mesh-build-go: let a check's go vet warm the shared Go build cache with cgo off, and say each linted directory as it ends

A check's lint recompiled every package from a cold layer on each ask (g14 linted
mesh-catalog's 8 directories in 141 s where ace, warm, took 17 s), and said nothing
until it ended. ADR 0541 §2 and ADR 0542 §1 decided this; the built speed-up on
feat/try-speed (S11) was parked. Ported here alone: CGO_ENABLED=0 in every check
command's go environment, vet runs with the shared caches writable, and the lint
says a phase per finished directory (issue 1921).

modules/mesh-build-go/cmd/mesh-build-go/fetch.go

@@ -296,6 +296,7 @@ func (m *Module) goEnv() []string {
 		modcache = m.GoModCache
 	}
 	return append([]string{
+		"CGO_ENABLED=0", // as a production build compiles (build.go), so a check tests what ships and the warm cache matches (ADR 0542 §1)
 		"GOTOOLCHAIN=local",
 		"GOFLAGS=-mod=mod",
 		"GOMODCACHE=" + modcache,

modules/mesh-build-go/cmd/mesh-build-go/isolate.go

@@ -45,7 +45,7 @@ func (m *Module) isolation(workspace string, write bool) buildstep.Isolation {
 }
 
 // stepRunner is the runner of a run step of the workspace: buildstep.Isolated over m.Run, so every command it runs is
-// shut off; write is true for a trunk build alone (the shared caches are written by trunk builds only). Inside is the
+// shut off; write is true for a trunk build and for a check's go vet, compile steps that run nothing of the change (ADR 0541 §2). Inside is the
 // integration level's own process, already running shut off (level.go): it runs its commands as they are.
 func (m *Module) stepRunner(workspace string, write bool) (forgekey.Runner, error) {
 	if m.Inside {

modules/mesh-build-go/cmd/mesh-build-go/isolate_test.go

@@ -130,8 +130,23 @@ func TestTheUnitLevelRunsEveryGoTestShutOff(t *testing.T) {
 	if ran == 0 || len(seen) != ran {
 		t.Fatalf("%d go commands ran, %d of them shut off: %v", ran, len(seen), *calls)
 	}
+	// A check compiles with cgo off as a build does (ADR 0542 §1), and the lint's go vet, a compile that runs nothing
+	// of the change, writes the shared caches; every other command reads them beneath its own layer (ADR 0541 §2).
+	for _, c := range *calls {
+		if strings.HasPrefix(c.line, "go ") && !slices.Contains(c.env, "CGO_ENABLED=0") {
+			t.Errorf("%s ran without CGO_ENABLED=0: %v", c.line, c.env)
+		}
+	}
+	vets := 0
 	for _, s := range seen {
-		checkShutOff(t, m, s, false)
+		vet := strings.Contains(s.command, " vet ")
+		if vet {
+			vets++
+		}
+		checkShutOff(t, m, s, vet)
+	}
+	if vets == 0 {
+		t.Error("no go vet ran shut off")
 	}
 }
 

modules/mesh-build-go/cmd/mesh-build-go/lint.go

@@ -13,6 +13,7 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"time"
 
 	"git.novox.be/novox/mesh-sdk/go/buildstep"
 	"git.novox.be/novox/mesh-sdk/go/forgekey"
@@ -144,6 +145,18 @@ func (m *Module) lintAll(ctx context.Context, run forgekey.Runner, tree string,
 	var wg sync.WaitGroup
 	var crash any // a panic of a lint goes on in the caller, as a panic of a test run does
 	var crashMu sync.Mutex
+	// Each directory's end is said as a phase, so a long lint is readable while it runs (issue 1921): which directories
+	// are done and how long the last one took.
+	var doneMu sync.Mutex
+	done := 0
+	said := func(d string, began time.Time) {
+		doneMu.Lock()
+		defer doneMu.Unlock()
+		done++
+		if done < len(dirs) {
+			buildstep.Phase(ctx, fmt.Sprintf("unit: lint of %d directories, %d done (%s in %s)", len(dirs), done, d, m.clock().Sub(began).Round(time.Second)))
+		}
+	}
 	for i, d := range dirs {
 		wg.Add(1)
 		slots <- struct{}{}
@@ -157,7 +170,9 @@ func (m *Module) lintAll(ctx context.Context, run forgekey.Runner, tree string,
 					crashMu.Unlock()
 				}
 			}()
+			began := m.clock()
 			answers[i].failed, answers[i].err = m.lint(ctx, run, tree, d, env(d))
+			said(d, began)
 		}()
 	}
 	wg.Wait()

modules/mesh-build-go/cmd/mesh-build-go/test.go

@@ -251,8 +251,15 @@ func (m *Module) runLevel(ctx context.Context, tree string, o levelOpts) (*TestR
 	if o.level == LevelUnit {
 		// gofmt and go vet of each directory the level runs come before its tests (ADR 0529 §1; issue 1887 D7-go): a
 		// directory that fails them fails the level at once, and their time is the result's Lint.
+		// go vet compiles and runs nothing of the change, with cgo off (goEnv), so it writes the shared cache and warms it
+		// for every later check, a pull request's head included (ADR 0541 §2, ADR 0542 §1); the tests run beneath their
+		// own throwaway layer.
+		compile, err := m.stepRunner(o.workspace, true)
+		if err != nil {
+			return nil, err
+		}
 		linted := m.clock()
-		failed, err := m.lintAll(ctx, run, tree, todo, func(d string) []string {
+		failed, err := m.lintAll(ctx, compile, tree, todo, func(d string) []string {
 			return append(m.goEnvFor(filepath.Join(tree, d)), o.env...)
 		})
 		if err != nil {