Make SpanContext truly immutable. (#165)

Author: carlosalberto

common/src/main/java/com/lightstep/tracer/shared/SpanContext.java

@@ -90,9 +90,10 @@ Map<String, String> getBaggage() {
     }
 
     SpanContext withBaggageItem(String key, String value) {
-        // This is really a "set" not a "with" but keeping as is to preserve behavior.
-        this.baggage.put(key, value);
-        return new SpanContext(this.getTraceId(), this.getSpanId(), this.baggage, this.foreignTraceId);
+        // Do NOT modify our own baggage, as SpanContext is expected to be immutable.
+        Map<String, String> newBaggage = new HashMap<String, String>(this.baggage);
+        newBaggage.put(key, value);
+        return new SpanContext(this.getTraceId(), this.getSpanId(), newBaggage, this.foreignTraceId);
     }
 
     @Override

common/src/test/java/com/lightstep/tracer/shared/SpanTest.java

@@ -49,6 +49,15 @@ public void setup() {
         undertest = new Span(abstractTracer, spanContext, grpcSpan, 0L);
     }
 
+    @Test
+    public void testContext_isImmutable() {
+      undertest.setBaggageItem("foo", "bar");
+      SpanContext anotherSpanContext = undertest.context();
+      assertNull(spanContext.getBaggageItem("foo"));
+      assertNotNull(anotherSpanContext.getBaggageItem("foo"));
+      assertNotNull(undertest.getBaggageItem("foo"));
+    }
+
     @Test
     public void testContext() {
         assertSame(spanContext, undertest.context());