state: Add 'version' file

There's a possibility that we make breaking changes
to the state store in the future.

To detect and upgrade it automatically, add a 'version' file
to the state store.

Its current value is "1",
and that's what we'll assume when the file is absent.

The binary will refuse to run against a store with a version
newer than the current binary.

.changes/unreleased/Added-20241220-194631.yaml

@@ -0,0 +1,6 @@
+kind: Added
+body: >-
+  state: Track version of the state store layout in use.
+  This should be a no-op for users, but it guards against corruption
+  in case of future changes to the layout.
+time: 2024-12-20T19:46:31.685098-05:00

internal/spice/state/storage/mem.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"iter"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -24,6 +26,17 @@ func NewMemBackend() *MemBackend {
 	}
 }
 
+// AddFiles adds the given files to the memory backend,
+// overwriting similarly named files.
+func (m *MemBackend) AddFiles(files iter.Seq2[string, []byte]) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	for name, body := range files {
+		m.items[name] = slices.Clone(body)
+	}
+}
+
 // Get retrieves a value from the store
 func (m *MemBackend) Get(ctx context.Context, key string, dst any) error {
 	m.mu.RLock()

internal/spice/state/store.go

@@ -24,6 +24,8 @@ type DB interface {
 
 var _ DB = (*storage.DB)(nil)
 
+//go:generate mockgen -destination mocks_test.go -package state -typed . DB
+
 // Store implements storage for state tracked by gs.
 type Store struct {
 	db  DB
@@ -105,11 +107,23 @@ func InitStore(ctx context.Context, req InitStoreRequest) (*Store, error) {
 		}
 	}
 
-	info := repoInfo{
-		Trunk:  req.Trunk,
-		Remote: req.Remote,
+	update := storage.UpdateRequest{
+		Sets: []storage.SetRequest{
+			{
+				Key: _repoJSON,
+				Value: repoInfo{
+					Trunk:  req.Trunk,
+					Remote: req.Remote,
+				},
+			},
+			{
+				Key:   _versionFile,
+				Value: LatestVersion,
+			},
+		},
+		Message: "initialize store",
 	}
-	if err := db.Set(ctx, _repoJSON, info, "initialize store"); err != nil {
+	if err := db.Update(ctx, update); err != nil {
 		return nil, fmt.Errorf("put repo state: %w", err)
 	}
 
@@ -165,6 +179,10 @@ func OpenStore(ctx context.Context, db DB, logger *log.Logger) (*Store, error) {
 		logger = log.New(io.Discard)
 	}
 
+	if err := checkVersion(ctx, db); err != nil {
+		return nil, fmt.Errorf("check store layout: %w", err)
+	}
+
 	var info repoInfo
 	if err := db.Get(ctx, _repoJSON, &info); err != nil {
 		if errors.Is(err, ErrNotExist) {