state: a state file without a section loads it empty (closes #267)

Load fills in each of domains, hostnames, ports and certificates that
the file leaves out or holds as null with an empty map. Before, such a
section loaded as a nil map, and the first check's write into it
panicked, so a state file holding {} stopped the watcher at every
start.

A state test loads {} and a file missing each section in turn, and
checks that every section is an empty map. A watcher test loads {}, and
a file without the certificates section, runs the first check against
live DNS, and checks that it saved domain, hostname, port and
certificate entries.

Model: opus-5-5

internal/state/state_test.go

@@ -695,6 +695,58 @@ func TestLoadMissingFile(t *testing.T) {
 	}
 }
 
+// TestLoadFileWithoutSections loads a state file holding {}, and files
+// that each leave out one section. Every section loads as an empty map.
+// TestFirstCheckAfterLoadingFileWithoutSections in the watcher tests
+// runs the first check after such a load.
+func TestLoadFileWithoutSections(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		file string
+	}{
+		{"no sections", `{}`},
+		{"no domains", `{"hostnames": {}, "ports": {}, "certificates": {}}`},
+		{"no hostnames", `{"domains": {}, "ports": {}, "certificates": {}}`},
+		{"no ports", `{"domains": {}, "hostnames": {}, "certificates": {}}`},
+		{"no certificates", `{"domains": {}, "hostnames": {}, "ports": {}}`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			dir := t.TempDir()
+
+			err := os.WriteFile(
+				filepath.Join(dir, "state.json"), []byte(tt.file), 0o600,
+			)
+			if err != nil {
+				t.Fatalf("writing state file: %v", err)
+			}
+
+			s := state.NewForTestWithDataDir(dir)
+
+			err = s.Load()
+			if err != nil {
+				t.Fatalf("Load() error: %v", err)
+			}
+
+			want := state.Snapshot{
+				Domains:      map[string]*state.DomainState{},
+				Hostnames:    map[string]*state.HostnameState{},
+				Ports:        map[string]*state.PortState{},
+				Certificates: map[string]*state.CertificateState{},
+			}
+
+			if got := s.GetSnapshot(); !reflect.DeepEqual(got, want) {
+				t.Errorf("loaded %#v, want %#v", got, want)
+			}
+		})
+	}
+}
+
 // TestLoadCorruptFile verifies that a corrupt state file returns an error.
 func TestLoadCorruptFile(t *testing.T) {
 	t.Parallel()