S4 sdk-seat-read: SeatRead splits a read of a seat holder's state

Builds: design 47 G A-116, A-128 (A-343, A-344)

Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h

go/catalogue/state.go

@@ -78,6 +78,25 @@ func (s StateDeclaration) MarshalJSON() ([]byte, error) {
 	return json.Marshal(plain(s))
 }
 
+// SeatReadPrefix opens a read of the state a seat's holder keeps: `seat:<seat>.<state>` (design 47 G A-116, A-128).
+const SeatReadPrefix = "seat:"
+
+// SeatRead splits a read of a seat holder's state, `seat:<seat>.<state>`, into the seat and the local name; ok is false for a
+// read that is not of this kind (a module's `<module>.<state>`, which ReadState splits), and err says why a `seat:` read is
+// malformed. The holder is not in it: it is resolved where the read is made, so a new holder is read with no change of the
+// manifest.
+func SeatRead(read string) (seat, local string, ok bool, err error) {
+	rest, isSeat := strings.CutPrefix(read, SeatReadPrefix)
+	if !isSeat {
+		return "", "", false, nil
+	}
+	at := strings.LastIndex(rest, ".")
+	if at <= 0 || at == len(rest)-1 || !stateName.MatchString(rest[:at]) || !stateName.MatchString(rest[at+1:]) {
+		return "", "", true, fmt.Errorf("%q does not name a seat and its state: a read of a seat holder's state is seat:<seat>.<name>", read)
+	}
+	return rest[:at], rest[at+1:], true, nil
+}
+
 // SeatViewPrefix opens a consumed view of a seat, `seat:<seat>.views.<state>` (design 47 G §616).
 const SeatViewPrefix = "seat:"
 

go/catalogue/state_test.go

@@ -51,3 +51,26 @@ func TestAStateMayHaveKeysThatExpireOneByOne(t *testing.T) {
 		t.Fatalf("read-only given twice: %v", err)
 	}
 }
+
+// A read of a seat holder's state is its own kind, `seat:<seat>.<state>`, kept apart from a module's read (design 47 G A-128).
+func TestSeatRead(t *testing.T) {
+	for _, row := range []struct {
+		read, seat, local string
+		ok, bad           bool
+	}{
+		{"seat:mesh-provisioner.plans", "mesh-provisioner", "plans", true, false},
+		{"mesh-provisioner.plans", "", "", false, false},
+		{"seat:mesh-provisioner", "", "", true, true},
+		{"seat:.plans", "", "", true, true},
+		{"seat:Mesh.plans", "", "", true, true},
+	} {
+		seat, local, ok, err := SeatRead(row.read)
+		if seat != row.seat || local != row.local || ok != row.ok || (err != nil) != row.bad {
+			t.Errorf("SeatRead(%q) = %q, %q, %v, %v", row.read, seat, local, ok, err)
+		}
+	}
+	// A module's read reads as today.
+	if module, local, err := ReadState("mesh-provisioner.plans"); module != "mesh-provisioner" || local != "plans" || err != nil {
+		t.Errorf("a module's read = %q, %q, %v", module, local, err)
+	}
+}