[GitHub] [arrow] westonpace commented on a change in pull request #11964: ARROW-15067: [C++] Add tracing spans to the scanner

[GitBox <gi...@apache.org>]

westonpace commented on a change in pull request #11964:
URL: https://github.com/apache/arrow/pull/11964#discussion_r778512018



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File path: cpp/src/arrow/util/tracing_internal.h
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@@ -97,6 +98,58 @@ AsyncGenerator<T> WrapAsyncGenerator(AsyncGenerator<T> wrapped,
     return fut;
   };
 }
+
+/// \brief Start a new span for each invocation of a generator.
+///
+/// The parent span of the new span will be the currently active span
+/// (if any) as of when WrapAsyncGenerator was itself called.
+template <typename T>
+AsyncGenerator<T> WrapAsyncGenerator(AsyncGenerator<T> wrapped,
+                                     const std::string& span_name) {
+  opentelemetry::trace::StartSpanOptions options;
+  options.parent = GetTracer()->GetCurrentSpan()->GetContext();
+  return WrapAsyncGenerator(std::move(wrapped), std::move(options), span_name);
+}
+
+/// \brief End the given span when the given async generator ends.
+///
+/// The span will be made the active span each time the generator is called.
+template <typename T>
+AsyncGenerator<T> TieSpanToAsyncGenerator(
+    AsyncGenerator<T> wrapped,
+    opentelemetry::nostd::shared_ptr<opentelemetry::trace::Span> span) {
+  return [=]() mutable -> Future<T> {
+    auto scope = GetTracer()->WithActiveSpan(span);
+    return wrapped().Then(
+        [span](const T& result) -> Result<T> {
+          span->SetStatus(opentelemetry::trace::StatusCode::kOk);
+          return result;
+        },
+        [span](const Status& status) -> Result<T> {
+          MarkSpan(status, span.get());
+          return status;
+        });
+  };
+}
+
+/// \brief Activate the given span on each invocation of an async generator.
+template <typename T>
+AsyncGenerator<T> PropagateSpanThroughAsyncGenerator(
+    AsyncGenerator<T> wrapped,
+    opentelemetry::nostd::shared_ptr<opentelemetry::trace::Span> span) {
+  return [=]() mutable -> Future<T> {
+    auto scope = GetTracer()->WithActiveSpan(span);
+    return wrapped();
+  };
+}

Review comment:
       On second glance this helper method seems a little off to me.  Is the "active span" a thread local concept?  Will this work even if `wrapped()` launches its task on a separate thread (e.g. an I/O operation)?




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