Build a merge's modules of a tier at once, and beside the check's tests where the judge cache allows (hq issue 716)

A check built its images one after another, after merge-check.sh. With a judge cache the
builds run during the tests, out of their reach; a failed check lets go of them.

internal/builder/check.go

@@ -15,6 +15,7 @@ import (
 	"regexp"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/novox/mesh-controller/internal/artifacts"
@@ -300,7 +301,17 @@ func ScriptToolchain(script []byte) string {
 // Check runs one merge check. An error is that it could not run; the verdict is then "error".
 func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry string, forge GitCredential,
 	log Log) (CheckVerdict, error) {
-	// Everything a check says goes to the build's log, which the bus keeps: said redacted (novox/hq issue 462).
+	// Everything a check says goes to the build's log, which the bus keeps: said redacted (novox/hq issue 462). A line
+	// at a time: its builds for the merge say theirs while its tests run (novox/hq issue 716).
+	if log != nil {
+		var saying sync.Mutex
+		inner := log
+		log = func(step, message string) {
+			saying.Lock()
+			defer saying.Unlock()
+			inner(step, message)
+		}
+	}
 	redact := redactorFor(forge)
 	say := logging(func(step, message string) {
 		if log != nil {
@@ -532,6 +543,25 @@ func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry
 	// Every clone is made: the credential is gone before anything of the check runs (novox/hq issue 462).
 	forget()
 
+	// **What the merge moves is built beside the tests** (novox/hq issue 716) when the build agent has a judge cache to
+	// build it in, where no container of the check reaches; without one, after the check has judged.
+	logged := func(step, message string) {
+		if log != nil {
+			log(step, redact.redact(message))
+		}
+	}
+	var beside *besideTheTests
+	if buildingTree != "" {
+		place, err := buildsBesideTheTests(spec.JudgeCache, workspace, spec.ID)
+		if err != nil {
+			return CheckVerdict{}, err
+		}
+		if place != "" {
+			beside = startBesideTheTests(ctx, run, spec, place, buildingTree, judged, forge, logged, say)
+			defer beside.abandon()
+		}
+	}
+
 	// The facts, and the versions they say the mesh runs.
 	if registry == "" {
 		return CheckVerdict{}, errors.New("no artifact store to read the facts snapshot from")
@@ -752,12 +782,11 @@ func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry
 
 	// **What the merge would move, built once, here** (novox/hq ADR 0350 §1): after the judging, from the merge it
 	// judged, into the scratch namespace — so the walk of the merge commit takes it rather than building again.
-	if len(spec.Builds) > 0 && spec.BuildWith != nil && len(spec.Group) == 0 {
-		v.Built = forTheMerge(ctx, run, spec, v, workspace, buildingTree, judged, forge, func(step, message string) {
-			if log != nil {
-				log(step, redact.redact(message))
-			}
-		}, say)
+	switch {
+	case beside != nil:
+		v.Built = beside.settle(ctx, v)
+	case len(spec.Builds) > 0 && spec.BuildWith != nil && len(spec.Group) == 0:
+		v.Built = forTheMerge(ctx, run, spec, v, workspace, buildingTree, judged, forge, logged, say)
 	}
 	return named(v), nil
 }