diff --git a/absl/synchronization/internal/graphcycles.h b/absl/synchronization/internal/graphcycles.h
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+// Copyright 2017 The Abseil Authors.
+//
+// Licensed 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
+//
+//      https://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.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_GRAPHCYCLES_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_GRAPHCYCLES_H_
+
+// GraphCycles detects the introduction of a cycle into a directed
+// graph that is being built up incrementally.
+//
+// Nodes are identified by small integers.  It is not possible to
+// record multiple edges with the same (source, destination) pair;
+// requests to add an edge where one already exists are silently
+// ignored.
+//
+// It is also not possible to introduce a cycle; an attempt to insert
+// an edge that would introduce a cycle fails and returns false.
+//
+// GraphCycles uses no internal locking; calls into it should be
+// serialized externally.
+
+// Performance considerations:
+//   Works well on sparse graphs, poorly on dense graphs.
+//   Extra information is maintained incrementally to detect cycles quickly.
+//   InsertEdge() is very fast when the edge already exists, and reasonably fast
+//   otherwise.
+//   FindPath() is linear in the size of the graph.
+// The current implemenation uses O(|V|+|E|) space.
+
+#include <cstdint>
+
+namespace absl {
+namespace synchronization_internal {
+
+// Opaque identifier for a graph node.
+struct GraphId {
+  uint64_t handle;
+
+  bool operator==(const GraphId& x) const { return handle == x.handle; }
+  bool operator!=(const GraphId& x) const { return handle != x.handle; }
+};
+
+// Return an invalid graph id that will never be assigned by GraphCycles.
+inline GraphId InvalidGraphId() {
+  return GraphId{0};
+}
+
+class GraphCycles {
+ public:
+  GraphCycles();
+  ~GraphCycles();
+
+  // Return the id to use for ptr, assigning one if necessary.
+  // Subsequent calls with the same ptr value will return the same id
+  // until Remove().
+  GraphId GetId(void* ptr);
+
+  // Remove "ptr" from the graph.  Its corresponding node and all
+  // edges to and from it are removed.
+  void RemoveNode(void* ptr);
+
+  // Return the pointer associated with id, or nullptr if id is not
+  // currently in the graph.
+  void* Ptr(GraphId id);
+
+  // Attempt to insert an edge from source_node to dest_node.  If the
+  // edge would introduce a cycle, return false without making any
+  // changes. Otherwise add the edge and return true.
+  bool InsertEdge(GraphId source_node, GraphId dest_node);
+
+  // Remove any edge that exists from source_node to dest_node.
+  void RemoveEdge(GraphId source_node, GraphId dest_node);
+
+  // Return whether node exists in the graph.
+  bool HasNode(GraphId node);
+
+  // Return whether there is an edge directly from source_node to dest_node.
+  bool HasEdge(GraphId source_node, GraphId dest_node) const;
+
+  // Return whether dest_node is reachable from source_node
+  // by following edges.
+  bool IsReachable(GraphId source_node, GraphId dest_node) const;
+
+  // Find a path from "source" to "dest".  If such a path exists,
+  // place the nodes on the path in the array path[], and return
+  // the number of nodes on the path.  If the path is longer than
+  // max_path_len nodes, only the first max_path_len nodes are placed
+  // in path[].  The client should compare the return value with
+  // max_path_len" to see when this occurs.  If no path exists, return
+  // 0.  Any valid path stored in path[] will start with "source" and
+  // end with "dest".  There is no guarantee that the path is the
+  // shortest, but no node will appear twice in the path, except the
+  // source and destination node if they are identical; therefore, the
+  // return value is at most one greater than the number of nodes in
+  // the graph.
+  int FindPath(GraphId source, GraphId dest, int max_path_len,
+               GraphId path[]) const;
+
+  // Update the stack trace recorded for id with the current stack
+  // trace if the last time it was updated had a smaller priority
+  // than the priority passed on this call.
+  //
+  // *get_stack_trace is called to get the stack trace.
+  void UpdateStackTrace(GraphId id, int priority,
+                        int (*get_stack_trace)(void**, int));
+
+  // Set *ptr to the beginning of the array that holds the recorded
+  // stack trace for id and return the depth of the stack trace.
+  int GetStackTrace(GraphId id, void*** ptr);
+
+  // Check internal invariants. Crashes on failure, returns true on success.
+  // Expensive: should only be called from graphcycles_test.cc.
+  bool CheckInvariants() const;
+
+  // ----------------------------------------------------
+  struct Rep;
+ private:
+  Rep *rep_;      // opaque representation
+  GraphCycles(const GraphCycles&) = delete;
+  GraphCycles& operator=(const GraphCycles&) = delete;
+};
+
+}  // namespace synchronization_internal
+}  // namespace absl
+
+#endif