Sample env carries a version marker; install appends settings missing since the writing version

internal/service/service_linux.go

@@ -140,7 +140,7 @@ func copyFile(src, dst string, mode os.FileMode) error {
 // *directory*, and granting the sandboxed service user write access to a
 // shared system directory would let a compromised service overwrite other
 // binaries. /var/lib/gpu-turnstile is exclusively ours.
-func Install(configPath string, copyBin bool) error {
+func Install(configPath string, copyBin bool, version string) error {
 	exe, err := os.Executable()
 	if err != nil {
 		return err
@@ -172,14 +172,23 @@ func Install(configPath string, copyBin bool) error {
 			}
 		}
 		exe = installedExe
-		if _, err := os.Stat(etcConfig); os.IsNotExist(err) {
+		data, readErr := os.ReadFile(etcConfig)
+		switch {
+		case os.IsNotExist(readErr):
 			if configPath != "" {
 				copyFile(configPath, etcConfig, 0o644) //nolint:errcheck // best effort
-			} else {
-				// No config at all: install a fully commented sample
-				// covering every setting. LOG_FILE stays commented —
-				// stderr goes to the journal on Linux.
-				if err := os.WriteFile(etcConfig, []byte(config.SampleEnv("")), 0o644); err != nil {
+			} else if err := os.WriteFile(etcConfig, []byte(config.SampleEnv(version, "")), 0o644); err != nil {
+				// Fully commented sample covering every setting; LOG_FILE
+				// stays commented — stderr goes to the journal on Linux.
+				return fmt.Errorf("write %s: %w", etcConfig, err)
+			}
+		case readErr != nil:
+			return fmt.Errorf("read %s: %w", etcConfig, readErr)
+		default:
+			// An installer-written config from an older version gets any
+			// new settings appended; hand-written files stay untouched.
+			if synced, changed := config.SyncSample(string(data), version, ""); changed {
+				if err := os.WriteFile(etcConfig, []byte(synced), 0o644); err != nil {
 					return fmt.Errorf("write %s: %w", etcConfig, err)
 				}
 			}

internal/service/service_other.go

@@ -29,7 +29,7 @@ func Run(run func(ctx context.Context) error) error {
 }
 
 // Install is unsupported on non-Windows, non-Linux platforms.
-func Install(string, bool) error { return errUnsupported }
+func Install(string, bool, string) error { return errUnsupported }
 
 // Remove is unsupported on non-Windows, non-Linux platforms.
 func Remove() error { return errUnsupported }

internal/service/service_windows.go

@@ -116,7 +116,7 @@ func (h *handler) Execute(_ []string, requests <-chan svc.ChangeRequest, status
 // the installed binary is refreshed only when the content differs, the
 // registration is updated only where it drifted, and the service is
 // started again only if it was running before.
-func Install(configPath string, copyBin bool) error {
+func Install(configPath string, copyBin bool, version string) error {
 	exe, err := os.Executable()
 	if err != nil {
 		return err
@@ -174,15 +174,26 @@ func Install(configPath string, copyBin bool) error {
 		}
 		// A service has no console: without LOG_FILE the output vanishes.
 		// With no config at all, install a fully commented sample covering
-		// every setting (only LOG_FILE active); an existing config just gets
-		// a LOG_FILE line appended if it has none.
+		// every setting (only LOG_FILE active). An existing installer-written
+		// config from an older version gets any new settings appended; any
+		// other existing config just gets a LOG_FILE line if it has none.
 		logPath := filepath.Join(dataDir, "gpu-turnstile.log")
-		if _, statErr := os.Stat(targetCfg); os.IsNotExist(statErr) {
-			if err := os.WriteFile(targetCfg, []byte(config.SampleEnv(logPath)), 0o644); err != nil {
+		data, readErr := os.ReadFile(targetCfg)
+		switch {
+		case os.IsNotExist(readErr):
+			if err := os.WriteFile(targetCfg, []byte(config.SampleEnv(version, logPath)), 0o644); err != nil {
 				return fmt.Errorf("write %s: %w", targetCfg, err)
 			}
-		} else if err := ensureLogFile(targetCfg, logPath); err != nil {
-			return err
+		case readErr != nil:
+			return fmt.Errorf("read %s: %w", targetCfg, readErr)
+		default:
+			if synced, changed := config.SyncSample(string(data), version, logPath); changed {
+				if err := os.WriteFile(targetCfg, []byte(synced), 0o644); err != nil {
+					return fmt.Errorf("write %s: %w", targetCfg, err)
+				}
+			} else if err := ensureLogFile(targetCfg, logPath); err != nil {
+				return err
+			}
 		}
 		configPath = targetCfg
 	}