make version check more robust

version/version.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"strings"
 	"time"
 
 	"github.com/goccy/go-json"
@@ -70,16 +71,29 @@ func getLatestVersion() (string, error) {
 	return getLatestVersionWithTimeout(updateCheckTimeout)
 }
 
+func latestVersionCommand(ctx context.Context) *exec.Cmd {
+	// Keep update checks independent of the caller's module and workspace
+	cmd := exec.CommandContext(ctx, "go", "list", "-m", "-mod=mod", "-json", modulePath+"@latest")
+	cmd.Dir = os.TempDir()
+	cmd.Env = append(os.Environ(), "GOWORK=off", "GOFLAGS=")
+	return cmd
+}
+
 func getLatestVersionWithTimeout(timeout time.Duration) (string, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), timeout)
 	defer cancel()
 
-	cmd := exec.CommandContext(ctx, "go", "list", "-m", "-json", modulePath+"@latest")
+	cmd := latestVersionCommand(ctx)
 	output, err := cmd.Output()
 	if ctxErr := ctx.Err(); ctxErr != nil {
 		return "", fmt.Errorf("latest version check failed: %w", ctxErr)
 	}
 	if err != nil {
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			if detail := strings.TrimSpace(string(exitErr.Stderr)); detail != "" {
+				return "", fmt.Errorf("failed to query latest version: %w: %s", err, detail)
+			}
+		}
 		return "", fmt.Errorf("failed to query latest version: %w", err)
 	}
 

version/version_test.go

@@ -6,6 +6,8 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"slices"
+	"strings"
 	"testing"
 	"time"
 )
@@ -31,3 +33,57 @@ func TestGetLatestVersionHasOverallTimeout(t *testing.T) {
 		t.Fatalf("version lookup exceeded timeout allowance: %s", elapsed)
 	}
 }
+
+func TestLatestVersionCommandIsIsolated(t *testing.T) {
+	t.Setenv("GOFLAGS", "-mod=vendor")
+	t.Setenv("GOWORK", filepath.Join(t.TempDir(), "go.work"))
+	t.Setenv("GOPROXY", "https://proxy.example.com")
+
+	cmd := latestVersionCommand(context.Background())
+	if cmd.Dir != os.TempDir() {
+		t.Fatalf("command directory = %q, want %q", cmd.Dir, os.TempDir())
+	}
+	if args := strings.Join(cmd.Args, " "); !strings.Contains(args, " -mod=mod ") {
+		t.Fatalf("command arguments %q do not force module mode", args)
+	}
+	if got := lastEnvValue(cmd.Env, "GOWORK"); got != "off" {
+		t.Fatalf("GOWORK = %q, want off", got)
+	}
+	if got := lastEnvValue(cmd.Env, "GOFLAGS"); got != "" {
+		t.Fatalf("GOFLAGS = %q, want empty", got)
+	}
+	if got := lastEnvValue(cmd.Env, "GOPROXY"); got != "https://proxy.example.com" {
+		t.Fatalf("GOPROXY = %q, want inherited value", got)
+	}
+}
+
+func lastEnvValue(env []string, key string) string {
+	prefix := key + "="
+	for _, entry := range slices.Backward(env) {
+		if after, ok := strings.CutPrefix(entry, prefix); ok {
+			return after
+		}
+	}
+	return ""
+}
+
+func TestGetLatestVersionIncludesGoDiagnostic(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("test uses a POSIX executable script")
+	}
+
+	dir := t.TempDir()
+	fakeGo := filepath.Join(dir, "go")
+	if err := os.WriteFile(fakeGo, []byte("#!/bin/sh\necho 'go: module lookup disabled by GOPROXY=off' >&2\nexit 1\n"), 0o755); err != nil {
+		t.Fatalf("create fake go command: %v", err)
+	}
+	t.Setenv("PATH", dir)
+
+	_, err := getLatestVersionWithTimeout(time.Second)
+	if err == nil {
+		t.Fatal("expected latest version lookup to fail")
+	}
+	if !strings.Contains(err.Error(), "module lookup disabled by GOPROXY=off") {
+		t.Fatalf("error %q does not include go command diagnostic", err)
+	}
+}