A-278: seatlist.Definition is the one seat-definition function AnswerSeat takes

The definition of a seat on the list a part follows, read through the part's reader at each call: none until the list
is read, none for a seat the list does not hold. Every holder passes it instead of writing its own.

Builds: design 47 G A-278, A-186, A-179

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

go/seatlist/part.go

@@ -6,6 +6,7 @@ import (
 
 	stdio "git.novox.be/novox/mesh-sdk/go"
 	"git.novox.be/novox/mesh-sdk/go/catalogue"
+	"git.novox.be/novox/mesh-sdk/go/seatdef"
 )
 
 // A part that reads the seat list wires its reader one way (novox/hq ADR 0636 §1, §7): started from the bootstrapped
@@ -57,6 +58,22 @@ func (p *Part) Reader() *Reader {
 	return p.reader.Load()
 }
 
+// Definition is the seat-definition function process.Conn.AnswerSeat takes, written once for every holder (design 47 G
+// A-278, A-186): the definition of seat on the list the part follows, read at each call through reader (a Part's
+// Reader, or a part's own reader), none until the list is read and none for a seat the list does not hold.
+func Definition(reader func() *Reader, seat string) func() (seatdef.Definition, bool) {
+	return func() (seatdef.Definition, bool) {
+		r := reader()
+		if r == nil || !r.Status().Heard {
+			return seatdef.Definition{}, false
+		}
+		r.mu.Lock()
+		defer r.mu.Unlock()
+		d, ok := r.defs[seat]
+		return d, ok
+	}
+}
+
 // MayAct is the reader's MayAct, false before it is started.
 func (p *Part) MayAct(seats ...string) bool {
 	r := p.Reader()

go/seatlist/seatlist_test.go

@@ -201,11 +201,25 @@ func TestTheBootstrapIsHeldAndNothingNewIsActedOnUntilTheSeatAnswers(t *testing.
 			t.Fatalf("waiting: MayAct(%v) = %v, want %v", a.seats, got, a.want)
 		}
 	}
+	// A-278: the seat-definition function AnswerSeat takes answers none until the list is read, and none without a reader.
+	reader := func() *Reader { return r }
+	if _, ok := Definition(reader, "node-engine")(); ok {
+		t.Fatal("Definition answered a bootstrapped definition before the seat list was read")
+	}
+	if _, ok := Definition(func() *Reader { return nil }, "node-engine")(); ok {
+		t.Fatal("Definition answered with no reader")
+	}
 	owner.down = false
 	owner.table = map[string]string{"node-b": def("node-b", "node", "put")}
 	if err := r.Reload(); err != nil {
 		t.Fatal(err)
 	}
+	if d, ok := Definition(reader, "node-b")(); !ok || d.Seat != "node-b" {
+		t.Fatalf("Definition(node-b) after the list was read = %+v, %v, want node-b's", d, ok)
+	}
+	if _, ok := Definition(reader, "node-files")(); ok {
+		t.Fatal("Definition answered a seat the list does not hold")
+	}
 	if st := r.Status(); !st.Heard || len(st.Waiting) != len(boot) || st.LastError != "" || st.Revision != 7 {
 		t.Fatalf("status after the seat answered %+v: want heard, the bootstrap's seats it does not hold still waiting, no error, revision 7", st)
 	}