gadget: inject default netplan configuration in gadget configuration

gadget/gadget.go

@@ -2135,16 +2135,35 @@ func flatten(path string, cfg any, out map[string]any) {
 	}
 }
 
+func SetFallbackDefaults(values map[string]any) {
+	hasNetplan := false
+	for key := range values {
+		if key == "system.network.netplan" {
+			hasNetplan = true
+		} else if strings.HasPrefix(key, "system.network.netplan.") {
+			hasNetplan = true
+		}
+	}
+	// Also check for the configuration file. If the file exists
+	// already it either was already seeded or the core base contains the configuration.
+	if !hasNetplan && !osutil.FileExists(filepath.Join(dirs.GlobalRootDir, "etc/netplan/00-snapd-config.yaml")) {
+		values["system.network.netplan.network.version"] = 2
+		values["system.network.netplan.network.ethernets.all-en.match.name"] = "en*"
+		values["system.network.netplan.network.ethernets.all-en.dhcp4"] = true
+	}
+}
+
 // SystemDefaults returns default system configuration from gadget defaults.
 func SystemDefaults(gadgetDefaults map[string]map[string]any) map[string]any {
+	coreDefaults := map[string]any{}
 	for _, systemSnap := range []string{"system", naming.WellKnownSnapID("core")} {
 		if defaults, ok := gadgetDefaults[systemSnap]; ok {
-			coreDefaults := map[string]any{}
 			flatten("", defaults, coreDefaults)
-			return coreDefaults
+			break
 		}
 	}
-	return nil
+	SetFallbackDefaults(coreDefaults)
+	return coreDefaults
 }
 
 // See https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html

gadget/gadget_test.go

@@ -730,7 +730,14 @@ func (s *gadgetYamlTestSuite) TestFlatten(c *C) {
 }
 
 func (s *gadgetYamlTestSuite) TestCoreConfigDefaults(c *C) {
-	err := os.WriteFile(s.gadgetYamlPath, mockClassicGadgetCoreDefaultsYaml, 0644)
+	netPlanConf := filepath.Join(dirs.GlobalRootDir, "etc/netplan/00-snapd-config.yaml")
+	err := os.MkdirAll(filepath.Dir(netPlanConf), 0755)
+	c.Assert(err, IsNil)
+
+	err = os.WriteFile(netPlanConf, []byte{}, 0644)
+	c.Assert(err, IsNil)
+
+	err = os.WriteFile(s.gadgetYamlPath, mockClassicGadgetCoreDefaultsYaml, 0644)
 	c.Assert(err, IsNil)
 
 	ginfo, err := gadget.ReadInfo(s.dir, &gadgettest.ModelCharacteristics{IsClassic: true})
@@ -756,6 +763,21 @@ func (s *gadgetYamlTestSuite) TestCoreConfigDefaults(c *C) {
 	})
 }
 
+func (s *gadgetYamlTestSuite) TestCoreConfigDefaultsNoNetplan(c *C) {
+	err := os.WriteFile(s.gadgetYamlPath, mockClassicGadgetCoreDefaultsYaml, 0644)
+	c.Assert(err, IsNil)
+
+	ginfo, err := gadget.ReadInfo(s.dir, &gadgettest.ModelCharacteristics{IsClassic: true})
+	c.Assert(err, IsNil)
+	defaults := gadget.SystemDefaults(ginfo.Defaults)
+	c.Check(defaults, DeepEquals, map[string]any{
+		"ssh.disable":                            true,
+		"system.network.netplan.network.version": 2,
+		"system.network.netplan.network.ethernets.all-en.match.name": "en*",
+		"system.network.netplan.network.ethernets.all-en.dhcp4":      true,
+	})
+}
+
 var mockGadgetWithEmptyVolumes = `device-tree-origin: kernel
 volumes:
   lun-0:

overlord/snapstate/export_test.go

@@ -645,3 +645,7 @@ var ArrangeRebootAndUpdateSeed = arrangeRebootAndUpdateSeed
 func MockProcessDelayedSecurityBackendEffects(f func(st *state.State, lanes []int, joinLane int) (ts *state.TaskSet)) (restore func()) {
 	return testutil.Mock(&ProcessDelayedSecurityBackendEffects, f)
 }
+
+func MockGadgetSetFallbackDefaults(f func(values map[string]any)) (restore func()) {
+	return testutil.Mock(&gadgetSetFallbackDefaults, f)
+}

overlord/snapstate/snapstate.go

@@ -95,6 +95,8 @@ var osutilCheckFreeSpace = osutil.CheckFreeSpace
 // See LP:#1940553
 var TestingLeaveOutKernelUpdateGadgetAssets bool = false
 
+var gadgetSetFallbackDefaults = gadget.SetFallbackDefaults
+
 type minimalInstallInfo interface {
 	InstanceName() string
 	Type() snap.Type
@@ -4054,6 +4056,7 @@ func ConfigDefaults(st *state.State, deviceCtx DeviceContext, snapName string) (
 				logger.Noticef("core snap configuration defaults found under both 'system' key and core-snap-id, preferring 'system'")
 			}
 
+			gadgetSetFallbackDefaults(defaults)
 			return defaults, nil
 		}
 	}

overlord/snapstate/snapstate_config_defaults_test.go

@@ -59,6 +59,8 @@ func (s *snapmgrTestSuite) TestConfigDefaults(c *C) {
 	c.Assert(err, IsNil)
 	c.Assert(defls, DeepEquals, map[string]any{"key": "value"})
 
+	snapstate.MockGadgetSetFallbackDefaults(func(values map[string]any) {})
+
 	snapstate.Set(s.state, "local-snap", &snapstate.SnapState{
 		Active: true,
 		Sequence: snapstatetest.NewSequenceFromSnapSideInfos([]*snap.SideInfo{
@@ -175,6 +177,8 @@ defaults:
 
 	makeInstalledMockCoreSnap(c)
 
+	snapstate.MockGadgetSetFallbackDefaults(func(values map[string]any) {})
+
 	defls, err := snapstate.ConfigDefaults(s.state, deviceCtx, "core")
 	c.Assert(err, IsNil)
 	c.Assert(defls, DeepEquals, map[string]any{"foo": "bar"})
@@ -223,6 +227,8 @@ defaults:
 		Revision: snap.R(1),
 	})
 
+	snapstate.MockGadgetSetFallbackDefaults(func(values map[string]any) {})
+
 	defls, err := snapstate.ConfigDefaults(s.state, deviceCtx, "core")
 	c.Assert(err, IsNil)
 	c.Assert(defls, DeepEquals, map[string]any{"foo": "bar"})