// Package metricsapi hosts the optional Prometheus scrape listener.
//
// The listener is enabled only when BACKEND_OTEL_METRICS_EXPORTER=prometheus
// and the configured listen address is non-empty. main.go wires this server
// into the application lifecycle only when Enabled returns true.
package metricsapi

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/http"
	"sync"

	"go.uber.org/zap"
)

// Server owns the optional Prometheus HTTP listener.
type Server struct {
	addr    string
	handler http.Handler
	logger  *zap.Logger

	stateMu  sync.RWMutex
	server   *http.Server
	listener net.Listener
}

// NewServer constructs a Prometheus scrape server bound to addr. A handler of
// nil is replaced with http.NotFoundHandler so the server can still serve
// 404s in unconfigured deployments.
func NewServer(addr string, handler http.Handler, logger *zap.Logger) *Server {
	if handler == nil {
		handler = http.NotFoundHandler()
	}
	if logger == nil {
		logger = zap.NewNop()
	}

	return &Server{
		addr:    addr,
		handler: handler,
		logger:  logger.Named("metricsapi"),
	}
}

// Enabled reports whether the metrics listener should run.
func (s *Server) Enabled() bool {
	return s != nil && s.addr != ""
}

// Run binds the listener and serves the scrape surface. A disabled server
// blocks until ctx is cancelled so the App lifecycle can still treat it as a
// regular Component.
func (s *Server) Run(ctx context.Context) error {
	if ctx == nil {
		return errors.New("run backend metrics server: nil context")
	}
	if err := ctx.Err(); err != nil {
		return err
	}
	if !s.Enabled() {
		<-ctx.Done()
		return nil
	}

	listener, err := net.Listen("tcp", s.addr)
	if err != nil {
		return fmt.Errorf("run backend metrics server: listen on %q: %w", s.addr, err)
	}

	server := &http.Server{
		Handler: s.handler,
	}

	s.stateMu.Lock()
	s.server = server
	s.listener = listener
	s.stateMu.Unlock()

	s.logger.Info("backend metrics server started", zap.String("addr", listener.Addr().String()))

	defer func() {
		s.stateMu.Lock()
		s.server = nil
		s.listener = nil
		s.stateMu.Unlock()
	}()

	err = server.Serve(listener)
	switch {
	case err == nil:
		return nil
	case errors.Is(err, http.ErrServerClosed):
		s.logger.Info("backend metrics server stopped")
		return nil
	default:
		return fmt.Errorf("run backend metrics server: serve on %q: %w", s.addr, err)
	}
}

// Shutdown gracefully stops the metrics listener within ctx.
func (s *Server) Shutdown(ctx context.Context) error {
	if ctx == nil {
		return errors.New("shutdown backend metrics server: nil context")
	}

	s.stateMu.RLock()
	server := s.server
	s.stateMu.RUnlock()

	if server == nil {
		return nil
	}

	if err := server.Shutdown(ctx); err != nil && !errors.Is(err, http.ErrServerClosed) {
		return fmt.Errorf("shutdown backend metrics server: %w", err)
	}
	return nil
}