View: build_usage answers how the build nodes are used in one call; the checker's tries name their asks

The operator's question (how much are the build nodes used, is ace eligible) took six calls over four tools.
build_usage reads it from the bus's states: seats held (have), capacity (mesh-config resolved), pauses, the
build modules' steps by kind with left-running keys, the last hour and day with total and median time and time
at capacity, the running tries, and the backlog the view program reads from the seats' streams. A try now names
the asks it put, and the checker announces its tries readable, so the view tells a try's asks from a check's.

modules/mesh-delivery-checker/internal/check/case.go

@@ -44,6 +44,8 @@ type Case struct {
 
 	notesMu sync.Mutex
 	notes   []string
+	// asks are the ids of the asks the check put on the build seats' streams, in order: a try names them (Try.Asks).
+	asks []string
 	// Progress says what the check is doing, so a check that ran past its bound names the step it was stuck at.
 	Progress *Progress
 	// Kind is what asked the check (KindNew, KindNodes, KindTrunk): a carry after a trunk or sibling move is asked on the
@@ -59,6 +61,20 @@ type Case struct {
 	Composition *delivery.Composition
 }
 
+// Asked records the id of an ask the check put.
+func (c *Case) Asked(id string) {
+	c.notesMu.Lock()
+	c.asks = append(c.asks, id)
+	c.notesMu.Unlock()
+}
+
+// Asks are the ids of the asks the check put, in order.
+func (c *Case) Asks() []string {
+	c.notesMu.Lock()
+	defer c.notesMu.Unlock()
+	return append([]string(nil), c.asks...)
+}
+
 // Progress is the step a check is at, written by the check and read when its bound passes.
 type Progress struct {
 	mu          sync.Mutex

modules/mesh-delivery-checker/internal/check/judge.go

@@ -423,6 +423,7 @@ func (st *state) askNeeding(ctx context.Context, t *target, merged *[]Fetched, l
 			what += " for a node with " + strings.Join(needs, ", ")
 		}
 		st.c.Progress.Set(what, "")
+		st.c.Asked(a.ID)
 		if err := st.j.Asks.Put(ctx, a); err != nil {
 			return fail(t.module, fmt.Sprintf("the check of %s#%d could not be asked: %v", p.Repository, p.Number, err))
 		}

modules/mesh-delivery-checker/internal/check/try.go

@@ -77,9 +77,12 @@ type Try struct {
 	Failed   []string                        `json:"failed,omitempty"`
 	Levels   map[string]delivery.LevelResult `json:"levels,omitempty"`
 	Machines []string                        `json:"machines,omitempty"`
-	Asked    time.Time                       `json:"asked"`
-	Started  time.Time                       `json:"started,omitzero"`
-	Ended    time.Time                       `json:"ended,omitzero"`
+	// Asks are the ids of the asks the try put on the build seats' streams, each ending on the step key <node>.<id> of its
+	// build module: the delivery view counts those steps as a try's, not a candidate's check (build_usage).
+	Asks    []string  `json:"asks,omitempty"`
+	Asked   time.Time `json:"asked"`
+	Started time.Time `json:"started,omitzero"`
+	Ended   time.Time `json:"ended,omitzero"`
 }
 
 // Over says whether the try came to its end.
@@ -409,7 +412,7 @@ func (tr *Tryer) run(ctx context.Context, key string) {
 	v := out.Verdict
 	t.Ended = tr.now().UTC()
 	t.Took = t.Ended.Sub(t.Started).Round(time.Second).String()
-	t.Levels, t.Machines = v.Levels, v.Machines
+	t.Levels, t.Machines, t.Asks = v.Levels, v.Machines, cs.Asks()
 	t.Said = Report(v, cs, cs.Pulls[0], out.Builds, out.Slowest, out.Judged)
 	for _, n := range cs.Notes() {
 		t.Said += " | " + n

modules/mesh-delivery-checker/internal/check/try_test.go

@@ -97,6 +97,12 @@ func TestATryIsJudgedAsAPullRequestsHeadAndOwesTheForgeNothing(t *testing.T) { /
 	if got.State != TryPass || got.Took == "" || got.Head != head1 || got.Trunk != trunkA || got.Said == "" || got.Levels[delivery.LevelUnit].Run != 2 {
 		t.Fatalf("try = %+v", got)
 	}
+	// The try names the asks it put, so the delivery view counts their steps as a try's (build_usage).
+	if len(got.Asks) != 1 {
+		t.Errorf("asks = %v, want the one check ask the try put", got.Asks)
+	} else if _, err := delivery.AskStepKey("shanks", got.Asks[0]); err != nil {
+		t.Errorf("the try names %q, which is no ask id: %v", got.Asks[0], err)
+	}
 	fetch := b.steps.tool("fetch")
 	if len(fetch) != 1 || fetch[0].args["commit"] != head1 || fetch[0].args["base"] != trunkA || fetch[0].args["ref"] != "" {
 		t.Errorf("fetch = %+v, want the head merged onto the trunk, with no pull request's ref", fetch)