Checker client: the codecs of the verdicts, kept and previews states the checker writes and others read (ruling 17:4x; ADR 0634 §2; issue 1966)

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

contracts/go/clients/meshdeliverycheckerclient/client_test.go

@@ -149,3 +149,46 @@ func TestVerdictKeyAsGoDeliveryWroteIt(t *testing.T) {
 		t.Error("a short hash keys")
 	}
 }
+
+// The codecs write a value at the kept values' version after its rules and read it back through them.
+func TestTheCodecsWriteAtTheVersionAndReadBackThroughTheRules(t *testing.T) {
+	raw, err := EncodeVerdict(passing())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, err := DecodeVerdict(raw); err != nil || got.V == 0 || got.Group != "g" {
+		t.Errorf("verdict read back = %+v, %v", got, err)
+	}
+	bad := passing()
+	bad.Heads = "x"
+	if _, err := EncodeVerdict(bad); err == nil {
+		t.Error("a verdict whose heads are no hash is written")
+	}
+	if _, err := DecodeVerdict([]byte(`{"v":99,"group":"g","heads":"0123456789ab","state":"running"}`)); err == nil {
+		t.Error("a verdict of an unknown version is read")
+	}
+	k := checker.KeptByLevelDigestValue{State: VerdictPass, Repository: "novox/mesh-sdk", Package: "./go/keys", Level: LevelUnit, Toolchain: "1.26.0", At: "2026-11-05T10:00:00Z"}
+	raw, err = EncodeKept(k)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, err := DecodeKept(raw); err != nil || got.Package != k.Package {
+		t.Errorf("kept read back = %+v, %v", got, err)
+	}
+	k.State = VerdictFail
+	if _, err := EncodeKept(k); err == nil {
+		t.Error("a failure is kept")
+	}
+	p := checker.PreviewsByRepositoryHeadTrunkValue{Repository: "novox/a", Head: "h1", Trunk: "t1", Conflicts: []string{"f.go"}}
+	raw, err = EncodePreview(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, err := DecodePreview(raw); err != nil || got.Head != "h1" {
+		t.Errorf("preview read back = %+v, %v", got, err)
+	}
+	p.Conflicts = nil
+	if _, err := EncodePreview(p); err == nil {
+		t.Error("a preview with no merge commit and no conflict is written")
+	}
+}

contracts/go/clients/meshdeliverycheckerclient/codec.go

@@ -0,0 +1,87 @@
+package meshdeliverycheckerclient
+
+import (
+	"encoding/json"
+	"fmt"
+
+	checker "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshdeliverychecker"
+	"git.novox.be/novox/mesh-sdk/go/keys"
+)
+
+// The codecs of the three states the checker writes and others read (the view and the gate read verdicts, the indexer reads
+// previews, the checker and the tools read kept verdicts): a value is written at the version of the kept values after its rules
+// (CheckVerdict, CheckKept, CheckPreview) and read back through the same rules. The checker's owed state is read by no other
+// part and keeps its own codec.
+
+// EncodeVerdict is v as JSON at the kept values' version, refused by CheckVerdict's rules.
+func EncodeVerdict(v checker.VerdictsByGroupHeadsValue) ([]byte, error) {
+	if err := CheckVerdict(v); err != nil {
+		return nil, err
+	}
+	v.V = keys.Version
+	return json.Marshal(v)
+}
+
+// DecodeVerdict reads a `verdicts` value; it refuses an unknown version (keys.ErrUnknownVersion) and what EncodeVerdict refuses.
+func DecodeVerdict(data []byte) (checker.VerdictsByGroupHeadsValue, error) {
+	var v checker.VerdictsByGroupHeadsValue
+	if err := decode(data, &v); err != nil {
+		return v, err
+	}
+	if err := CheckVerdict(v); err != nil {
+		return checker.VerdictsByGroupHeadsValue{}, err
+	}
+	return v, nil
+}
+
+// EncodeKept is k as JSON at the kept values' version, refused by CheckKept's rules.
+func EncodeKept(k checker.KeptByLevelDigestValue) ([]byte, error) {
+	if err := CheckKept(k); err != nil {
+		return nil, err
+	}
+	k.V = keys.Version
+	return json.Marshal(k)
+}
+
+// DecodeKept reads a `kept` value; it refuses an unknown version and what EncodeKept refuses.
+func DecodeKept(data []byte) (checker.KeptByLevelDigestValue, error) {
+	var k checker.KeptByLevelDigestValue
+	if err := decode(data, &k); err != nil {
+		return k, err
+	}
+	if err := CheckKept(k); err != nil {
+		return checker.KeptByLevelDigestValue{}, err
+	}
+	return k, nil
+}
+
+// EncodePreview is p as JSON at the kept values' version, refused by CheckPreview's rules.
+func EncodePreview(p checker.PreviewsByRepositoryHeadTrunkValue) ([]byte, error) {
+	if err := CheckPreview(p); err != nil {
+		return nil, err
+	}
+	p.V = keys.Version
+	return json.Marshal(p)
+}
+
+// DecodePreview reads a `previews` value; it refuses an unknown version and what EncodePreview refuses.
+func DecodePreview(data []byte) (checker.PreviewsByRepositoryHeadTrunkValue, error) {
+	var p checker.PreviewsByRepositoryHeadTrunkValue
+	if err := decode(data, &p); err != nil {
+		return p, err
+	}
+	if err := CheckPreview(p); err != nil {
+		return checker.PreviewsByRepositoryHeadTrunkValue{}, err
+	}
+	return p, nil
+}
+
+func decode(data []byte, into any) error {
+	if err := keys.CheckVersion(data); err != nil {
+		return err
+	}
+	if err := json.Unmarshal(data, into); err != nil {
+		return fmt.Errorf("meshdeliverycheckerclient: value does not fit its shape: %w", err)
+	}
+	return nil
+}