add registry package

go.mod

@@ -0,0 +1,3 @@
+module github.com/recipeer/go/registry
+
+go 1.21.3

registry.go

@@ -0,0 +1,52 @@
+package transport
+
+import (
+	"errors"
+)
+
+// Registry error.
+var (
+	ErrKeyLocked = errors.New("key is locked")
+)
+
+// Registry is a simple interface allowing untyped common values to be shared across packages.
+// This is particularly useful for resolving import cycles.
+type Registry struct {
+	Data  map[string]any
+	Locks map[string]bool
+}
+
+// Get the value of a key.
+// This function returns nil if no value exists.
+func (reg *Registry) Get(key string) any {
+	return reg.Data[key]
+}
+
+// Has checks whether a key is non-nil.
+func (reg *Registry) Has(key string) bool {
+	return reg.Data[key] == nil
+}
+
+// Lock a key.
+// This prevents the value being overwritten by Set.
+func (reg *Registry) Lock(key string) {
+	reg.Locks[key] = true
+}
+
+// Set the value for a key.
+// This function returns an error if the key is locked.
+func (reg *Registry) Set(key string, value any) error {
+	if reg.Locks[key] {
+		return ErrKeyLocked
+	}
+	reg.Data[key] = value
+	return nil
+}
+
+// New creates a new registry.
+func New() *Registry {
+	return &Registry{
+		Data:  map[string]any{},
+		Locks: map[string]bool{},
+	}
+}