Support key lookup in span context extraction. (#25)

* Support key lookup in span context extraction.

* Undo clang-formating change.

include/opentracing/propagation.h

@@ -80,6 +80,15 @@ const std::error_code invalid_carrier_error(2, propagation_error_category());
 const std::error_code span_context_corrupted_error(
     3, propagation_error_category());
 
+// `key_not_found_error` occurs when TextMapReader::LookupKey fails to find
+// an entry for the provided key.
+const std::error_code key_not_found_error(4, propagation_error_category());
+
+// `lookup_key_not_supported_error` occurs when TextMapReader::LookupKey is
+// not supported for the provided key.
+const std::error_code lookup_key_not_supported_error(
+    5, propagation_error_category());
+
 // TextMapWriter is the Inject() carrier for the TextMap builtin format. With
 // it, the caller can encode a SpanContext for propagation as entries in a map
 // of unicode strings.
@@ -89,6 +98,18 @@ class TextMapReader {
  public:
   virtual ~TextMapReader() = default;
 
+  // LookupKey returns the value for the specified `key` if available. If no
+  // such key is present, it returns `key_not_found_error`.
+  //
+  // TextMapReaders are not required to implement this method. If not supported,
+  // the function returns `lookup_key_not_supported_error`.
+  //
+  // Tracers may use this as an alternative to `ForeachKey` as a faster way to
+  // extract span context.
+  virtual expected<string_view> LookupKey(string_view /*key*/) const {
+    return make_unexpected(lookup_key_not_supported_error);
+  }
+
   // ForeachKey returns TextMap contents via repeated calls to the `f`
   // function. If any call to `f` returns an error, ForeachKey terminates and
   // returns that error.

src/propagation.cpp

@@ -8,9 +8,7 @@ class PropagationErrorCategory : public std::error_category {
   // Needed to fix bug in macOS build
   // (https://travis-ci.org/isaachier/hunter/jobs/281868518).
   // See https://stackoverflow.com/a/7411708/1930331 for justification.
-  PropagationErrorCategory()
-  {
-  }
+  PropagationErrorCategory() {}
 
   const char* name() const noexcept override {
     return "OpenTracingPropagationError";
@@ -27,6 +25,12 @@ class PropagationErrorCategory : public std::error_category {
     if (code == span_context_corrupted_error.value()) {
       return std::make_error_condition(std::errc::invalid_argument);
     }
+    if (code == key_not_found_error.value()) {
+      return std::make_error_condition(std::errc::invalid_argument);
+    }
+    if (code == lookup_key_not_supported_error.value()) {
+      return std::make_error_condition(std::errc::not_supported);
+    }
     return std::error_condition(code, *this);
   }
 
@@ -40,6 +44,12 @@ class PropagationErrorCategory : public std::error_category {
     if (code == span_context_corrupted_error.value()) {
       return "opentracing: SpanContext data corrupted in Extract carrier";
     }
+    if (code == key_not_found_error.value()) {
+      return "opentracing: SpanContext key not found";
+    }
+    if (code == lookup_key_not_supported_error.value()) {
+      return "opentracing: Lookup for the given key is not supported";
+    }
     return "opentracing: unknown propagation error";
   }
 };