Keep a try off the forge and bound how many run (hq issue 686)

Review of #314: a try's start named its commit, which the forge's holder
matches to an open pull request's head and marks pending. Its start now
names no repository or commit. One try per commit and four in all, a try
never ages ahead of deliveries or pull request checks, and its id is
checked before it is a bus subject.

internal/catalogue/verbs.go

@@ -369,11 +369,14 @@ var ControllerVerbs = []Verb{
 		"once with an id; asked again with that id it waits up to 20 s and answers pass, fail or error, how long it " +
 		"took and the lines naming what failed — or that it is still waiting or running, to ask again. Sets no " +
 		"status on the forge, registers and builds nothing, runs no merge gate, counts as a check in the build queue " +
-		"(after deliveries) and is bounded as one. Only for a repository of the mesh; the ref must be on the forge.",
+		"(after deliveries and after every pull request's check, however long it waits) and is bounded as one. One " +
+		"try of a commit at a time — a second ask answers the first's id — and at most four waiting or running in " +
+		"all: a fifth is refused. Only for a repository of the mesh; the ref must be on the forge.",
 		Input: schema(map[string]string{
 			"repository": "the repository on the forge, owner/name; with ref",
 			"ref":        "the branch or commit to try, as pushed to the forge",
-			"base":       "the branch it is merged into (the repository's default branch when none is named)",
+			"base": "the branch it is merged into (the repository's default branch when none is named). The " +
+				"merge-check.sh that runs is that branch's, never the ref's own: the same trust as a pull request's base",
 			"tests": "only the tests this `go test -run` pattern names, where the repository's merge-check.sh reads " +
 				"MESH_CHECK_TESTS; a script that does not runs whole",
 			"store-only": "\"true\": only the repository's store tests, where its merge-check.sh reads " +

internal/link/builds.go

@@ -96,8 +96,27 @@ type BuildStart struct {
 	// an earlier taker left unfinished — most often a build agent replaced mid-build — and started
 	// again from the beginning (novox/hq issue 594). 0 from a machine that predates it.
 	Delivery int `json:"delivery,omitempty"`
+	// Try marks the start of a branch tried before its pull request (novox/hq issue 686), which names no
+	// repository and no ref (startOf).
+	Try bool `json:"try,omitempty"`
 }
 
+// startOf is what a build machine says of the ask it took. **A try's start names no repository and no commit**
+// (novox/hq issue 686): the forge's holder writes mesh/merge-gate pending on an open pull request whose head is
+// the commit a start names, and a try of that head is no check of the pull request — nothing on the forge moves
+// for a try. Its id, machine and time are all the queue reads of a start.
+func startOf(r BuildRequest, on string, delivery int, at time.Time) BuildStart {
+	s := BuildStart{ID: r.ID, Repository: r.Repository, Path: r.Path, Ref: r.Ref, On: on,
+		At: at.UTC().Format(time.RFC3339Nano), Delivery: delivery}
+	if IsTry(r) {
+		s.Repository, s.Path, s.Ref, s.Try = "", "", "", true
+	}
+	return s
+}
+
+// IsTry says an ask is a branch tried before its pull request (novox/hq issue 686).
+func IsTry(r BuildRequest) bool { return r.Check != nil && r.Check.Try }
+
 // Restarted is how a build handed out again says so (novox/hq issue 594), in its log and in its check's
 // report, so a second run never reads as the first. Neutral on why: a holder replaced mid-build, a paused
 // holder handing an ask back as it arrived, and a bus that held its acknowledgements past the wait all

internal/link/builds_nats.go

@@ -401,10 +401,7 @@ func (b *natsBuild) Done() error { return b.msg.Ack() }
 // Began publishes that this machine has taken the build, into the stream like the outcome, so a
 // reader that asks afterwards sees when it started as well as how it ended.
 func (b *natsBuild) Began(ctx context.Context) error {
-	body, err := json.Marshal(BuildStart{
-		ID: b.request.ID, Repository: b.request.Repository, Path: b.request.Path,
-		Ref: b.request.Ref, On: b.on, At: time.Now().UTC().Format(time.RFC3339Nano), Delivery: b.delivery,
-	})
+	body, err := json.Marshal(startOf(b.request, b.on, b.delivery, time.Now()))
 	if err != nil {
 		return err
 	}

internal/link/queue.go

@@ -427,6 +427,9 @@ func ReadCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, pul
 // safeKey is an id that can be a key, and a subject token, as it is: a build id, never a pattern.
 var safeKey = regexp.MustCompile(`^[A-Za-z0-9_-]+$`)
 
+// SafeBuildID says an id can be a subject token as it is: a build's id, never a pattern.
+func SafeBuildID(id string) bool { return safeKey.MatchString(id) }
+
 // Cancelled is what the controller writes for an ask it cancelled.
 type Cancelled struct {
 	At  string `json:"at"`

internal/link/queue_order.go

@@ -26,6 +26,8 @@ import (
 //   - a check that has waited CheckWaitsAtMost goes before every delivery asked after it, so no check
 //     waits for ever behind a stream of deliveries;
 //   - within a kind, first come, first served, by when it was asked;
+//   - a branch tried before its pull request goes after everything else, however long it has waited: it is
+//     never promoted ahead of a delivery, and never ahead of a pull request's check (novox/hq issue 686);
 //   - an ask whose kind cannot be told counts as a check: a delivery taken late breaks a budget, a check
 //     taken late breaks nothing.
 //
@@ -64,8 +66,12 @@ func (a QueuedAsk) askedOrStored() time.Time {
 // QueueOrder is the order the waiting asks are to be handed out in, now: the deliveries and every check
 // that has waited CheckWaitsAtMost, by when each was asked, then the other checks, by when each was asked.
 func QueueOrder(waiting []QueuedAsk, now time.Time) []QueuedAsk {
-	var first, then []QueuedAsk
+	var first, then, tries []QueuedAsk
 	for _, a := range waiting {
+		if IsTry(a.Request) {
+			tries = append(tries, a)
+			continue
+		}
 		if KindOf(a.Request) == KindCheck && now.Sub(a.askedOrStored()) < CheckWaitsAtMost {
 			then = append(then, a)
 			continue
@@ -82,7 +88,8 @@ func QueueOrder(waiting []QueuedAsk, now time.Time) []QueuedAsk {
 	}
 	byAsking(first)
 	byAsking(then)
-	return append(first, then...)
+	byAsking(tries)
+	return append(append(first, then...), tries...)
 }
 
 // toMove is the asks to move to the tail, in this order, for the queue to stand as order says: everything

internal/link/try_test.go

@@ -0,0 +1,55 @@
+package link
+
+import (
+	"encoding/json"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+)
+
+// The start of a try names no repository and no commit (novox/hq issue 686): the forge's holder writes
+// mesh/merge-gate pending on the open pull request whose head a start names, and nothing on the forge moves
+// for a try. A pull request's check still names both.
+func TestATrysStartNamesNoCommitTheForgeCouldMatch(t *testing.T) {
+	sha := strings.Repeat("c0ffee", 7)[:40]
+	at := time.Date(2026, 10, 14, 0, 0, 0, 0, time.UTC)
+	try := BuildRequest{ID: "build-1", Repository: "https://forge/novox/a.git", Ref: sha,
+		Check: &CheckRequest{Owner: "novox", Repo: "a", Try: true}}
+	body, _ := json.Marshal(startOf(try, "ace", 1, at))
+	if strings.Contains(string(body), sha) || strings.Contains(string(body), "novox/a") {
+		t.Fatalf("a try's start names what the forge matches a pull request by: %s", body)
+	}
+	if s := startOf(try, "ace", 2, at); s.ID != "build-1" || s.On != "ace" || !s.Try || s.Delivery != 2 || s.At == "" {
+		t.Fatalf("a try's start lost what the queue reads: %+v", s)
+	}
+	pull := try
+	pull.Check = &CheckRequest{Owner: "novox", Repo: "a", Number: 7}
+	if s := startOf(pull, "ace", 1, at); s.Ref != sha || s.Repository == "" || s.Try {
+		t.Fatalf("a pull request's check's start: %+v", s)
+	}
+}
+
+// A try stays behind every delivery and every pull request's check, however long it has waited: the ten
+// minutes that put a check ahead of later deliveries never promote a try (novox/hq issue 686).
+func TestATryIsNeverHandedOutBeforeADeliveryOrAPullRequestsCheck(t *testing.T) {
+	now := time.Date(2026, 10, 12, 12, 0, 0, 0, time.UTC)
+	try := func(seq uint64, id string, asked time.Time) QueuedAsk {
+		a := anAsk(seq, id, asked, true)
+		a.Request.Check.Try = true
+		return a
+	}
+	waiting := []QueuedAsk{
+		try(1, "try-old", now.Add(-40*time.Minute)),
+		try(2, "try-young", now.Add(-time.Minute)),
+		anAsk(3, "check-young", now.Add(-2*time.Minute), true),
+		anAsk(4, "delivery", now.Add(-time.Minute), false),
+	}
+	want := []string{"delivery", "check-young", "try-old", "try-young"}
+	if got := idsOf(QueueOrder(waiting, now)); !reflect.DeepEqual(got, want) {
+		t.Fatalf("handed out as %v, want %v", got, want)
+	}
+	if !SafeBuildID("build-20261014T000000-1a2b") || SafeBuildID("x.>") || SafeBuildID("*") || SafeBuildID("") {
+		t.Fatal("an id that is a pattern passes for a build's id")
+	}
+}