Fix health check port binding conflict (issue #892)

The Docker health check was attempting to bind to ports already in use by
the running application, causing "address already in use" errors. This fix
adds a health check mode that skips network binding operations while still
validating output configuration (listen addresses, paths, etc.).

Changes:
- Add health check mode flag in pkg/poller/outputs.go
- Update prometheus and webserver DebugOutput() to skip port binding in health check mode
- Maintain full configuration validation without network conflicts

Fixes #892

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

pkg/poller/commands.go

@@ -127,6 +127,10 @@ func (u *UnifiPoller) DebugIO() error {
 // It validates configuration and checks if inputs/outputs are properly configured.
 // Returns nil (exit 0) if healthy, error (exit 1) if unhealthy.
 func (u *UnifiPoller) HealthCheck() error {
+	// Enable health check mode to skip network binding in output validation.
+	SetHealthCheckMode(true)
+	defer SetHealthCheckMode(false)
+
 	// Silence output for health checks (Docker doesn't need verbose logs).
 	u.Quiet = true
 
@@ -170,7 +174,8 @@ func (u *UnifiPoller) HealthCheck() error {
 		return fmt.Errorf("health check failed: no enabled output plugins")
 	}
 
-	// Perform basic validation checks on enabled outputs.
+	// Perform configuration validation on enabled outputs.
+	// Network binding checks will be skipped automatically due to health check mode.
 	for _, output := range outputs {
 		if !output.Enabled() {
 			continue

pkg/poller/outputs.go

@@ -11,6 +11,8 @@ var (
 	outputSync          sync.RWMutex // nolint: gochecknoglobals
 	errNoOutputPlugins  = fmt.Errorf("no output plugins imported")
 	errAllOutputStopped = fmt.Errorf("all output plugins have stopped, or none enabled")
+	// healthCheckMode indicates when we're running in health check mode to skip network operations
+	healthCheckMode     bool         // nolint: gochecknoglobals
 )
 
 // Collect is passed into output packages so they may collect metrics to output.
@@ -50,6 +52,17 @@ func NewOutput(o *Output) {
 	outputs = append(outputs, o)
 }
 
+// SetHealthCheckMode enables or disables health check mode.
+// When enabled, output plugins should skip network operations in DebugOutput().
+func SetHealthCheckMode(enabled bool) {
+	healthCheckMode = enabled
+}
+
+// IsHealthCheckMode returns true if we're running in health check mode.
+func IsHealthCheckMode() bool {
+	return healthCheckMode
+}
+
 // Poller returns the poller config.
 func (u *UnifiPoller) Poller() Poller {
 	return *u.Config.Poller

pkg/promunifi/collector.go

@@ -139,6 +139,12 @@ func (u *promUnifi) DebugOutput() (bool, error) {
 		return false, fmt.Errorf("invalid listen address: %s (must be of the form \"IP:Port\"", u.HTTPListen)
 	}
 
+	// Skip network binding check during health checks to avoid "address already in use"
+	// errors when the main application is already running and bound to the port.
+	if poller.IsHealthCheckMode() {
+		return true, nil
+	}
+
 	ln, err := net.Listen("tcp", u.HTTPListen)
 	if err != nil {
 		return false, err

pkg/webserver/server.go

@@ -117,6 +117,12 @@ func (s *Server) DebugOutput() (bool, error) {
 		return false, fmt.Errorf("problem with HTML path: %w", err)
 	}
 
+	// Skip network binding check during health checks to avoid "address already in use"
+	// errors when the main application is already running and bound to the port.
+	if poller.IsHealthCheckMode() {
+		return true, nil
+	}
+
 	// check the port
 	ln, err := net.Listen("tcp", fmt.Sprintf(":%d", s.Port))
 	if err != nil {