Prevent cycles in inferred spans

CHANGELOG.asciidoc

@@ -31,6 +31,10 @@ Use subheadings with the "=====" level for adding notes for unreleased changes:
 
 === Unreleased
 
+[float]
+===== Bug fixes
+* Fixed edge case where inferred spans could cause cycles in the trace parent-child relationships, subsequently resulting in the UI crashing - {pull}3588[#3588]
+
 [[release-notes-1.x]]
 === Java Agent version 1.x
 

apm-agent-core/src/main/java/co/elastic/apm/agent/impl/transaction/TraceContext.java

@@ -64,7 +64,7 @@ public class TraceContext implements Recyclable, co.elastic.apm.agent.tracer.Tra
     public static final String ELASTIC_TRACE_PARENT_TEXTUAL_HEADER_NAME = "elastic-apm-traceparent";
     public static final String W3C_TRACE_PARENT_TEXTUAL_HEADER_NAME = "traceparent";
     public static final String TRACESTATE_HEADER_NAME = "tracestate";
-    public static final int SERIALIZED_LENGTH = 42;
+    public static final int SERIALIZED_LENGTH = 51;
     private static final int TEXT_HEADER_EXPECTED_LENGTH = 55;
     private static final int TEXT_HEADER_TRACE_ID_OFFSET = 3;
     private static final int TEXT_HEADER_PARENT_ID_OFFSET = 36;
@@ -227,6 +227,7 @@ private TraceContext(ElasticApmTracer tracer, Id id) {
      * <p>
      * Note: the {@link #traceId} will still be 128 bit
      * </p>
+     *
      * @param tracer a valid tracer
      */
     public static TraceContext with64BitId(ElasticApmTracer tracer) {
@@ -621,7 +622,7 @@ public boolean equals(Object o) {
         if (o == null || getClass() != o.getClass()) return false;
         TraceContext that = (TraceContext) o;
         return id.equals(that.id) &&
-            traceId.equals(that.traceId);
+               traceId.equals(that.traceId);
     }
 
     public boolean idEquals(@Nullable TraceContext o) {
@@ -650,6 +651,8 @@ public void serialize(byte[] buffer) {
         offset = traceId.toBytes(buffer, offset);
         offset = id.toBytes(buffer, offset);
         offset = transactionId.toBytes(buffer, offset);
+        buffer[offset++] = parentId.isEmpty() ? (byte) 0 : (byte) 1;
+        offset = parentId.toBytes(buffer, offset);
         buffer[offset++] = flags;
         buffer[offset++] = (byte) (discardable ? 1 : 0);
         ByteUtils.putLong(buffer, offset, clock.getOffset());
@@ -660,6 +663,12 @@ public void deserialize(byte[] buffer, @Nullable String serviceName, @Nullable S
         offset += traceId.fromBytes(buffer, offset);
         offset += id.fromBytes(buffer, offset);
         offset += transactionId.fromBytes(buffer, offset);
+        if (buffer[offset++] != 0) {
+            offset += parentId.fromBytes(buffer, offset);
+        } else {
+            parentId.resetState();
+            offset += 8;
+        }
         flags = buffer[offset++];
         discardable = buffer[offset++] == (byte) 1;
         clock.init(ByteUtils.getLong(buffer, offset));
@@ -672,6 +681,10 @@ public static long getSpanId(byte[] serializedTraceContext) {
         return ByteUtils.getLong(serializedTraceContext, 16);
     }
 
+    public static long getParentId(byte[] serializedTraceContext) {
+        return ByteUtils.getLong(serializedTraceContext, 33);
+    }
+
     public boolean traceIdAndIdEquals(byte[] serialized) {
         return id.dataEquals(serialized, traceId.getLength()) && traceId.dataEquals(serialized, 0);
     }

apm-agent-plugins/apm-profiling-plugin/src/main/java/co/elastic/apm/agent/profiler/CallTree.java

@@ -79,9 +79,9 @@ public class CallTree implements Recyclable {
      * @see co.elastic.apm.agent.impl.transaction.AbstractSpan#childIds
      */
     @Nullable
-    private LongList childIds;
+    private ChildList childIds;
     @Nullable
-    private LongList maybeChildIds;
+    private ChildList maybeChildIds;
 
     public CallTree() {
     }
@@ -375,28 +375,29 @@ private void toString(Appendable out, int level) throws IOException {
         }
     }
 
-    int spanify(CallTree.Root root, TraceContext parentContext) {
+    int spanify(CallTree.Root root, TraceContext parentContext, TraceContext nonInferredParentContext) {
         int createdSpans = 0;
         if (activeContextOfDirectParent != null) {
             parentContext = activeContextOfDirectParent;
+            nonInferredParentContext = activeContextOfDirectParent;
         }
         Span span = null;
         if (!isPillar() || isLeaf()) {
             createdSpans++;
-            span = asSpan(root, parentContext);
+            span = asSpan(root, parentContext, nonInferredParentContext);
             this.isSpan = true;
         }
         List<CallTree> children = getChildren();
         for (int i = 0, size = children.size(); i < size; i++) {
-            createdSpans += children.get(i).spanify(root, span != null ? span.getTraceContext() : parentContext);
+            createdSpans += children.get(i).spanify(root, span != null ? span.getTraceContext() : parentContext, nonInferredParentContext);
         }
         if (span != null) {
             span.end(span.getTimestamp() + getDurationUs());
         }
         return createdSpans;
     }
 
-    protected Span asSpan(Root root, TraceContext parentContext) {
+    protected Span asSpan(Root root, TraceContext parentContext, TraceContext nonInferredParentContext) {
         transferMaybeChildIdsToChildIds();
         Span span = parentContext.createSpan(root.getEpochMicros(this.start))
             .withType("app")
@@ -410,7 +411,21 @@ protected Span asSpan(Root root, TraceContext parentContext) {
         }
         span.appendToName("#");
         span.appendToName(frame.getMethodName());
-        span.withChildIds(childIds);
+
+        LongList childSpanIds = null;
+        if (childIds != null) {
+            long expectedParent = nonInferredParentContext.getId().readLong(0);
+            childSpanIds = new LongList(childIds.getSize());
+            for (int i = 0; i < childIds.getSize(); i++) {
+                // to avoid cycles, we only insert child-ids if the parent of the child is also
+                // the parent of the stack of inferred spans inserted
+                if (childIds.getParentId(i) == expectedParent) {
+                    childSpanIds.add(childIds.getId(i));
+                }
+            }
+        }
+
+        span.withChildIds(childSpanIds);
 
         // we're not interested in the very bottom of the stack which contains things like accepting and handling connections
         if (!root.rootContext.idEquals(parentContext)) {
@@ -498,19 +513,20 @@ public void resetState() {
      * </p>
      *
      * @param id the child span id to add to this call tree element
+     * @param parentId the parent id of the span represented by the id parameter
      */
-    public void addMaybeChildId(long id) {
+    public void addMaybeChildId(long id, long parentId) {
         if (maybeChildIds == null) {
-            maybeChildIds = new LongList();
+            maybeChildIds = new ChildList();
         }
-        maybeChildIds.add(id);
+        maybeChildIds.add(id, parentId);
     }
 
-    public void addChildId(long id) {
+    public void addChildId(long id, long parentId) {
         if (childIds == null) {
-            childIds = new LongList();
+            childIds = new ChildList();
         }
-        childIds.add(id);
+        childIds.add(id, parentId);
     }
 
     public boolean hasChildIds() {
@@ -541,7 +557,11 @@ void giveChildIdsTo(CallTree giveTo) {
 
     void giveLastChildIdTo(CallTree giveTo) {
         if (childIds != null && !childIds.isEmpty()) {
-            giveTo.addChildId(childIds.remove(childIds.getSize() - 1));
+            int size = childIds.getSize();
+            long id = childIds.getId(size - 1);
+            long parentId = childIds.getParentId(size - 1);
+            giveTo.addChildId(id, parentId);
+            childIds.removeLast();
         }
     }
 
@@ -619,7 +639,7 @@ public void onActivation(byte[] active, long timestamp) {
                 long spanId = TraceContext.getSpanId(active);
                 activeSet.add(spanId);
                 if (!isNestedActivation(topOfStack)) {
-                    topOfStack.addMaybeChildId(spanId);
+                    topOfStack.addMaybeChildId(spanId, TraceContext.getParentId(active));
                 }
             }
         }
@@ -628,12 +648,12 @@ private boolean isNestedActivation(CallTree topOfStack) {
             return isAnyActive(topOfStack.childIds) || isAnyActive(topOfStack.maybeChildIds);
         }
 
-        private boolean isAnyActive(@Nullable LongList spanIds) {
+        private boolean isAnyActive(@Nullable ChildList spanIds) {
             if (spanIds == null) {
                 return false;
             }
             for (int i = 0, size = spanIds.getSize(); i < size; i++) {
-                if (activeSet.contains(spanIds.get(i))) {
+                if (activeSet.contains(spanIds.getId(i))) {
                     return true;
                 }
             }
@@ -719,7 +739,7 @@ public int spanify() {
             int createdSpans = 0;
             List<CallTree> callTrees = getChildren();
             for (int i = 0, size = callTrees.size(); i < size; i++) {
-                createdSpans += callTrees.get(i).spanify(this, rootContext);
+                createdSpans += callTrees.get(i).spanify(this, rootContext, rootContext);
             }
             return createdSpans;
         }

apm-agent-plugins/apm-profiling-plugin/src/main/java/co/elastic/apm/agent/profiler/ChildList.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package co.elastic.apm.agent.profiler;
+
+
+import co.elastic.apm.agent.sdk.internal.collections.LongList;
+
+/** List for maintaining pairs of (spanId,parentIds) both represented as longs. */
+public class ChildList {
+
+    // this list contains the (spanId,parentIds) flattened
+    private final LongList idsWithParentIds = new LongList();
+
+    public void add(long id, long parentId) {
+        idsWithParentIds.add(id);
+        idsWithParentIds.add(parentId);
+    }
+
+    public long getId(int index) {
+        return idsWithParentIds.get(index * 2);
+    }
+
+    public long getParentId(int index) {
+        return idsWithParentIds.get(index * 2 + 1);
+    }
+
+    public int getSize() {
+        return idsWithParentIds.getSize() / 2;
+    }
+
+    public void addAll(ChildList other) {
+        idsWithParentIds.addAll(other.idsWithParentIds);
+    }
+
+    public void clear() {
+        idsWithParentIds.clear();
+    }
+
+    public boolean isEmpty() {
+        return getSize() == 0;
+    }
+
+    public void removeLast() {
+        int size = idsWithParentIds.getSize();
+        idsWithParentIds.remove(size - 1);
+        idsWithParentIds.remove(size - 2);
+    }
+}

apm-agent-plugins/apm-profiling-plugin/src/main/java/co/elastic/apm/agent/profiler/SamplingProfiler.java

@@ -106,7 +106,7 @@
  * and at least one stack trace.
  * Once {@linkplain ActivationEvent#handleDeactivationEvent(SamplingProfiler) handling the deactivation event} of the root span in a thread
  * (after which {@link ElasticApmTracer#getActive()} would return {@code null}),
- * the {@link CallTree} is {@linkplain CallTree#spanify(CallTree.Root, TraceContext) converted into regular spans}.
+ * the {@link CallTree} is {@linkplain CallTree#spanify(CallTree.Root, TraceContext, TraceContext) converted into regular spans}.
  * </p>
  * <p>
  * Overall, the allocation rate does not depend on the number of {@link ActivationEvent}s but only on

apm-agent-plugins/apm-profiling-plugin/src/test/java/co/elastic/apm/agent/profiler/CallTreeSpanifyTest.java

@@ -35,6 +35,7 @@
 import org.stagemonitor.configuration.ConfigurationRegistry;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 import java.util.concurrent.TimeUnit;
@@ -141,8 +142,62 @@ void testCallTreeWithActiveSpan() {
         root.spanify();
 
         assertThat(reporter.getSpans()).hasSize(2);
-        assertThat(reporter.getSpans().get(1).getTraceContext().isChildOf(spanContext));
-        assertThat(reporter.getSpans().get(0).getTraceContext().isChildOf(rootContext));
+        assertThat(reporter.getSpans().get(0).getTraceContext().isChildOf(spanContext)).isTrue();
+        assertThat(reporter.getSpans().get(1).getTraceContext().isChildOf(rootContext)).isTrue();
+    }
+
+
+    @Test
+    void testSpanWithInvertedActivation() {
+        TraceContext rootContext = CallTreeTest.rootTraceContext(tracer);
+
+        TraceContext childSpanContext = TraceContext.with64BitId(tracer);
+        TraceContext.fromParentContext().asChildOf(childSpanContext, rootContext);
+
+        NoopObjectPool<CallTree.Root> rootPool = NoopObjectPool.ofRecyclable(() -> new CallTree.Root(tracer));
+        NoopObjectPool<CallTree> childPool = NoopObjectPool.ofRecyclable(CallTree::new);
+
+        CallTree.Root root = CallTree.createRoot(rootPool, childSpanContext.serialize(), rootContext.getServiceName(), rootContext.getServiceVersion(), 0);
+        root.addStackTrace(tracer, Collections.singletonList(StackFrame.of("A", "a")), 10_000, childPool, 0);
+
+        root.onActivation(rootContext.serialize(), 20_000);
+        root.onDeactivation(rootContext.serialize(), childSpanContext.serialize(), 30_000);
+
+        root.addStackTrace(tracer, Collections.singletonList(StackFrame.of("A", "a")), 40_000, childPool, 0);
+        root.end(childPool, 0);
+
+        root.spanify();
+
+        assertThat(reporter.getSpans()).hasSize(1);
+        assertThat(reporter.getSpans().get(0).getTraceContext().isChildOf(childSpanContext)).isTrue();
+        // the inferred span should not have any span links because this
+        // span link would cause a cycle in the trace
+        assertThat(reporter.getSpans().get(0).getChildIds().getSize()).isEqualTo(0L);
+    }
+
+    @Test
+    void testSpanWithNestedActivation() {
+        TraceContext rootContext = CallTreeTest.rootTraceContext(tracer);
+
+        NoopObjectPool<CallTree.Root> rootPool = NoopObjectPool.ofRecyclable(() -> new CallTree.Root(tracer));
+        NoopObjectPool<CallTree> childPool = NoopObjectPool.ofRecyclable(CallTree::new);
+
+        CallTree.Root root = CallTree.createRoot(rootPool, rootContext.serialize(), rootContext.getServiceName(), rootContext.getServiceVersion(), 0);
+        root.addStackTrace(tracer, Collections.singletonList(StackFrame.of("A", "a")), 10_000, childPool, 0);
+
+        root.onActivation(rootContext.serialize(), 20_000);
+        root.onDeactivation(rootContext.serialize(), rootContext.serialize(), 30_000);
+
+        root.addStackTrace(tracer, Collections.singletonList(StackFrame.of("A", "a")), 40_000, childPool, 0);
+        root.end(childPool, 0);
+
+        root.spanify();
+
+        assertThat(reporter.getSpans()).hasSize(1);
+        assertThat(reporter.getSpans().get(0).getTraceContext().isChildOf(rootContext)).isTrue();
+        // the inferred span should not have any span links because this
+        // span link would cause a cycle in the trace
+        assertThat(reporter.getSpans().get(0).getChildIds().getSize()).isEqualTo(0L);
     }
 
 }

apm-agent-plugins/apm-profiling-plugin/src/test/java/co/elastic/apm/agent/profiler/CallTreeTest.java

@@ -120,7 +120,7 @@ void testCallTree() {
     @Test
     void testGiveEmptyChildIdsTo() {
         CallTree rich = new CallTree();
-        rich.addChildId(42);
+        rich.addChildId(42, 0L);
         CallTree robinHood = new CallTree();
         CallTree poor = new CallTree();