Separate Go discovery configuration from diagnostics

internal/gotool/gotool.go

@@ -187,18 +187,18 @@ func discoveryEnvironment() map[string]string {
 }
 
 func discover(ctx context.Context, directory string, environment map[string]string, args ...string) ([]byte, error) {
-	captured := &combinedWriter{}
+	var configuration, diagnostics bytes.Buffer
 	err := process.Run(ctx, process.Invocation{
 		Args:   args,
 		Dir:    directory,
 		Env:    environment,
-		Stdout: captured,
-		Stderr: captured,
+		Stdout: &configuration,
+		Stderr: &diagnostics,
 	})
-	contents := captured.Bytes()
+	contents := configuration.Bytes()
 	if err != nil {
-		if len(contents) != 0 {
-			return contents, fmt.Errorf("%s: %w: %s", strings.Join(args, " "), err, strings.TrimSpace(string(contents)))
+		if output := strings.TrimSpace(configuration.String() + diagnostics.String()); output != "" {
+			return contents, fmt.Errorf("%s: %w: %s", strings.Join(args, " "), err, output)
 		}
 		return contents, fmt.Errorf("%s: %w", strings.Join(args, " "), err)
 	}

internal/gotool/gotool_test.go

@@ -47,6 +47,18 @@ func runGoPreparationFixture() int {
 		return 11
 	}
 	switch os.Getenv("RELEASEKIT_GO_FIXTURE_MODE") {
+	case "discovery":
+		settings := make(map[string]string)
+		for _, key := range os.Args[3:] {
+			settings[key] = os.Getenv(key)
+		}
+		settings["GOHOSTOS"] = runtime.GOOS
+		settings["GOHOSTARCH"] = runtime.GOARCH
+		fmt.Fprintln(os.Stderr, "fixture discovery diagnostic")
+		if err := json.NewEncoder(os.Stdout).Encode(settings); err != nil {
+			return 12
+		}
+		return 0
 	case "fail":
 		fmt.Fprintln(os.Stderr, "fixture dependency failure")
 		return 23
@@ -59,6 +71,30 @@ func runGoPreparationFixture() int {
 	}
 }
 
+func TestDiscoveryAcceptsDiagnosticsWithValidConfiguration(t *testing.T) {
+	repository, _ := installGoPreparationFixture(t)
+	t.Setenv("RELEASEKIT_GO_FIXTURE_MODE", "discovery")
+	cache := filepath.Join(t.TempDir(), "cache")
+	t.Setenv("GOCACHE", cache)
+	tool, err := New(context.Background(), repository)
+	if err != nil {
+		t.Fatalf("New() rejected successful discovery: %v", err)
+	}
+	if tool.HostTarget() != (Target{OS: runtime.GOOS, Arch: runtime.GOARCH}) || tool.settings["GOCACHE"] != cache {
+		t.Fatalf("discovered host=%v cache=%q", tool.HostTarget(), tool.settings["GOCACHE"])
+	}
+}
+
+func TestDiscoveryPreservesFailureDiagnosticsAndCause(t *testing.T) {
+	repository, _ := installGoPreparationFixture(t)
+	t.Setenv("RELEASEKIT_GO_FIXTURE_MODE", "fail")
+	_, err := New(context.Background(), repository)
+	var exitError *exec.ExitError
+	if !errors.As(err, &exitError) || exitError.ExitCode() != 23 || !strings.Contains(err.Error(), "fixture dependency failure") {
+		t.Fatalf("New() error = %v", err)
+	}
+}
+
 func mustGetwd() string {
 	directory, err := os.Getwd()
 	if err != nil {