refactor: manage instance ID in the KV table (#1579)

backend/internal/instanceid/instanceid.go

@@ -0,0 +1,50 @@
+package instanceid
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+// Load the instance ID for the Pocket ID cluster from the "kv" table
+// If no instance ID exists yet, a new one is generated and persisted atomically
+func Load(parentCtx context.Context, db *gorm.DB) (string, error) {
+	// Candidate value used only if there's no instance ID stored yet
+	newInstanceID := uuid.NewString()
+
+	// We use a raw query because gorm can't build it for us in this atomic way
+	// The syntax is valid for both SQLite and Postgres
+	var value string
+	ctx, cancel := context.WithTimeout(parentCtx, 10*time.Second)
+	defer cancel()
+	err := db.
+		WithContext(ctx).
+		Raw(
+			`INSERT INTO kv (key, value)
+			VALUES ('instance_id', ?)
+			ON CONFLICT (key) DO UPDATE SET
+				value = CASE
+					WHEN kv.value = '' THEN excluded.value
+					ELSE kv.value
+				END
+			RETURNING value`,
+			newInstanceID,
+		).
+		Scan(&value).
+		Error
+	if err != nil {
+		return "", fmt.Errorf("failed to load instance ID from the database: %w", err)
+	}
+
+	// We should have an instance ID now
+	// This case should never happen
+	if value == "" {
+		return "", errors.New("retrieved instance ID is unexpectedly empty")
+	}
+
+	return value, nil
+}