OpenTracing Shim: Allow invalid but sampled SpanContext to be returne…

…d. (open-telemetry#3471)

This is done to support the `jaeger-debug-id` functionality, which
allows invalid SpanContext with debug information to be propagated.

This came up through an issue in Java:
open-telemetry/opentelemetry-java#5339

CHANGELOG.md

@@ -19,6 +19,9 @@ release.
 
 ### Compatibility
 
+- OpenTracing Shim: Allow invalid but sampled SpanContext to be returned.
+  ([#3471](https://github.com/open-telemetry/opentelemetry-specification/pull/3471))
+
 ### SDK Configuration
 
 ### Common

specification/compatibility/opentracing.md

@@ -194,12 +194,18 @@ registered or the global OpenTelemetry `Propagator`s, as configured at construct
 - `TextMap` and `HttpHeaders` formats MUST use their explicitly specified `TextMapPropagator`,
   if any, or else use the global `TextMapPropagator`.
 
-If the extracted `SpanContext` is invalid AND the extracted `Baggage` is empty, this operation
-MUST return a null value, and otherwise it MUST return a `SpanContext` Shim instance with
-the extracted values.
+The operation MUST return a `SpanContext` Shim instance with the extracted values if any of these conditions are met:
+
+* `SpanContext` is valid.
+* `SpanContext` is sampled.
+* `SpanContext` contains non-empty extracted `Baggage`.
+
+Otherwise, the operation MUST return null or empty value.
 
 ```java
-if (!extractedSpanContext.isValid() && extractedBaggage.isEmpty()) {
+if (!extractedSpanContext.isValid()
+    && !extractedSpanContext.isSampled()
+    && extractedBaggage.isEmpty()) {
   return null;
 }
 
@@ -210,6 +216,10 @@ Errors MAY be raised if either the `Format` is not recognized
 or no value could be extracted, depending on the specific OpenTracing Language API
 (e.g. Go and Python do, but Java may not).
 
+Note: Invalid but sampled `SpanContext` instances are returned as a way to support
+`jaeger-debug-id` [headers](https://github.com/jaegertracing/jaeger-client-java#via-http-headers),
+which are used to force propagation of debug information.
+
 ## Close
 
 OPTIONAL operation. If this operation is implemented for a specific OpenTracing language,