feat(state): add runtime state snapshot

internal/state/state.go

@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: MIT
+
+// Package state persists the runtime snapshot prx needs to describe and undo
+// the currently applied routing configuration.
+package state
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"time"
+
+	"prx/internal/fsx"
+)
+
+// State is the on-disk snapshot.
+type State struct {
+	Enabled          bool      `json:"enabled"`
+	WANInterface     string    `json:"wan_interface"`
+	Gateway4         string    `json:"gateway_v4,omitempty"`
+	Gateway6         string    `json:"gateway_v6,omitempty"`
+	Mark             uint32    `json:"mark"`
+	TableV4          int       `json:"table_v4"`
+	TableV6          int       `json:"table_v6"`
+	PriorityV4       int       `json:"priority_v4"`
+	PriorityV6       int       `json:"priority_v6"`
+	BypassTableV4    int       `json:"bypass_table_v4"`
+	BypassTableV6    int       `json:"bypass_table_v6"`
+	BypassPrioV4     int       `json:"bypass_priority_v4"`
+	BypassPrioV6     int       `json:"bypass_priority_v6"`
+	BypassCIDRs      []string  `json:"bypass_cidrs,omitempty"`
+	EngineManaged    bool      `json:"engine_managed"`
+	EngineWasRunning bool      `json:"engine_was_running"`
+	EnabledAt        time.Time `json:"enabled_at"`
+	Version          string    `json:"version,omitempty"`
+}
+
+// Load reads the state. A missing file yields a zero (disabled) state.
+//
+// The returned state is never nil, even alongside an error, so callers cannot
+// dereference a nil pointer after ignoring a read failure.
+func Load(path string) (*State, error) {
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return &State{}, nil
+		}
+		return &State{}, fmt.Errorf("read state %s: %w", path, err)
+	}
+	var s State
+	if err := json.Unmarshal(raw, &s); err != nil {
+		return &State{}, fmt.Errorf("parse state %s: %w", path, err)
+	}
+	return &s, nil
+}
+
+// Save writes the state atomically, creating the parent directory.
+func Save(path string, s *State) error {
+	raw, err := json.MarshalIndent(s, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encode state: %w", err)
+	}
+	raw = append(raw, '\n')
+	if err := fsx.WriteFileAtomic(path, raw, 0o644); err != nil {
+		return fmt.Errorf("write state %s: %w", path, err)
+	}
+	return nil
+}
+
+// Remove deletes the state file. A missing file is not an error.
+func Remove(path string) error {
+	err := os.Remove(path)
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+	return nil
+}