metadata: add Merge func

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

metadata/context.go

@@ -35,15 +35,5 @@ func MergeContext(ctx context.Context, pmd Metadata, overwrite bool) context.Con
 	if !ok {
 		return context.WithValue(ctx, metadataKey{}, Copy(pmd))
 	}
-	nmd := Copy(md)
+	return context.WithValue(ctx, metadataKey{}, Merge(md, pmd, overwrite))
-	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)
 }

metadata/metadata.go

@@ -120,3 +120,18 @@ func New(size int) Metadata {
 	}
 	return make(Metadata, size)
 }
+
+// Merge merges metadata to existing metadata, overwriting if specified
+func Merge(omd Metadata, mmd Metadata, overwrite bool) Metadata {
+	nmd := Copy(omd)
+	for key, val := range mmd {
+		if _, ok := nmd[key]; ok && !overwrite {
+			// skip
+		} else if val != "" {
+			nmd.Set(key, val)
+		} else {
+			nmd.Del(key)
+		}
+	}
+	return nmd
+}

metadata/metadata_test.go

@@ -7,6 +7,20 @@ import (
 	"testing"
 )
 
+func TestMerge(t *testing.T) {
+	omd := Metadata{
+		"key1": "val1",
+	}
+	mmd := Metadata{
+		"key2": "val2",
+	}
+
+	nmd := Merge(omd, mmd, true)
+	if len(nmd) != 2 {
+		t.Fatalf("merge failed: %v", nmd)
+	}
+}
+
 func TestIterator(t *testing.T) {
 	md := Metadata{
 		"1Last":   "last",