Add span links capabilities to the OTel API

src/DDTrace/OpenTelemetry/Context.php

@@ -8,6 +8,7 @@
 use DDTrace\Tag;
 use DDTrace\Util\ObjectKVStore;
 use OpenTelemetry\API\Trace as API;
+use OpenTelemetry\SDK\Common\Attribute\Attributes;
 use OpenTelemetry\SDK\Common\Attribute\AttributesFactory;
 use OpenTelemetry\SDK\Common\Instrumentation\InstrumentationScopeFactory;
 use OpenTelemetry\SDK\Common\Instrumentation\InstrumentationScopeInterface;
@@ -176,6 +177,18 @@ private static function activateParent(?SpanData $currentSpan): ContextInterface
             : null;
         $traceState = new API\TraceState($traceContext['tracestate'] ?? null);
 
+        // Check for span links
+        $links = [];
+        foreach ($currentSpan->links as $spanLink) {
+            $linkSpanContext = API\SpanContext::create(
+                $spanLink->traceId,
+                $spanLink->spanId,
+                API\TraceFlags::DEFAULT,
+                new API\TraceState($spanLink->traceState ?? null),
+            );
+            $links[] = new SDK\Link($linkSpanContext, Attributes::create($spanLink->attributes));
+        }
+
         $OTelCurrentSpan = SDK\Span::startSpan(
             $currentSpan,
             API\SpanContext::create($currentTraceId, $currentSpanId, $traceFlags, $traceState), // $context
@@ -186,8 +199,8 @@ private static function activateParent(?SpanData $currentSpan): ContextInterface
             NoopSpanProcessor::getInstance(), // $spanProcessor
             ResourceInfoFactory::defaultResource(), // $resource
             (new AttributesFactory())->builder(), // $attributesBuilder
-            [], // TODO: Handle Span Links
-            0, // TODO: Handle Span Links
+            $links, // $links
+            count($links), // $totalRecordedLinks
             false // The span was created using the DD Api
         );
         ObjectKVStore::put($currentSpan, 'otel_span', $OTelCurrentSpan);

src/DDTrace/OpenTelemetry/Span.php

@@ -5,6 +5,7 @@
 namespace OpenTelemetry\SDK\Trace;
 
 use DDTrace\SpanData;
+use DDTrace\SpanLink;
 use DDTrace\Tag;
 use DDTrace\Util\Convention;
 use DDTrace\Util\ObjectKVStore;
@@ -35,6 +36,16 @@ final class Span extends API\Span implements ReadWriteSpanInterface
     /** @readonly */
     private SpanProcessorInterface $spanProcessor;
 
+    /**
+     * @readonly
+     *
+     * @var list<LinkInterface>
+     */
+    private array $links;
+
+    /** @readonly */
+    private int $totalRecordedLinks;
+
     /** @readonly */
     private int $kind;
 
@@ -48,6 +59,8 @@ final class Span extends API\Span implements ReadWriteSpanInterface
 
     private string $operationNameConvention = "";
 
+    private string $uniqueIdentifier = "";
+
     private function __construct(
         SpanData $span,
         API\SpanContextInterface $context,
@@ -56,6 +69,8 @@ private function __construct(
         API\SpanContextInterface $parentSpanContext,
         SpanProcessorInterface $spanProcessor,
         ResourceInfo $resource,
+        array $links = [],
+        int $totalRecordedLinks = 0,
         bool $isRemapped = true
     ) {
         $this->span = $span;
@@ -65,6 +80,8 @@ private function __construct(
         $this->parentSpanContext = $parentSpanContext;
         $this->spanProcessor = $spanProcessor;
         $this->resource = $resource;
+        $this->links = $links;
+        $this->totalRecordedLinks = $totalRecordedLinks;
 
         $this->status = StatusData::unset();
 
@@ -75,6 +92,27 @@ private function __construct(
             // done in serializer.c:ddtrace_set_root_span_properties (as of v0.92.0)
             $span->name = $this->operationNameConvention = Convention::defaultOperationName($span);
         }
+
+        // Set the span links
+        if ($isRemapped) {
+            // At initialization time (now), only set the links if the span was created using the OTel API
+            // Otherwise, the links were already set in DD's OpenTelemetry\Context\Context
+            foreach ($links as $link) {
+                /** @var LinkInterface $link */
+                $linkContext = $link->getSpanContext();
+
+                $spanLink = new SpanLink();
+                $spanLink->traceId = $linkContext->getTraceId();
+                $spanLink->spanId = $linkContext->getSpanId();
+                $spanLink->traceState = (string)$linkContext->getTraceState(); // __toString()
+                $spanLink->attributes = $link->getAttributes()->toArray();
+                $spanLink->droppedAttributesCount = 0; // Attributes limit aren't supported/meaningful in DD
+
+                // Save the link
+                ObjectKVStore::put($spanLink, "link", $link);
+                $span->links[] = $spanLink;
+            }
+        }
     }
 
     /**
@@ -116,6 +154,8 @@ public static function startSpan(
             $parentSpan->getContext(),
             $spanProcessor,
             $resource,
+            $links,
+            $totalRecordedLinks,
             $isRemapped
         );
 
@@ -162,10 +202,12 @@ public function toSpanData(): SpanDataInterface
     {
         $hasEnded = $this->hasEnded();
 
+        $this->updateSpanLinks();
+
         return new ImmutableSpan(
             $this,
             $this->getName(),
-            [], // TODO: Handle Span Links
+            $this->links,
             [], // TODO: Handle Span Events
             Attributes::create(array_merge($this->span->meta, $this->span->metrics)),
             0,
@@ -206,7 +248,7 @@ public function getStartEpochNanos(): int
 
     public function getTotalRecordedLinks(): int
     {
-        return 0;
+        return $this->totalRecordedLinks;
     }
 
     public function getTotalRecordedEvents(): int
@@ -400,4 +442,36 @@ public function getDDSpan(): SpanData
     {
         return $this->span;
     }
+
+    private function updateSpanLinks()
+    {
+        // Important: Span Links are supposed immutable
+        $datadogSpanLinks = $this->span->links;
+
+        $otel = [];
+        foreach ($datadogSpanLinks as $datadogSpanLink) {
+            // Check if the link relationship exists
+            $link = ObjectKVStore::get($datadogSpanLink, "link");
+            if ($link === null) {
+                // Create the link
+                $link = new Link(
+                    API\SpanContext::create(
+                        $datadogSpanLink->traceId,
+                        $datadogSpanLink->spanId,
+                        $this->context->getTraceFlags(),
+                        new API\TraceState($datadogSpanLink->traceState ?? null)
+                    ),
+                    Attributes::create($datadogSpanLink->attributes ?? [])
+                );
+
+                // Save the link
+                ObjectKVStore::put($datadogSpanLink, "link", $link);
+            }
+            $otel[] = $link;
+        }
+
+        // Update the links
+        $this->links = $otel;
+        $this->totalRecordedLinks = count($otel);
+    }
 }

src/DDTrace/OpenTelemetry/SpanBuilder.php

@@ -69,7 +69,20 @@ public function setParent($context): API\SpanBuilderInterface
 
     public function addLink(SpanContextInterface $context, iterable $attributes = []): SpanBuilderInterface
     {
-        // TODO: Span Links are future works
+        if (!$context->isValid()) {
+            return $this;
+        }
+
+        $this->totalNumberOfLinksAdded++;
+
+        $this->links[] = new Link(
+            $context,
+            $this->tracerSharedState
+                ->getSpanLimits()
+                ->getLinkAttributesFactory()
+                ->builder($attributes)
+                ->build(),
+        );
 
         return $this;
     }
@@ -186,7 +199,7 @@ public function startSpan(): SpanInterface
             $this->tracerSharedState->getResource(),
             $attributesBuilder,
             $this->links,
-            0,
+            $this->totalNumberOfLinksAdded,
         );
     }