {
 	mu       sync.Mutex
 	agent    bool
 	noHooks  bool
+	noHostDB bool // like Kea with a configuration file only: add/del need operation-target memory
 	hosts    []obj
 	commands []string
 	written  int
@@ -89,6 +90,13 @@ func (f *fakeKea) handler() http.Handler {
 				return
 			}
 			answer(obj{"result": 0, "arguments": obj{"hosts": f.hosts}})
+		case "reservation-add", "reservation-del":
+			if f.noHostDB && args["operation-target"] != "memory" {
+				answer(obj{"result": 1, "text": "Host database not available, cannot add host."})
+				return
+			}
+		}
+		switch req.Command {
 		case "reservation-add":
 			res := args["reservation"].(map[string]any)
 			for _, h := range f.hosts {
@@ -122,7 +130,7 @@ func keaFor(t *testing.T, f *fakeKea) (*keaProvider, func()) {
 
 func TestKeaReservations(t *testing.T) {
 	for _, agent := range []bool{false, true} {
-		f := &fakeKea{agent: agent, hosts: []obj{
+		f := &fakeKea{agent: agent, noHostDB: agent, hosts: []obj{
 			{"hw-address": "52:54:00:aa:aa:aa", "ip-address": "10.214.0.9", "hostname": "printer"}, // someone else's
 			{"hw-address": "52:54:00:00:00:09", "ip-address": "10.214.0.99", "user-context": obj{"rollcall": "m-old"}},
 		}}
@@ -146,13 +154,17 @@ func TestKeaReservations(t *testing.T) {
 				added = h
 			}
 		}
-		if dig(added, "user-context", "rollcall") != "m-1" || added["hostname"] != "fresh01" || added["subnet-id"] != float64(7) || f.written != 1 {
+		wantWrites := 0 // a hosts database keeps reservations by itself
+		if f.noHostDB {
+			wantWrites = 1 // in memory: config-write saves them
+		}
+		if dig(added, "user-context", "rollcall") != "m-1" || added["hostname"] != "fresh01" || added["subnet-id"] != float64(7) || f.written != wantWrites {
 			t.Errorf("agent=%v added %v written %d", agent, added, f.written)
 		}
 		// a second pass changes nothing
 		rep = dhcpResReport{}
 		a.dhcpResWith(context.Background(), want, []string{"10.214.0.0/24"}, &rep)
-		if len(rep.Put)+len(rep.Deleted) != 0 || f.written != 1 {
+		if len(rep.Put)+len(rep.Deleted) != 0 || f.written != wantWrites {
 			t.Errorf("agent=%v second pass %+v", agent, rep)
 		}
 		done()