mmm: lockfile to prevent multiple instances.

eventstore/mmm/mmmm.go

@@ -56,6 +56,15 @@ func (b *MultiMmapManager) Init() error {
 		b.Logger = &nopLogger
 	}
 
+	// create lockfile to prevent multiple instances
+	lockfilePath := filepath.Join(b.Dir, "mmmm.lock")
+	if _, err := os.OpenFile(lockfilePath, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0644); err != nil {
+		if os.IsExist(err) {
+			return fmt.Errorf("database at %s is already in use by another instance", b.Dir)
+		}
+		return fmt.Errorf("failed to create lockfile %s: %w", lockfilePath, err)
+	}
+
 	// create directory if it doesn't exist
 	dbpath := filepath.Join(b.Dir, "mmmm")
 	if err := os.MkdirAll(dbpath, 0755); err != nil {
@@ -333,4 +342,8 @@ func (b *MultiMmapManager) Close() {
 	}
 
 	syscall.Munmap(b.mmapf)
+
+	// remove lockfile
+	lockfilePath := filepath.Join(b.Dir, "mmmm.lock")
+	os.Remove(lockfilePath)
 }