Handle 8 byte trace ids (#365)

Nginx uses 8 byte trace ids whereas OpenCensus expects trace ids which are 16 bytes.  When we encounter an 8 byte trace id, we discard it, breaking the trace tree.

When reading trace id headers, we now left-pad trace ids with 0s to 16 bytes.  This is consistent with the way that OpenCensus handles trace ids less than 16 bytes: https://github.com/census-instrumentation/opencensus-go/blob/master/plugin/ochttp/propagation/b3/b3.go#L66

We also now log the appropriate error when we fail to emit a span.  This will allow us to distinguish between the scenario when the span receiver has been dropped vs when span conversion fails.

Signed-off-by: Alex Leong <alex@buoyant.io>

lib/linkerd2-trace-context/src/layer.rs

@@ -178,8 +178,8 @@ where
             span.end = SystemTime::now();
             response_labels(&mut span.labels, &inner);
             trace!(message = "emitting span", ?span);
-            if sink.try_send(span).is_err() {
-                warn!("span dropped due to backpressure");
+            if let Err(error) = sink.try_send(span) {
+                warn!(message = "span dropped", %error);
             }
         }
         Ok(Async::Ready(inner))

lib/linkerd2-trace-context/src/propagation.rs

@@ -179,8 +179,8 @@ fn increment_grpc_span_id<B>(request: &mut http::Request<B>, context: &TraceCont
 }
 
 fn unpack_http_trace_context<B>(request: &http::Request<B>) -> Option<TraceContext> {
-    let parent_id = parse_header_id(request, HTTP_SPAN_ID_HEADER)?;
-    let trace_id = parse_header_id(request, HTTP_TRACE_ID_HEADER)?;
+    let parent_id = parse_header_id(request, HTTP_SPAN_ID_HEADER, 8)?;
+    let trace_id = parse_header_id(request, HTTP_TRACE_ID_HEADER, 16)?;
     let flags = match get_header_str(request, HTTP_SAMPLED_HEADER) {
         Some("1") => Flags(1),
         _ => Flags(0),
@@ -215,10 +215,20 @@ fn get_header_str<'a, B>(request: &'a http::Request<B>, header: &str) -> Option<
         .ok()
 }
 
-fn parse_header_id<B>(request: &http::Request<B>, header: &str) -> Option<Id> {
+fn parse_header_id<B>(request: &http::Request<B>, header: &str, pad_to: usize) -> Option<Id> {
     let header_value = get_header_str(request, header)?;
     hex::decode(header_value)
-        .map(|data| Id(data))
+        .map(|mut data| {
+            if data.len() < pad_to {
+                let padding = pad_to - data.len();
+                let mut padded = Vec::with_capacity(padding);
+                padded.resize(padding, 0u8);
+                padded.append(&mut data);
+                Id(padded)
+            } else {
+                Id(data)
+            }
+        })
         .map_err(|e| warn!("Header {} does not contain a hex value: {}", header, e))
         .ok()
 }