[o11y] Log a useful warning when reporting span without current IncomingRequest

This should help us understand which spans are affected.

Author: fhanau

src/workerd/io/io-context.h

@@ -401,6 +401,13 @@ class IoContext final: public kj::Refcounted, private kj::TaskSet::ErrorHandler
   // True if this is the IoContext for the current thread (same as `hasCurrent() && tcx == current()`).
   bool isCurrent();
 
+  // Check if a current request is available. Used to provide better diagnostics when this is
+  // unexpectedly absent when reporting a user span.
+  // TODO(cleanup): This is a hack, remove after addressing the underlying issue.
+  bool hasCurrentIncomingRequest() {
+    return !incomingRequests.empty();
+  }
+
   // Like requireCurrent() but throws a JS error if this IoContext is not the current.
   void requireCurrentOrThrowJs();
 

src/workerd/io/tracer.c++

@@ -465,7 +465,29 @@ void BaseTracer::adjustSpanTime(CompleteSpan& span) {
   // missing topLevelInvocationSpanContext right after).
   if (weakIoContext != kj::none) {
     auto& weakIoCtx = KJ_ASSERT_NONNULL(weakIoContext);
-    weakIoCtx->runIfAlive([&span](IoContext& context) { span.endTime = context.now(); });
+    weakIoCtx->runIfAlive([this, &span](IoContext& context) {
+      if (context.hasCurrentIncomingRequest()) {
+        span.endTime = context.now();
+      } else {
+        // We have an IOContext, but there's no current IncomingRequest. Always log a warning here,
+        // this should not be happening. Still report completeTime as a useful timestamp if
+        // available.
+        bool hasCompleteTime = false;
+        if (completeTime != kj::UNIX_EPOCH) {
+          span.endTime = completeTime;
+          hasCompleteTime = true;
+        } else {
+          span.endTime = span.startTime;
+        }
+        if (isPredictableModeForTest()) {
+          KJ_FAIL_ASSERT(
+              "reported span without current request", span.operationName, hasCompleteTime);
+        } else {
+          KJ_LOG(WARNING, "reported span without current request", span.operationName,
+              hasCompleteTime);
+        }
+      }
+    });
     if (!weakIoCtx->isValid()) {
       // This can happen if we start a customEvent from this event and cancel it after this IoContext
       // gets destroyed. In that case we no longer have an IoContext available and can't get the