From 2ef32813748b754e58e3b9336fd257aec967515b Mon Sep 17 00:00:00 2001
From: Caleb Doxsey <caleb@datadoghq.com>
Date: Mon, 6 Aug 2018 14:05:05 -0400
Subject: [PATCH] contrib/google.golang.org/grpc: fix race condition when
 setting metadata (#302)

---
 contrib/google.golang.org/grpc/client.go | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/contrib/google.golang.org/grpc/client.go b/contrib/google.golang.org/grpc/client.go
index 842976d6f8..b0960ffc18 100644
--- a/contrib/google.golang.org/grpc/client.go
+++ b/contrib/google.golang.org/grpc/client.go
@@ -162,7 +162,10 @@ func injectSpanIntoContext(ctx context.Context) context.Context {
 		return ctx
 	}
 	md, ok := metadata.FromIncomingContext(ctx)
-	if !ok {
+	if ok {
+		// we have to copy the metadata because its not safe to modify
+		md = md.Copy()
+	} else {
 		md = metadata.MD{}
 	}
 	if err := tracer.Inject(span.Context(), grpcutil.MDCarrier(md)); err != nil {