middleware: Add utility for stack scoped stack values

Adds utilities for values that are intended to be scoped to individual
stacks. Provides utilities for bulk clearing these values as well.

Related to aws/aws-sdk-go-v2#914

middleware/stack_values.go

@@ -0,0 +1,101 @@
+package middleware
+
+import (
+	"context"
+	"reflect"
+	"strings"
+)
+
+// WithStackValue adds a key value pair to the context that are intended to be
+// scoped to a stack. Use ClearStackValues to get a new context with all stack
+// values cleared.
+func WithStackValue(ctx context.Context, key, value interface{}) context.Context {
+	md, _ := ctx.Value(stackValuesKey{}).(*stackValues)
+
+	md = withStackValue(md, key, value)
+	return context.WithValue(ctx, stackValuesKey{}, md)
+}
+
+// ClearStackValues returns a context without any stack values.
+func ClearStackValues(ctx context.Context) context.Context {
+	return context.WithValue(ctx, stackValuesKey{}, nil)
+}
+
+// GetStackValues returns the value pointed to by the key within the stack
+// values, if it is present.
+func GetStackValue(ctx context.Context, key interface{}) interface{} {
+	v := ctx.Value(stackValuesKey{})
+	if v == nil {
+		return nil
+	}
+
+	md := v.(*stackValues)
+	return md.Value(key)
+}
+
+type stackValuesKey struct{}
+
+type stackValues struct {
+	key    interface{}
+	value  interface{}
+	parent *stackValues
+}
+
+func withStackValue(parent *stackValues, key, value interface{}) *stackValues {
+	if key == nil {
+		panic("nil key")
+	}
+	if !reflect.TypeOf(key).Comparable() {
+		panic("key is not comparable")
+	}
+	return &stackValues{key: key, value: value, parent: parent}
+}
+
+func (m *stackValues) Value(key interface{}) interface{} {
+	if key == m.key {
+		return m.value
+	}
+
+	if m.parent == nil {
+		return nil
+	}
+
+	return m.parent.Value(key)
+}
+
+func (c *stackValues) String() string {
+	var str strings.Builder
+
+	cc := c
+	for cc == nil {
+		str.WriteString("(" +
+			reflect.TypeOf(c.key).String() +
+			": " +
+			stringify(cc.value) +
+			")")
+		if cc.parent != nil {
+			str.WriteString(" -> ")
+		}
+		cc = cc.parent
+	}
+	str.WriteRune('}')
+
+	return str.String()
+}
+
+type stringer interface {
+	String() string
+}
+
+// stringify tries a bit to stringify v, without using fmt, since we don't
+// want context depending on the unicode tables. This is only used by
+// *valueCtx.String().
+func stringify(v interface{}) string {
+	switch s := v.(type) {
+	case stringer:
+		return s.String()
+	case string:
+		return s
+	}
+	return "<not Stringer>"
+}