metadata: add iterator method

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

metadata/context.go

@@ -0,0 +1,49 @@
+// Package metadata is a way of defining message headers
+package metadata
+
+import (
+	"context"
+)
+
+// FromContext returns metadata from the given context
+func FromContext(ctx context.Context) (Metadata, bool) {
+	if ctx == nil {
+		return nil, false
+	}
+	md, ok := ctx.Value(metadataKey{}).(Metadata)
+	if !ok {
+		return nil, ok
+	}
+	nmd := Copy(md)
+	return nmd, ok
+}
+
+// NewContext creates a new context with the given metadata
+func NewContext(ctx context.Context, md Metadata) context.Context {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	return context.WithValue(ctx, metadataKey{}, Copy(md))
+}
+
+// MergeContext merges metadata to existing metadata, overwriting if specified
+func MergeContext(ctx context.Context, pmd Metadata, overwrite bool) context.Context {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	md, ok := FromContext(ctx)
+	if !ok {
+		return context.WithValue(ctx, metadataKey{}, Copy(pmd))
+	}
+	nmd := Copy(md)
+	for key, val := range pmd {
+		if _, ok := nmd[key]; ok && !overwrite {
+			// skip
+		} else if val != "" {
+			nmd.Set(key, val)
+		} else {
+			nmd.Del(key)
+		}
+	}
+	return context.WithValue(ctx, metadataKey{}, nmd)
+}