// Package agent owns the in-process flock-agent runtime: IPAM, netns, state,
// anycast, and NetworkPolicy. This file implements the durable per-node
// allocation file at /var/lib/flock/allocations.json.
package agent

import (
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"sync"
	"time"
)

// AllocationState is the lifecycle marker for an entry in allocations.json.
//
// pending  — IPAM picked addresses; netlink work may be incomplete.
//            On agent startup these are GC'd: addrs/routes/veth removed.
// committed — netlink ops finished; entry is the source of truth.
type AllocationState string

const (
	StatePending   AllocationState = "pending"
	StateCommitted AllocationState = "committed"
)

// Allocation is a single per-pod entry persisted in allocations.json.
type Allocation struct {
	ContainerID string          `json:"container_id"`
	Namespace   string          `json:"namespace"`
	PodName     string          `json:"pod_name"`
	OwnerUID    string          `json:"owner_uid"`
	IP6         string          `json:"ip6,omitempty"`
	IP4         string          `json:"ip4,omitempty"`
	Anycast     []string        `json:"anycast,omitempty"`
	State       AllocationState `json:"state"`
	AllocatedAt time.Time       `json:"allocated_at"`
}

// State is the on-disk file shape. Version is bumped on incompatible changes.
type State struct {
	Version     int          `json:"version"`
	Node        string       `json:"node"`
	Allocations []Allocation `json:"allocations"`
}

const stateVersion = 1

// Store is the durable allocation store.
//
// All public methods are safe for concurrent use. They serialize through
// a single mutex so that the on-disk file is always consistent and so that
// the CNI ADD/DEL critical sections (which mutate kernel state alongside the
// file) can rely on snapshot semantics.
type Store struct {
	mu   sync.Mutex
	path string
	node string
	data State
}

// NewStore opens (or creates) a per-node store. The directory containing
// `path` must already exist; we do not create it because in production it is
// /var/lib/flock managed by the DaemonSet, not the agent process.
func NewStore(path, node string) (*Store, error) {
	s := &Store{path: path, node: node}
	if err := s.load(); err != nil {
		return nil, err
	}
	return s, nil
}

func (s *Store) load() error {
	b, err := os.ReadFile(s.path)
	if os.IsNotExist(err) {
		s.data = State{Version: stateVersion, Node: s.node, Allocations: []Allocation{}}
		return nil
	}
	if err != nil {
		return fmt.Errorf("read state: %w", err)
	}
	if err := json.Unmarshal(b, &s.data); err != nil {
		return fmt.Errorf("parse state: %w", err)
	}
	if s.data.Version != stateVersion {
		return fmt.Errorf("state version %d, want %d", s.data.Version, stateVersion)
	}
	if s.data.Allocations == nil {
		s.data.Allocations = []Allocation{}
	}
	return nil
}

// flushLocked writes the in-memory state to disk durably:
//
//	1. write to <path>.tmp
//	2. fsync(tmpfd)
//	3. rename to <path>
//	4. fsync(parent dir) — required so the rename survives power loss.
//
// Caller MUST hold s.mu.
func (s *Store) flushLocked() error {
	b, err := json.MarshalIndent(s.data, "", "  ")
	if err != nil {
		return fmt.Errorf("marshal state: %w", err)
	}

	tmp := s.path + ".tmp"
	f, err := os.OpenFile(tmp, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0o600)
	if err != nil {
		return fmt.Errorf("open tmp: %w", err)
	}
	if _, err := f.Write(b); err != nil {
		f.Close()
		os.Remove(tmp)
		return fmt.Errorf("write tmp: %w", err)
	}
	if err := f.Sync(); err != nil {
		f.Close()
		os.Remove(tmp)
		return fmt.Errorf("fsync tmp: %w", err)
	}
	if err := f.Close(); err != nil {
		os.Remove(tmp)
		return fmt.Errorf("close tmp: %w", err)
	}
	if err := os.Rename(tmp, s.path); err != nil {
		os.Remove(tmp)
		return fmt.Errorf("rename: %w", err)
	}

	dir, err := os.Open(filepath.Dir(s.path))
	if err != nil {
		return fmt.Errorf("open parent: %w", err)
	}
	defer dir.Close()
	if err := dir.Sync(); err != nil {
		return fmt.Errorf("fsync parent: %w", err)
	}
	return nil
}

// Get returns the entry for containerID, ok=false if absent. Returned value
// is a copy; mutations do not affect store state.
func (s *Store) Get(containerID string) (Allocation, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, a := range s.data.Allocations {
		if a.ContainerID == containerID {
			return a, true
		}
	}
	return Allocation{}, false
}

// Upsert inserts or replaces the entry for a.ContainerID and flushes.
func (s *Store) Upsert(a Allocation) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	for i, ex := range s.data.Allocations {
		if ex.ContainerID == a.ContainerID {
			s.data.Allocations[i] = a
			return s.flushLocked()
		}
	}
	s.data.Allocations = append(s.data.Allocations, a)
	return s.flushLocked()
}

// Delete removes the entry for containerID (no-op if absent) and flushes.
func (s *Store) Delete(containerID string) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	for i, a := range s.data.Allocations {
		if a.ContainerID == containerID {
			s.data.Allocations = append(s.data.Allocations[:i], s.data.Allocations[i+1:]...)
			return s.flushLocked()
		}
	}
	return nil
}

// Snapshot returns a defensive copy of all allocations.
func (s *Store) Snapshot() []Allocation {
	s.mu.Lock()
	defer s.mu.Unlock()
	out := make([]Allocation, len(s.data.Allocations))
	copy(out, s.data.Allocations)
	return out
}

// PendingContainerIDs returns container IDs whose entries are State==pending.
// Used by agent startup GC.
func (s *Store) PendingContainerIDs() []string {
	s.mu.Lock()
	defer s.mu.Unlock()
	var out []string
	for _, a := range s.data.Allocations {
		if a.State == StatePending {
			out = append(out, a.ContainerID)
		}
	}
	return out
}