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diff --git a/src/include/gnunet_setu_service.h b/src/include/gnunet_setu_service.h
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+++ b/src/include/gnunet_setu_service.h
@@ -0,0 +1,390 @@
+/*
+      This file is part of GNUnet
+      Copyright (C) 2013, 2014, 2020 GNUnet e.V.
+
+      GNUnet is free software: you can redistribute it and/or modify it
+      under the terms of the GNU Affero General Public License as published
+      by the Free Software Foundation, either version 3 of the License,
+      or (at your option) any later version.
+
+      GNUnet is distributed in the hope that it will be useful, but
+      WITHOUT ANY WARRANTY; without even the implied warranty of
+      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+      Affero General Public License for more details.
+
+      You should have received a copy of the GNU Affero General Public License
+      along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+     SPDX-License-Identifier: AGPL3.0-or-later
+ */
+/**
+ * @author Florian Dold
+ * @author Christian Grothoff
+ *
+ * @file
+ * Two-peer set union operations
+ *
+ * @defgroup set  Set union service
+ * Two-peer set operations
+ *
+ * @{
+ */
+
+#ifndef GNUNET_SETU_SERVICE_H
+#define GNUNET_SETU_SERVICE_H
+
+#ifdef __cplusplus
+extern "C"
+{
+#if 0                           /* keep Emacsens' auto-indent happy */
+}
+#endif
+#endif
+
+#include "gnunet_common.h"
+#include "gnunet_time_lib.h"
+#include "gnunet_configuration_lib.h"
+
+
+/**
+ * Maximum size of a context message for set operation requests.
+ */
+#define GNUNET_SETU_CONTEXT_MESSAGE_MAX_SIZE ((1 << 16) - 1024)
+
+/**
+ * Opaque handle to a set.
+ */
+struct GNUNET_SETU_Handle;
+
+/**
+ * Opaque handle to a set operation request from another peer.
+ */
+struct GNUNET_SETU_Request;
+
+/**
+ * Opaque handle to a listen operation.
+ */
+struct GNUNET_SETU_ListenHandle;
+
+/**
+ * Opaque handle to a set operation.
+ */
+struct GNUNET_SETU_OperationHandle;
+
+
+/**
+ * Status for the result callback
+ */
+enum GNUNET_SETU_Status
+{
+
+  /**
+   * Element should be added to the result set of the local peer, i.e. the
+   * local peer is missing an element.
+   */
+  GNUNET_SETU_STATUS_ADD_LOCAL,
+
+  /**
+   * Element should be added to the result set of the remote peer, i.e. the
+   * remote peer is missing an element. Only used if
+   * #GNUNET_SETU_OPTION_SYMMETRIC is set.
+   */
+  GNUNET_SETU_STATUS_ADD_REMOTE,
+
+  /**
+   * The other peer refused to do the operation with us, or something went
+   * wrong.
+   */
+  GNUNET_SETU_STATUS_FAILURE,
+
+  /**
+   * Success, all elements have been sent (and received).
+   */
+  GNUNET_SETU_STATUS_DONE
+};
+
+
+/**
+ * Element stored in a set.
+ */
+struct GNUNET_SETU_Element
+{
+  /**
+   * Number of bytes in the buffer pointed to by data.
+   */
+  uint16_t size;
+
+  /**
+   * Application-specific element type.
+   */
+  uint16_t element_type;
+
+  /**
+   * Actual data of the element
+   */
+  const void *data;
+};
+
+
+/**
+ * Possible options to pass to a set operation.
+ *
+ * Used as tag for struct #GNUNET_SETU_Option.
+ */
+enum GNUNET_SETU_OptionType
+{
+  /**
+   * List terminator.
+   */
+  GNUNET_SETU_OPTION_END=0,
+
+  /**
+   * Fail set operations when the other peer shows weird behavior
+   * that might by a Byzantine fault.
+   *
+   * For set union, 'v.num' is a lower bound on elements that the other peer
+   * must have in common with us.
+   */
+  GNUNET_SETU_OPTION_BYZANTINE=1,
+
+  /**
+   * Do not use the optimized set operation, but send full sets.  Might
+   * trigger Byzantine fault detection.
+   */
+  GNUNET_SETU_OPTION_FORCE_FULL=2,
+
+  /**
+   * Only use optimized set operations, even though for this particular set
+   * operation they might be much slower.  Might trigger Byzantine fault
+   * detection.
+   */
+  GNUNET_SETU_OPTION_FORCE_DELTA=4,
+
+  /**
+   * Notify client also if we are sending a value to the other peer.
+   */
+  GNUNET_SETU_OPTION_SYMMETRIC = 8
+};
+
+
+/**
+ * Option for set operations.
+ */
+struct GNUNET_SETU_Option
+{
+  /**
+   * Type of the option.
+   */
+  enum GNUNET_SETU_OptionType type;
+
+  /**
+   * Value for the option, only used with some options.
+   */
+  union
+  {
+    uint64_t num;
+  } v;
+};
+
+
+/**
+ * Callback for set union operation results. Called for each element
+ * in the result set.
+ *
+ * @param cls closure
+ * @param element a result element, only valid if status is #GNUNET_SETU_STATUS_OK
+ * @param current_size current set size
+ * @param status see `enum GNUNET_SETU_Status`
+ */
+typedef void
+(*GNUNET_SETU_ResultIterator) (void *cls,
+                               const struct GNUNET_SETU_Element *element,
+                               uint64_t current_size,
+                               enum GNUNET_SETU_Status status);
+
+
+/**
+ * Called when another peer wants to do a set operation with the
+ * local peer. If a listen error occurs, the @a request is NULL.
+ *
+ * @param cls closure
+ * @param other_peer the other peer
+ * @param context_msg message with application specific information from
+ *        the other peer
+ * @param request request from the other peer (never NULL), use GNUNET_SETU_accept()
+ *        to accept it, otherwise the request will be refused
+ *        Note that we can't just return value from the listen callback,
+ *        as it is also necessary to specify the set we want to do the
+ *        operation with, whith sometimes can be derived from the context
+ *        message. It's necessary to specify the timeout.
+ */
+typedef void
+(*GNUNET_SETU_ListenCallback) (void *cls,
+                               const struct GNUNET_PeerIdentity *other_peer,
+                               const struct GNUNET_MessageHeader *context_msg,
+                               struct GNUNET_SETU_Request *request);
+
+
+/**
+ * Create an empty set, supporting the specified operation.
+ *
+ * @param cfg configuration to use for connecting to the
+ *        set service
+ * @return a handle to the set
+ */
+struct GNUNET_SETU_Handle *
+GNUNET_SETU_create (const struct GNUNET_CONFIGURATION_Handle *cfg);
+
+
+/**
+ * Add an element to the given set.
+ *
+ * @param set set to add element to
+ * @param element element to add to the set
+ * @param cb function to call when finished, can be NULL
+ * @param cb_cls closure for @a cb
+ * @return #GNUNET_OK on success, #GNUNET_SYSERR if the
+ *         set is invalid (e.g. the set service crashed)
+ */
+int
+GNUNET_SETU_add_element (struct GNUNET_SETU_Handle *set,
+                         const struct GNUNET_SETU_Element *element,
+                         GNUNET_SCHEDULER_TaskCallback cb,
+                         void *cb_cls);
+
+
+/**
+ * Destroy the set handle, and free all associated resources.  Operations may
+ * still be pending when a set is destroyed (and will be allowed to complete).
+ *
+ * @param set set to destroy
+ */
+void
+GNUNET_SETU_destroy (struct GNUNET_SETU_Handle *set);
+
+
+/**
+ * Prepare a set operation to be evaluated with another peer.  The evaluation
+ * will not start until the client provides a local set with
+ * GNUNET_SETU_commit().
+ *
+ * @param other_peer peer with the other set
+ * @param app_id hash for the application using the set
+ * @param context_msg additional information for the request
+ * @param options options to use when processing the request
+ * @param result_cb called on error or success
+ * @param result_cls closure for @a result_cb
+ * @return a handle to cancel the operation
+ */
+struct GNUNET_SETU_OperationHandle *
+GNUNET_SETU_prepare (const struct GNUNET_PeerIdentity *other_peer,
+                     const struct GNUNET_HashCode *app_id,
+                     const struct GNUNET_MessageHeader *context_msg,
+                     const struct GNUNET_SETU_Option options[],
+                     GNUNET_SETU_ResultIterator result_cb,
+                     void *result_cls);
+
+
+/**
+ * Wait for set operation requests for the given application ID.
+ * If the connection to the set service is lost, the listener is
+ * re-created transparently with exponential backoff.
+ *
+ * @param cfg configuration to use for connecting to
+ *            the set service
+ * @param app_id id of the application that handles set operation requests
+ * @param listen_cb called for each incoming request matching the operation
+ *                  and application id
+ * @param listen_cls handle for @a listen_cb
+ * @return a handle that can be used to cancel the listen operation
+ */
+struct GNUNET_SETU_ListenHandle *
+GNUNET_SETU_listen (const struct GNUNET_CONFIGURATION_Handle *cfg,
+                    const struct GNUNET_HashCode *app_id,
+                    GNUNET_SETU_ListenCallback listen_cb,
+                    void *listen_cls);
+
+
+/**
+ * Cancel the given listen operation.  After calling cancel, the
+ * listen callback for this listen handle will not be called again.
+ * Note that cancelling a listen operation will automatically reject
+ * all operations that have not yet been accepted.
+ *
+ * @param lh handle for the listen operation
+ */
+void
+GNUNET_SETU_listen_cancel (struct GNUNET_SETU_ListenHandle *lh);
+
+
+/**
+ * Accept a request we got via GNUNET_SETU_listen().  Must be called during
+ * GNUNET_SETU_listen(), as the `struct GNUNET_SETU_Request` becomes invalid
+ * afterwards.
+ * Call GNUNET_SETU_commit() to provide the local set to use for the operation,
+ * and to begin the exchange with the remote peer.
+ *
+ * @param request request to accept
+ * @param options options to use when processing the request
+ * @param result_cb callback for the results
+ * @param result_cls closure for @a result_cb
+ * @return a handle to cancel the operation
+ */
+struct GNUNET_SETU_OperationHandle *
+GNUNET_SETU_accept (struct GNUNET_SETU_Request *request,
+                    const struct GNUNET_SETU_Option options[],
+                    GNUNET_SETU_ResultIterator result_cb,
+                    void *result_cls);
+
+
+/**
+ * Commit a set to be used with a set operation.
+ * This function is called once we have fully constructed
+ * the set that we want to use for the operation.  At this
+ * time, the P2P protocol can then begin to exchange the
+ * set information and call the result callback with the
+ * result information.
+ *
+ * @param oh handle to the set operation
+ * @param set the set to use for the operation
+ * @return #GNUNET_OK on success, #GNUNET_SYSERR if the
+ *         set is invalid (e.g. the set service crashed)
+ */
+int
+GNUNET_SETU_commit (struct GNUNET_SETU_OperationHandle *oh,
+                    struct GNUNET_SETU_Handle *set);
+
+
+/**
+ * Cancel the given set operation.  May not be called after the operation's
+ * `GNUNET_SETU_ResultIterator` has been called with a status of
+ * #GNUNET_SETU_STATUS_FAILURE or #GNUNET_SETU_STATUS_DONE.
+ *
+ * @param oh set operation to cancel
+ */
+void
+GNUNET_SETU_operation_cancel (struct GNUNET_SETU_OperationHandle *oh);
+
+
+/**
+ * Hash a set element.
+ *
+ * @param element the element that should be hashed
+ * @param[out] ret_hash a pointer to where the hash of @a element
+ *        should be stored
+ */
+void
+GNUNET_SETU_element_hash (const struct GNUNET_SETU_Element *element,
+                          struct GNUNET_HashCode *ret_hash);
+
+
+#if 0                           /* keep Emacsens' auto-indent happy */
+{
+#endif
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+
+/** @} */  /* end of group */