@Test void backfillsMissingParentIdOnSharedSpan() {
    List<Span> trace = List.of(
      span("a", null, "a", Kind.SERVER, "frontend", null, false),
      span("a", "a", "b", Kind.CLIENT, "frontend", null, false),
      // below the parent ID is null as it wasn't propagated
      span("a", null, "b", Kind.SERVER, "backend", null, true)
    );

    assertThat(Trace.merge(trace)).usingFieldByFieldElementComparator().containsExactlyInAnyOrder(
      span("a", null, "a", Kind.SERVER, "frontend", null, false),
      span("a", "a", "b", Kind.CLIENT, "frontend", null, false),
      span("a", "a", "b", Kind.SERVER, "backend", null, true)
    );
  }
Some don't propagate the server's parent ID which creates a race condition. Try to unwind it.

<p>See https://github.com/openzipkin/zipkin/pull/1745
zipkin/src/test/java/zipkin2/internal/TraceTest.java