// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

// Package server accepts connections from NFS clients.
//
// Each accepted connection reaches the Handle hook, which owns the
// connection for its whole lifetime; a nil Handle closes it at once. The
// nfsd command wires the hook to the NFSv4.2 dispatcher.
package server

import (
	"context"
	"errors"
	"net"
	"sync/atomic"
	"syscall"
	"time"
)

// acceptRetryPause is the pause before the next accept after a
// transient resource error, so a connection storm cannot spin the loop.
const acceptRetryPause = 10 * time.Millisecond

// A Server accepts connections from NFS clients.
type Server struct {
	// Handle serves one accepted connection. It runs on its own goroutine
	// and owns the connection for its whole lifetime, closing it when the
	// session ends. A nil Handle closes the connection at once.
	Handle func(conn net.Conn)

	// MaxConns caps the connections served at once. Zero means no cap. A
	// connection offered above the cap closes at once, and the client
	// sees an immediate end of file.
	MaxConns int

	live atomic.Int64
}

// transientAccept reports whether the accept error is survivable: the
// listener stays usable and the next accept is worth trying. A storm of
// aborted connections or a momentary file table exhaustion must not
// take the daemon down with every client on it.
func transientAccept(err error) bool {
	return errors.Is(err, syscall.ECONNABORTED) ||
		errors.Is(err, syscall.EMFILE) ||
		errors.Is(err, syscall.ENFILE) ||
		errors.Is(err, syscall.EAGAIN) ||
		errors.Is(err, syscall.EINTR)
}

// Serve accepts connections on ln until the listener fails or ctx is
// cancelled. A cancellation closes the listener and Serve returns nil;
// a closed listener returns nil; a transient accept error is waited out;
// any other listener failure returns the error as is. In flight
// connections are not drained: NFS clients retry through their session
// replay caches, so an immediate return is the correct shutdown.
func (s *Server) Serve(ctx context.Context, ln net.Listener) error {
	stop := make(chan struct{})
	defer close(stop)
	go func() {
		select {
		case <-ctx.Done():
			ln.Close()
		case <-stop:
		}
	}()
	for {
		conn, err := ln.Accept()
		if err == nil {
			if s.Handle == nil {
				conn.Close()
				continue
			}
			if s.MaxConns > 0 && s.live.Load() >= int64(s.MaxConns) {
				conn.Close()
				continue
			}
			s.live.Add(1)
			go func() {
				defer s.live.Add(-1)
				s.Handle(conn)
			}()
			continue
		}
		if ctx.Err() != nil || errors.Is(err, net.ErrClosed) {
			return nil
		}
		if transientAccept(err) {
			time.Sleep(acceptRetryPause)
			continue
		}
		return err
	}
}