Run validation in the selected module

internal/gotool/gotool.go

@@ -34,8 +34,9 @@ type Target struct {
 // Tool holds the host target and effective operational settings resolved from
 // the process environment and Go's persistent environment file.
 type Tool struct {
-	host     Target
-	settings map[string]string
+	host         Target
+	settings     map[string]string
+	settingsFile string
 }
 
 var operationalSettings = []string{
@@ -53,10 +54,11 @@ var architectureFeatures = []string{
 // configuration from being read.
 func New(ctx context.Context, directory string) (*Tool, error) {
 	var host struct {
-		OS   string `json:"GOHOSTOS"`
-		Arch string `json:"GOHOSTARCH"`
+		OS           string `json:"GOHOSTOS"`
+		Arch         string `json:"GOHOSTARCH"`
+		SettingsFile string `json:"GOENV"`
 	}
-	output, err := discover(ctx, directory, discoveryEnvironment(), "go", "env", "-json", "GOHOSTOS", "GOHOSTARCH")
+	output, err := discover(ctx, directory, discoveryEnvironment(), "go", "env", "-json", "GOHOSTOS", "GOHOSTARCH", "GOENV")
 	if err != nil {
 		return nil, fmt.Errorf("identify Go host: %w", err)
 	}
@@ -66,7 +68,7 @@ func New(ctx context.Context, directory string) (*Tool, error) {
 	if host.OS == "" || host.Arch == "" {
 		return nil, fmt.Errorf("Go returned an incomplete host target: %s", strings.TrimSpace(string(output)))
 	}
-	tool := &Tool{host: Target{OS: host.OS, Arch: host.Arch}}
+	tool := &Tool{host: Target{OS: host.OS, Arch: host.Arch}, settingsFile: host.SettingsFile}
 
 	environment := discoveryEnvironment()
 	environment["GOOS"] = tool.host.OS

internal/gotool/gotool_test.go

@@ -301,6 +301,72 @@ func TestPersistentSettingsPreserveCachesAndNativeCTools(t *testing.T) {
 	}
 }
 
+func TestOwnerReusesSettingsUntilInheritedEnvironmentChanges(t *testing.T) {
+	clearGoSettings(t)
+	t.Setenv("GOENV", "off")
+	owner := NewOwner(t.TempDir())
+
+	first, err := owner.Acquire(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := owner.Acquire(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if first != second {
+		t.Fatal("unchanged Go inputs caused settings discovery to be repeated")
+	}
+
+	cache := filepath.Join(t.TempDir(), "changed-cache")
+	t.Setenv("GOCACHE", cache)
+	if err := owner.Refresh(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	third, err := owner.Acquire(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if third == first || third.settings["GOCACHE"] != cache {
+		t.Fatalf("changed inherited environment kept stale settings: tool=%p GOCACHE=%q", third, third.settings["GOCACHE"])
+	}
+}
+
+func TestOwnerRefreshesWhenPersistentSettingsChange(t *testing.T) {
+	clearGoSettings(t)
+	firstCache := filepath.Join(t.TempDir(), "first-cache")
+	secondCache := filepath.Join(t.TempDir(), "second-cache")
+	settingsFile := filepath.Join(t.TempDir(), "go-env")
+	writeSettings := func(cache string) {
+		t.Helper()
+		if err := os.WriteFile(settingsFile, []byte("GOCACHE="+cache+"\n"), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	writeSettings(firstCache)
+	t.Setenv("GOENV", settingsFile)
+	owner := NewOwner(t.TempDir())
+
+	first, err := owner.Acquire(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if first.settings["GOCACHE"] != firstCache {
+		t.Fatalf("initial GOCACHE = %q, want %q", first.settings["GOCACHE"], firstCache)
+	}
+	writeSettings(secondCache)
+	if err := owner.Refresh(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	second, err := owner.Acquire(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if second == first || second.settings["GOCACHE"] != secondCache {
+		t.Fatalf("changed persistent settings were not rediscovered: tool=%p GOCACHE=%q", second, second.settings["GOCACHE"])
+	}
+}
+
 func readSettings(t *testing.T, tool *Tool, mode Mode, target Target, names ...string) map[string]string {
 	t.Helper()
 	args := append([]string{"env", "-json"}, names...)

internal/gotool/owner.go

@@ -0,0 +1,136 @@
+package gotool
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// Owner lazily discovers and reuses one Go tool while its inherited
+// environment and persistent settings file remain unchanged.
+type Owner struct {
+	directory string
+	tool      *Tool
+	env       map[string]string
+	file      string
+	contents  []byte
+	fileFound bool
+}
+
+// NewOwner creates an operation-local Go tool owner without running Go.
+func NewOwner(directory string) *Owner {
+	return &Owner{directory: directory}
+}
+
+// Acquire returns the operation's Go tool, rediscovering settings when their
+// process or persistent inputs have changed.
+func (owner *Owner) Acquire(ctx context.Context) (*Tool, error) {
+	if owner.tool != nil {
+		changed, err := owner.inputsChanged()
+		if err == nil && !changed {
+			return owner.tool, nil
+		}
+	}
+	if err := owner.discover(ctx); err != nil {
+		return nil, err
+	}
+	return owner.tool, nil
+}
+
+// Refresh checks the owner's inputs after a consumer callback and rediscovers
+// settings when the callback changed the environment or persistent file.
+func (owner *Owner) Refresh(ctx context.Context) error {
+	if owner.tool == nil {
+		return nil
+	}
+	changed, err := owner.inputsChanged()
+	if err == nil && !changed {
+		return nil
+	}
+	return owner.discover(ctx)
+}
+
+func (owner *Owner) discover(ctx context.Context) error {
+	environment := inheritedEnvironment()
+	tool, err := New(ctx, owner.directory)
+	if err != nil {
+		return err
+	}
+	settingsFile, err := resolveSettingsFile(owner.directory, tool.settingsFile)
+	if err != nil {
+		return err
+	}
+	contents, found, err := readSettingsFile(settingsFile)
+	if err != nil {
+		return fmt.Errorf("read Go settings file %q: %w", settingsFile, err)
+	}
+	owner.tool = tool
+	owner.env = environment
+	owner.file = settingsFile
+	owner.contents = contents
+	owner.fileFound = found
+	return nil
+}
+
+func (owner *Owner) inputsChanged() (bool, error) {
+	if !sameEnvironment(owner.env, inheritedEnvironment()) {
+		return true, nil
+	}
+	contents, found, err := readSettingsFile(owner.file)
+	if err != nil {
+		return true, err
+	}
+	return found != owner.fileFound || !bytes.Equal(contents, owner.contents), nil
+}
+
+func inheritedEnvironment() map[string]string {
+	values := make(map[string]string)
+	for _, entry := range os.Environ() {
+		key, value, ok := strings.Cut(entry, "=")
+		if ok {
+			values[key] = value
+		}
+	}
+	return values
+}
+
+func sameEnvironment(left, right map[string]string) bool {
+	if len(left) != len(right) {
+		return false
+	}
+	for key, value := range left {
+		other, ok := right[key]
+		if !ok || other != value {
+			return false
+		}
+	}
+	return true
+}
+
+func resolveSettingsFile(directory, filename string) (string, error) {
+	if filename == "" || filename == "off" {
+		return "", nil
+	}
+	if !filepath.IsAbs(filename) {
+		filename = filepath.Join(directory, filename)
+	}
+	return filepath.Abs(filename)
+}
+
+func readSettingsFile(filename string) ([]byte, bool, error) {
+	if filename == "" {
+		return nil, false, nil
+	}
+	contents, err := os.ReadFile(filename)
+	if errors.Is(err, os.ErrNotExist) {
+		return nil, false, nil
+	}
+	if err != nil {
+		return nil, false, err
+	}
+	return contents, true, nil
+}