implement order claiming

1 files changed, 1089 insertions, 0 deletions

diff --git a/src/backend/taler-merchant-httpd_private-post-transfers.c b/src/backend/taler-merchant-httpd_private-post-transfers.c
new file mode 100644
index 00000000..7f55c917
--- /dev/null
+++ b/src/backend/taler-merchant-httpd_private-post-transfers.c
@@ -0,0 +1,1089 @@
+/*
+  This file is part of TALER
+  (C) 2014-2020 Taler Systems SA
+
+  TALER 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, or (at your option) any later version.
+
+  TALER 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 General Public License for more details.
+
+  You should have received a copy of the GNU General Public License along with
+  TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+*/
+/**
+ * @file backend/taler-merchant-httpd_track-transfer.c
+ * @brief implement API for tracking transfers and wire transfers
+ * @author Marcello Stanisci
+ * @author Christian Grothoff
+ */
+#include "platform.h"
+#include <jansson.h>
+#include <taler/taler_signatures.h>
+#include <taler/taler_json_lib.h>
+#include "taler-merchant-httpd.h"
+#include "taler-merchant-httpd_mhd.h"
+#include "taler-merchant-httpd_auditors.h"
+#include "taler-merchant-httpd_exchanges.h"
+#include "taler-merchant-httpd_track-transfer.h"
+
+
+/**
+ * How long to wait before giving up processing with the exchange?
+ */
+#define TRACK_TIMEOUT (GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, \
+                                                      30))
+
+/**
+ * How often do we retry the simple INSERT database transaction?
+ */
+#define MAX_RETRIES 3
+
+/**
+ * Context used for handing /track/transfer requests.
+ */
+struct TrackTransferContext
+{
+
+  /**
+   * This MUST be first!
+   */
+  struct TM_HandlerContext hc;
+
+  /**
+   * Handle to the exchange.
+   */
+  struct TALER_EXCHANGE_Handle *eh;
+
+  /**
+   * Handle for the /wire/transfers request.
+   */
+  struct TALER_EXCHANGE_TransfersGetHandle *wdh;
+
+  /**
+   * For which merchant instance is this tracking request?
+   */
+  struct MerchantInstance *mi;
+
+  /**
+   * HTTP connection we are handling.
+   */
+  struct MHD_Connection *connection;
+
+  /**
+   * Response to return upon resume.
+   */
+  struct MHD_Response *response;
+
+  /**
+   * Handle for operation to lookup /keys (and auditors) from
+   * the exchange used for this transaction; NULL if no operation is
+   * pending.
+   */
+  struct TMH_EXCHANGES_FindOperation *fo;
+
+  /**
+   * Task run on timeout.
+   */
+  struct GNUNET_SCHEDULER_Task *timeout_task;
+
+  /**
+   * URL of the exchange.
+   */
+  char *url;
+
+  /**
+   * Wire method used for the transfer.
+   */
+  char *wire_method;
+
+  /**
+   * Pointer to the detail that we are currently
+   * checking in #check_transfer().
+   */
+  const struct TALER_TrackTransferDetails *current_detail;
+
+  /**
+   * Argument for the /wire/transfers request.
+   */
+  struct TALER_WireTransferIdentifierRawP wtid;
+
+  /**
+   * Full original response we are currently processing.
+   */
+  const json_t *original_response;
+
+  /**
+   * Modified response to return to the frontend.
+   */
+  json_t *deposits_response;
+
+  /**
+   * Which transaction detail are we currently looking at?
+   */
+  unsigned int current_offset;
+
+  /**
+   * Response code to return.
+   */
+  unsigned int response_code;
+
+  /**
+   * #GNUNET_NO if we did not find a matching coin.
+   * #GNUNET_SYSERR if we found a matching coin, but the amounts do not match.
+   * #GNUNET_OK if we did find a matching coin.
+   */
+  int check_transfer_result;
+};
+
+
+/**
+ * Represents an entry in the table used to sum up
+ * individual deposits for each h_contract_terms.
+ */
+struct Entry
+{
+
+  /**
+   * Sum accumulator for deposited value.
+   */
+  struct TALER_Amount deposit_value;
+
+  /**
+   * Sum accumulator for deposit fee.
+   */
+  struct TALER_Amount deposit_fee;
+
+};
+
+
+/**
+ * Free the @a rctx.
+ *
+ * @param rctx data to free
+ */
+static void
+free_transfer_track_context (struct TrackTransferContext *rctx)
+{
+  if (NULL != rctx->fo)
+  {
+    TMH_EXCHANGES_find_exchange_cancel (rctx->fo);
+    rctx->fo = NULL;
+  }
+  if (NULL != rctx->timeout_task)
+  {
+    GNUNET_SCHEDULER_cancel (rctx->timeout_task);
+    rctx->timeout_task = NULL;
+  }
+  if (NULL != rctx->wdh)
+  {
+    TALER_EXCHANGE_transfers_get_cancel (rctx->wdh);
+    rctx->wdh = NULL;
+  }
+  if (NULL != rctx->url)
+  {
+    GNUNET_free (rctx->url);
+    rctx->url = NULL;
+  }
+  if (NULL != rctx->wire_method)
+  {
+    GNUNET_free (rctx->wire_method);
+    rctx->wire_method = NULL;
+  }
+  GNUNET_free (rctx);
+}
+
+
+/**
+ * Callback that frees all the elements in the hashmap
+ *
+ * @param cls closure, NULL
+ * @param key current key
+ * @param value a `struct Entry`
+ * @return #GNUNET_YES if the iteration should continue,
+ *         #GNUNET_NO otherwise.
+ */
+static int
+hashmap_free (void *cls,
+              const struct GNUNET_HashCode *key,
+              void *value)
+{
+  struct TALER_Entry *entry = value;
+
+  (void) cls;
+  (void) key;
+  GNUNET_free (entry);
+  return GNUNET_YES;
+}
+
+
+/**
+ * Builds JSON response containing the summed-up amounts
+ * from individual deposits.
+ *
+ * @param cls closure
+ * @param key map's current key
+ * @param map's current value
+ * @return #GNUNET_YES if iteration is to be continued,
+ *         #GNUNET_NO otherwise.
+ */
+static int
+build_deposits_response (void *cls,
+                         const struct GNUNET_HashCode *key,
+                         void *value)
+{
+  struct TrackTransferContext *rctx = cls;
+  struct Entry *entry = value;
+  json_t *element;
+  json_t *contract_terms;
+  json_t *order_id;
+
+  db->preflight (db->cls);
+  if (GNUNET_DB_STATUS_SUCCESS_ONE_RESULT !=
+      db->find_contract_terms_from_hash (db->cls,
+                                         &contract_terms,
+                                         key,
+                                         &rctx->mi->pubkey))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_NO;
+  }
+
+  order_id = json_object_get (contract_terms,
+                              "order_id");
+  if (NULL == order_id)
+  {
+    GNUNET_break_op (0);
+    json_decref (contract_terms);
+    return GNUNET_NO;
+  }
+  element = json_pack ("{s:O, s:o, s:o}",
+                       "order_id", order_id,
+                       "deposit_value", TALER_JSON_from_amount (
+                         &entry->deposit_value),
+                       "deposit_fee", TALER_JSON_from_amount (
+                         &entry->deposit_fee));
+  json_decref (contract_terms);
+  if (NULL == element)
+  {
+    GNUNET_break_op (0);
+    return GNUNET_NO;
+  }
+  GNUNET_break (0 ==
+                json_array_append_new (rctx->deposits_response,
+                                       element));
+  return GNUNET_YES;
+}
+
+
+/**
+ * Transform /track/transfer result as gotten from the exchange
+ * and transforms it in a format liked by the backoffice Web interface.
+ *
+ * @param result response from exchange's /track/transfer
+ * @result pointer to new JSON, or NULL upon errors.
+ */
+static json_t *
+transform_response (const json_t *result,
+                    struct TrackTransferContext *rctx)
+{
+  json_t *deposits;
+  json_t *value;
+  json_t *result_mod = NULL;
+  size_t index;
+  const char *key;
+  struct GNUNET_HashCode h_key;
+  struct GNUNET_CONTAINER_MultiHashMap *map;
+  struct TALER_Amount iter_value;
+  struct TALER_Amount iter_fee;
+  struct Entry *current_entry;
+  struct GNUNET_JSON_Specification spec[] = {
+    TALER_JSON_spec_amount ("deposit_value", &iter_value),
+    TALER_JSON_spec_amount ("deposit_fee", &iter_fee),
+    GNUNET_JSON_spec_string ("h_contract_terms", &key),
+    GNUNET_JSON_spec_end ()
+  };
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Transforming /track/transfer response.\n");
+  map = GNUNET_CONTAINER_multihashmap_create (1, GNUNET_NO);
+  deposits = json_object_get (result,
+                              "deposits");
+
+  json_array_foreach (deposits, index, value)
+  {
+    if (GNUNET_OK !=
+        GNUNET_JSON_parse (value,
+                           spec,
+                           NULL,
+                           NULL))
+    {
+      GNUNET_break_op (0);
+      return NULL;
+    }
+    GNUNET_CRYPTO_hash_from_string (key,
+                                    &h_key);
+
+    if (NULL != (current_entry =
+                   GNUNET_CONTAINER_multihashmap_get (map,
+                                                      &h_key)))
+    {
+      /* The map already knows this h_contract_terms*/
+      if ( (0 >
+            TALER_amount_add (&current_entry->deposit_value,
+                              &current_entry->deposit_value,
+                              &iter_value)) ||
+           (0 >
+            TALER_amount_add (&current_entry->deposit_fee,
+                              &current_entry->deposit_fee,
+                              &iter_fee)) )
+      {
+        GNUNET_JSON_parse_free (spec);
+        goto cleanup;
+      }
+    }
+    else
+    {
+      /* First time in the map for this h_contract_terms*/
+      current_entry = GNUNET_new (struct Entry);
+      current_entry->deposit_value = iter_value;
+      current_entry->deposit_fee = iter_fee;
+
+      if (GNUNET_SYSERR ==
+          GNUNET_CONTAINER_multihashmap_put (map,
+                                             &h_key,
+                                             current_entry,
+                                             GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY))
+      {
+        GNUNET_JSON_parse_free (spec);
+        goto cleanup;
+      }
+    }
+    GNUNET_JSON_parse_free (spec);
+  }
+  rctx->deposits_response = json_array ();
+
+  if (GNUNET_SYSERR ==
+      GNUNET_CONTAINER_multihashmap_iterate (map,
+                                             &build_deposits_response,
+                                             rctx))
+    goto cleanup;
+
+  result_mod = json_copy ((struct json_t *) result);
+  json_object_del (result_mod,
+                   "deposits");
+  json_object_set_new (result_mod,
+                       "deposits_sums",
+                       rctx->deposits_response);
+  rctx->deposits_response = NULL;
+cleanup:
+  GNUNET_CONTAINER_multihashmap_iterate (map,
+                                         &hashmap_free,
+                                         NULL);
+  GNUNET_CONTAINER_multihashmap_destroy (map);
+  return result_mod;
+}
+
+
+/**
+ * Resume the given /track/transfer operation and send the given response.
+ * Stores the response in the @a rctx and signals MHD to resume
+ * the connection.  Also ensures MHD runs immediately.
+ *
+ * @param rctx transfer tracking context
+ * @param response_code response code to use
+ * @param response response data to send back
+ */
+static void
+resume_track_transfer_with_response (struct TrackTransferContext *rctx,
+                                     unsigned int response_code,
+                                     struct MHD_Response *response)
+{
+  rctx->response_code = response_code;
+  rctx->response = response;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Resuming /track/transfer handling as exchange interaction is done (%u)\n",
+              response_code);
+  if (NULL != rctx->timeout_task)
+  {
+    GNUNET_SCHEDULER_cancel (rctx->timeout_task);
+    rctx->timeout_task = NULL;
+  }
+  MHD_resume_connection (rctx->connection);
+  TMH_trigger_daemon (); /* we resumed, kick MHD */
+}
+
+
+/**
+ * Custom cleanup routine for a `struct TrackTransferContext`.
+ *
+ * @param hc the `struct TrackTransferContext` to clean up.
+ */
+static void
+track_transfer_cleanup (struct TM_HandlerContext *hc)
+{
+  struct TrackTransferContext *rctx = (struct TrackTransferContext *) hc;
+
+  free_transfer_track_context (rctx);
+}
+
+
+/**
+ * This function checks that the information about the coin which
+ * was paid back by _this_ wire transfer matches what _we_ (the merchant)
+ * knew about this coin.
+ *
+ * @param cls closure with our `struct TrackTransferContext *`
+ * @param transaction_id of the contract
+ * @param coin_pub public key of the coin
+ * @param exchange_url URL of the exchange that issued @a coin_pub
+ * @param amount_with_fee amount the exchange will transfer for this coin
+ * @param deposit_fee fee the exchange will charge for this coin
+ * @param refund_fee fee the exchange will charge for refunding this coin
+ * @param exchange_proof proof from exchange that coin was accepted
+ */
+static void
+check_transfer (void *cls,
+                const struct GNUNET_HashCode *h_contract_terms,
+                const struct TALER_CoinSpendPublicKeyP *coin_pub,
+                const char *exchange_url,
+                const struct TALER_Amount *amount_with_fee,
+                const struct TALER_Amount *deposit_fee,
+                const struct TALER_Amount *refund_fee,
+                const struct TALER_Amount *wire_fee,
+                const json_t *exchange_proof)
+{
+  struct TrackTransferContext *rctx = cls;
+  const struct TALER_TrackTransferDetails *ttd = rctx->current_detail;
+
+  if (GNUNET_SYSERR == rctx->check_transfer_result)
+    return; /* already had a serious issue; odd that we're called more than once as well... */
+  if ( (0 != TALER_amount_cmp (amount_with_fee,
+                               &ttd->coin_value)) ||
+       (0 != TALER_amount_cmp (deposit_fee,
+                               &ttd->coin_fee)) )
+  {
+    /* Disagreement between the exchange and us about how much this
+       coin is worth! */
+    GNUNET_break_op (0);
+    rctx->check_transfer_result = GNUNET_SYSERR;
+    /* Build the `TrackTransferConflictDetails` */
+    rctx->response
+      = TALER_MHD_make_json_pack (
+          "{s:I, s:s, s:o, s:I, s:o, s:o, s:s, s:o, s:o}",
+          "code", (json_int_t) TALER_EC_TRACK_TRANSFER_CONFLICTING_REPORTS,
+          "hint", "disagreement about deposit valuation",
+          "exchange_deposit_proof", exchange_proof,
+          "conflict_offset", (json_int_t) rctx->current_offset,
+          "exchange_transfer_proof", rctx->original_response,
+          "coin_pub", GNUNET_JSON_from_data_auto (coin_pub),
+          "h_contract_terms", GNUNET_JSON_from_data_auto (
+            &ttd->h_contract_terms),
+          "amount_with_fee", TALER_JSON_from_amount (amount_with_fee),
+          "deposit_fee", TALER_JSON_from_amount (deposit_fee));
+    return;
+  }
+  rctx->check_transfer_result = GNUNET_OK;
+}
+
+
+/**
+ * Check that the given @a wire_fee is what the
+ * @a exchange_pub should charge at the @a execution_time.
+ * If the fee is correct (according to our database),
+ * return #GNUNET_OK.  If we do not have the fee structure
+ * in our DB, we just accept it and return #GNUNET_NO;
+ * if we have proof that the fee is bogus, we respond with
+ * the proof to the client and return #GNUNET_SYSERR.
+ *
+ * @param rctx context of the transfer to respond to
+ * @param json response from the exchange
+ * @param execution_time time of the wire transfer
+ * @param wire_fee fee claimed by the exchange
+ * @return #GNUNET_SYSERR if we returned hard proof of
+ *   missbehavior from the exchange to the client
+ */
+static int
+check_wire_fee (struct TrackTransferContext *rctx,
+                const json_t *json,
+                struct GNUNET_TIME_Absolute execution_time,
+                const struct TALER_Amount *wire_fee)
+{
+  const struct TALER_MasterPublicKeyP *master_pub;
+  struct GNUNET_HashCode h_wire_method;
+  struct TALER_Amount expected_fee;
+  struct TALER_Amount closing_fee;
+  struct TALER_MasterSignatureP master_sig;
+  struct GNUNET_TIME_Absolute start_date;
+  struct GNUNET_TIME_Absolute end_date;
+  enum GNUNET_DB_QueryStatus qs;
+  const struct TALER_EXCHANGE_Keys *keys;
+
+  keys = TALER_EXCHANGE_get_keys (rctx->eh);
+  if (NULL == keys)
+  {
+    GNUNET_break (0);
+    return GNUNET_NO;
+  }
+  master_pub = &keys->master_pub;
+  GNUNET_CRYPTO_hash (rctx->wire_method,
+                      strlen (rctx->wire_method) + 1,
+                      &h_wire_method);
+  db->preflight (db->cls);
+  qs = db->lookup_wire_fee (db->cls,
+                            master_pub,
+                            &h_wire_method,
+                            execution_time,
+                            &expected_fee,
+                            &closing_fee,
+                            &start_date,
+                            &end_date,
+                            &master_sig);
+  if (0 >= qs)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "Failed to find wire fee for `%s' and method `%s' at %s in DB, accepting blindly that the fee is %s\n",
+                TALER_B2S (master_pub),
+                rctx->wire_method,
+                GNUNET_STRINGS_absolute_time_to_string (execution_time),
+                TALER_amount2s (wire_fee));
+    return GNUNET_NO;
+  }
+  if (0 <= TALER_amount_cmp (&expected_fee,
+                             wire_fee))
+    return GNUNET_OK; /* expected_fee >= wire_fee */
+
+  /* Wire fee check failed, export proof to client */
+  resume_track_transfer_with_response (
+    rctx,
+    MHD_HTTP_FAILED_DEPENDENCY,
+    TALER_MHD_make_json_pack (
+      "{s:I, s:o, s:o, s:o, s:o, s:o, s:o, s:o, s:o, s:O}",
+      "code", (json_int_t) TALER_EC_TRACK_TRANSFER_JSON_BAD_WIRE_FEE,
+      "wire_fee", TALER_JSON_from_amount (wire_fee),
+      "execution_time", GNUNET_JSON_from_time_abs (execution_time),
+      "expected_wire_fee", TALER_JSON_from_amount (&expected_fee),
+      "expected_closing_fee", TALER_JSON_from_amount (&closing_fee),
+      "start_date", GNUNET_JSON_from_time_abs (start_date),
+      "end_date", GNUNET_JSON_from_time_abs (end_date),
+      "master_sig", GNUNET_JSON_from_data_auto (&master_sig),
+      "master_pub", GNUNET_JSON_from_data_auto (master_pub),
+      "json", json));
+  return GNUNET_SYSERR;
+}
+
+
+/**
+ * Function called with detailed wire transfer data, including all
+ * of the coin transactions that were combined into the wire transfer.
+ *
+ * @param cls closure
+ * @param hr HTTP response details
+ * @param exchange_pub public key of the exchange used to sign @a json
+ * @param h_wire hash of the wire transfer address the transfer went to, or NULL on error
+ * @param execution_time time when the exchange claims to have performed the wire transfer
+ * @param total_amount total amount of the wire transfer, or NULL if the exchange could
+ *             not provide any @a wtid (set only if @a http_status is #MHD_HTTP_OK)
+ * @param wire_fee wire fee that was charged by the exchange
+ * @param details_length length of the @a details array
+ * @param details array with details about the combined transactions
+ */
+static void
+wire_transfer_cb (void *cls,
+                  const struct TALER_EXCHANGE_HttpResponse *hr,
+                  const struct TALER_ExchangePublicKeyP *exchange_pub,
+                  const struct GNUNET_HashCode *h_wire,
+                  struct GNUNET_TIME_Absolute execution_time,
+                  const struct TALER_Amount *total_amount,
+                  const struct TALER_Amount *wire_fee,
+                  unsigned int details_length,
+                  const struct TALER_TrackTransferDetails *details)
+{
+  struct TrackTransferContext *rctx = cls;
+  json_t *jresponse;
+  enum GNUNET_DB_QueryStatus qs;
+
+  rctx->wdh = NULL;
+  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+              "Got response code %u from exchange for /track/transfer\n",
+              hr->http_status);
+  if (MHD_HTTP_OK != hr->http_status)
+  {
+    resume_track_transfer_with_response (
+      rctx,
+      MHD_HTTP_FAILED_DEPENDENCY,
+      TALER_MHD_make_json_pack (
+        "{s:I, s:I, s:I, s:O}",
+        "code", (json_int_t) TALER_EC_TRACK_TRANSFER_EXCHANGE_ERROR,
+        "exchange_code", (json_int_t) hr->ec,
+        "exchange_http_status", (json_int_t) hr->http_status,
+        "exchange_reply", hr->reply));
+    return;
+  }
+  for (unsigned int i = 0; i<MAX_RETRIES; i++)
+  {
+    db->preflight (db->cls);
+    qs = db->store_transfer_to_proof (db->cls,
+                                      rctx->url,
+                                      &rctx->wtid,
+                                      execution_time,
+                                      exchange_pub,
+                                      hr->reply);
+    if (GNUNET_DB_STATUS_SOFT_ERROR != qs)
+      break;
+  }
+  if (0 > qs)
+  {
+    /* Special report if retries insufficient */
+    GNUNET_break (GNUNET_DB_STATUS_SOFT_ERROR != qs);
+    /* Always report on hard error as well to enable diagnostics */
+    GNUNET_break (GNUNET_DB_STATUS_HARD_ERROR == qs);
+    resume_track_transfer_with_response
+      (rctx,
+      MHD_HTTP_INTERNAL_SERVER_ERROR,
+      TALER_MHD_make_json_pack ("{s:I, s:s}",
+                                "code",
+                                (json_int_t)
+                                TALER_EC_TRACK_TRANSFER_DB_STORE_TRANSFER_ERROR,
+                                "details",
+                                "failed to store response from exchange to local database"));
+    return;
+  }
+  rctx->original_response = hr->reply;
+
+  if (GNUNET_SYSERR ==
+      check_wire_fee (rctx,
+                      hr->reply,
+                      execution_time,
+                      wire_fee))
+    return;
+
+  /* Now we want to double-check that any (Taler coin) deposit
+   * which is accounted into _this_ wire transfer, does exist
+   * into _our_ database.  This is the rationale: if the
+   * exchange paid us for it, we must have received it _beforehands_!
+   *
+   * details_length is how many (Taler coin) deposits have been
+   * aggregated into _this_ wire transfer.
+   *///
+  for (unsigned int i = 0; i<details_length; i++)
+  {
+    rctx->current_offset = i;
+    rctx->current_detail = &details[i];
+    /* Set the coin as "never seen" before. */
+    rctx->check_transfer_result = GNUNET_NO;
+    db->preflight (db->cls);
+    qs = db->find_payments_by_hash_and_coin (db->cls,
+                                             &details[i].h_contract_terms,
+                                             &rctx->mi->pubkey,
+                                             &details[i].coin_pub,
+                                             &check_transfer,
+                                             rctx);
+    if (0 > qs)
+    {
+      /* single, read-only SQL statements should never cause
+         serialization problems */
+      GNUNET_break (GNUNET_DB_STATUS_SOFT_ERROR != qs);
+      /* Always report on hard error as well to enable diagnostics */
+      GNUNET_break (GNUNET_DB_STATUS_HARD_ERROR == qs);
+      resume_track_transfer_with_response
+        (rctx,
+        MHD_HTTP_INTERNAL_SERVER_ERROR,
+        TALER_MHD_make_json_pack ("{s:I, s:s}",
+                                  "code",
+                                  (json_int_t)
+                                  TALER_EC_TRACK_TRANSFER_DB_FETCH_DEPOSIT_ERROR,
+                                  "details",
+                                  "failed to obtain deposit data from local database"));
+      return;
+    }
+    if (GNUNET_DB_STATUS_SUCCESS_NO_RESULTS == qs)
+    {
+      /* The exchange says we made this deposit, but WE do not
+         recall making it (corrupted / unreliable database?)!
+         Well, let's say thanks and accept the money! */
+      GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                  "Failed to find payment data in DB\n");
+      rctx->check_transfer_result = GNUNET_OK;
+    }
+    if (GNUNET_NO == rctx->check_transfer_result)
+    {
+      /* Internal error: how can we have called #check_transfer()
+         but still have no result? */
+      GNUNET_break (0);
+      resume_track_transfer_with_response
+        (rctx,
+        MHD_HTTP_INTERNAL_SERVER_ERROR,
+        TALER_MHD_make_json_pack ("{s:I, s:s, s:I, s:s}",
+                                  "code",
+                                  (json_int_t)
+                                  TALER_EC_TRACK_TRANSFER_DB_INTERNAL_LOGIC_ERROR,
+                                  "details", "internal logic error",
+                                  "line", (json_int_t) __LINE__,
+                                  "file", __FILE__));
+      return;
+    }
+    if (GNUNET_SYSERR == rctx->check_transfer_result)
+    {
+      /* #check_transfer() failed, report conflict! */
+      GNUNET_break_op (0);
+      GNUNET_assert (NULL != rctx->response);
+      resume_track_transfer_with_response
+        (rctx,
+        MHD_HTTP_FAILED_DEPENDENCY,
+        rctx->response);
+      rctx->response = NULL;
+      return;
+    }
+    /* Response is consistent with the /deposit we made,
+       remember it for future reference */
+    for (unsigned int r = 0; r<MAX_RETRIES; r++)
+    {
+      db->preflight (db->cls);
+      qs = db->store_coin_to_transfer (db->cls,
+                                       &details[i].h_contract_terms,
+                                       &details[i].coin_pub,
+                                       &rctx->wtid);
+      if (GNUNET_DB_STATUS_SOFT_ERROR != qs)
+        break;
+    }
+    if (0 > qs)
+    {
+      /* Special report if retries insufficient */
+      GNUNET_break (GNUNET_DB_STATUS_SOFT_ERROR != qs);
+      /* Always report on hard error as well to enable diagnostics */
+      GNUNET_break (GNUNET_DB_STATUS_HARD_ERROR == qs);
+      resume_track_transfer_with_response
+        (rctx,
+        MHD_HTTP_INTERNAL_SERVER_ERROR,
+        TALER_MHD_make_json_pack ("{s:I, s:s}",
+                                  "code",
+                                  (json_int_t)
+                                  TALER_EC_TRACK_TRANSFER_DB_STORE_COIN_ERROR,
+                                  "details",
+                                  "failed to store response from exchange to local database"));
+      return;
+    }
+  }
+  rctx->original_response = NULL;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "About to call tracks transformator.\n");
+
+  if (NULL == (jresponse =
+                 transform_response (hr->reply,
+                                     rctx)))
+  {
+    resume_track_transfer_with_response
+      (rctx,
+      MHD_HTTP_INTERNAL_SERVER_ERROR,
+      TALER_MHD_make_error (TALER_EC_TRACK_TRANSFER_JSON_RESPONSE_ERROR,
+                            "Fail to elaborate the response."));
+    return;
+  }
+
+  resume_track_transfer_with_response (rctx,
+                                       MHD_HTTP_OK,
+                                       TALER_MHD_make_json (jresponse));
+  json_decref (jresponse);
+}
+
+
+/**
+ * Function called with the result of our exchange lookup.
+ *
+ * @param cls the `struct TrackTransferContext`
+ * @param hr HTTP response details
+ * @param eh NULL if exchange was not found to be acceptable
+ * @param wire_fee NULL (we did not specify a wire method)
+ * @param exchange_trusted #GNUNET_YES if this exchange is trusted by config
+ */
+static void
+process_track_transfer_with_exchange (void *cls,
+                                      const struct
+                                      TALER_EXCHANGE_HttpResponse *hr,
+                                      struct TALER_EXCHANGE_Handle *eh,
+                                      const struct TALER_Amount *wire_fee,
+                                      int exchange_trusted)
+{
+  struct TrackTransferContext *rctx = cls;
+
+  rctx->fo = NULL;
+  if (MHD_HTTP_OK != hr->http_status)
+  {
+    /* The request failed somehow */
+    GNUNET_break_op (0);
+    resume_track_transfer_with_response (
+      rctx,
+      MHD_HTTP_FAILED_DEPENDENCY,
+      TALER_MHD_make_json_pack (
+        (NULL != hr->reply)
+        ? "{s:s, s:I, s:I, s:I, s:O}"
+        : "{s:s, s:I, s:I, s:I}",
+        "hint", "failed to obtain meta-data from exchange",
+        "code", (json_int_t) TALER_EC_TRACK_TRANSFER_EXCHANGE_KEYS_FAILURE,
+        "exchange_http_status", (json_int_t) hr->http_status,
+        "exchange_code", (json_int_t) hr->ec,
+        "exchange_reply", hr->reply));
+    return;
+  }
+  rctx->eh = eh;
+  rctx->wdh = TALER_EXCHANGE_transfers_get (eh,
+                                            &rctx->wtid,
+                                            &wire_transfer_cb,
+                                            rctx);
+  if (NULL == rctx->wdh)
+  {
+    GNUNET_break (0);
+    resume_track_transfer_with_response
+      (rctx,
+      MHD_HTTP_INTERNAL_SERVER_ERROR,
+      TALER_MHD_make_json_pack ("{s:I, s:s}",
+                                "code",
+                                (json_int_t)
+                                TALER_EC_TRACK_TRANSFER_REQUEST_ERROR,
+                                "error",
+                                "failed to run /transfers/ GET on exchange"));
+  }
+}
+
+
+/**
+ * Handle a timeout for the processing of the track transfer request.
+ *
+ * @param cls closure
+ */
+static void
+handle_track_transfer_timeout (void *cls)
+{
+  struct TrackTransferContext *rctx = cls;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Resuming /track/transfer with error after timeout\n");
+  rctx->timeout_task = NULL;
+
+  if (NULL != rctx->fo)
+  {
+    TMH_EXCHANGES_find_exchange_cancel (rctx->fo);
+    rctx->fo = NULL;
+  }
+  resume_track_transfer_with_response (rctx,
+                                       MHD_HTTP_SERVICE_UNAVAILABLE,
+                                       TALER_MHD_make_error (
+                                         TALER_EC_TRACK_TRANSFER_EXCHANGE_TIMEOUT,
+                                         "exchange not reachable"));
+}
+
+
+/**
+ * Function called with information about a wire transfer identifier.
+ * Generate a response based on the given @a proof.
+ *
+ * @param cls closure
+ * @param proof proof from exchange about what the wire transfer was for.
+ *              should match the `TrackTransactionResponse` format
+ *              of the exchange
+ */
+static void
+proof_cb (void *cls,
+          const json_t *proof)
+{
+  struct TrackTransferContext *rctx = cls;
+  json_t *transformed_response;
+
+  if (NULL == (transformed_response =
+                 transform_response (proof,
+                                     rctx)))
+  {
+    rctx->response_code = MHD_HTTP_INTERNAL_SERVER_ERROR;
+    rctx->response
+      = TALER_MHD_make_error (TALER_EC_TRACK_TRANSFER_JSON_RESPONSE_ERROR,
+                              "Fail to elaborate response.");
+    return;
+  }
+
+  rctx->response_code = MHD_HTTP_OK;
+  rctx->response = TALER_MHD_make_json (transformed_response);
+  json_decref (transformed_response);
+}
+
+
+/**
+ * Manages a /track/transfer call, thus it calls the /track/wtid
+ * offered by the exchange in order to return the set of transfers
+ * (of coins) associated with a given wire transfer.
+ *
+ * @param rh context of the handler
+ * @param connection the MHD connection to handle
+ * @param[in,out] connection_cls the connection's closure (can be updated)
+ * @param upload_data upload data
+ * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param mi merchant backend instance, never NULL
+ * @return MHD result code
+ */
+MHD_RESULT
+MH_handler_track_transfer (struct TMH_RequestHandler *rh,
+                           struct MHD_Connection *connection,
+                           void **connection_cls,
+                           const char *upload_data,
+                           size_t *upload_data_size,
+                           struct MerchantInstance *mi)
+{
+  struct TrackTransferContext *rctx;
+  const char *str;
+  const char *url;
+  const char *wire_method;
+  MHD_RESULT ret;
+  enum GNUNET_DB_QueryStatus qs;
+
+  if (NULL == *connection_cls)
+  {
+    rctx = GNUNET_new (struct TrackTransferContext);
+    rctx->hc.cc = &track_transfer_cleanup;
+    rctx->connection = connection;
+    *connection_cls = rctx;
+  }
+  else
+  {
+    /* not first call, recover state */
+    rctx = *connection_cls;
+  }
+
+  if (0 != rctx->response_code)
+  {
+    /* We are *done* processing the request, just queue the response (!) */
+    if (UINT_MAX == rctx->response_code)
+    {
+      GNUNET_break (0);
+      return MHD_NO; /* hard error */
+    }
+    ret = MHD_queue_response (connection,
+                              rctx->response_code,
+                              rctx->response);
+    if (NULL != rctx->response)
+    {
+      MHD_destroy_response (rctx->response);
+      rctx->response = NULL;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Queueing response (%u) for /track/transfer (%s).\n",
+                (unsigned int) rctx->response_code,
+                ret ? "OK" : "FAILED");
+    return ret;
+  }
+  if ( (NULL != rctx->fo) ||
+       (NULL != rctx->eh) )
+  {
+    /* likely old MHD version */
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Not sure why we are here, should be suspended\n");
+    return MHD_YES; /* still work in progress */
+  }
+
+  url = MHD_lookup_connection_value (connection,
+                                     MHD_GET_ARGUMENT_KIND,
+                                     "exchange");
+  if (NULL == url)
+    return TALER_MHD_reply_with_error (connection,
+                                       MHD_HTTP_BAD_REQUEST,
+                                       TALER_EC_PARAMETER_MISSING,
+                                       "exchange");
+  rctx->url = GNUNET_strdup (url);
+
+  /* FIXME: change again: we probably don't want the wire_method
+     but rather the _account_ (section) here! */
+  wire_method = MHD_lookup_connection_value (connection,
+                                             MHD_GET_ARGUMENT_KIND,
+                                             "wire_method");
+  if (NULL == wire_method)
+  {
+    if (1)
+    {
+      /* temporary work-around until demo is adjusted... */
+      GNUNET_break (0);
+      wire_method = "x-taler-bank";
+      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                  "Client needs fixing, see API change for #4943!\n");
+    }
+    else
+      return TALER_MHD_reply_with_error (connection,
+                                         MHD_HTTP_BAD_REQUEST,
+                                         TALER_EC_PARAMETER_MISSING,
+                                         "wire_method");
+  }
+  rctx->wire_method = GNUNET_strdup (wire_method);
+  rctx->mi = mi;
+  str = MHD_lookup_connection_value (connection,
+                                     MHD_GET_ARGUMENT_KIND,
+                                     "wtid");
+  if (NULL == str)
+    return TALER_MHD_reply_with_error (connection,
+                                       MHD_HTTP_BAD_REQUEST,
+                                       TALER_EC_PARAMETER_MISSING,
+                                       "wtid");
+  if (GNUNET_OK !=
+      GNUNET_STRINGS_string_to_data (str,
+                                     strlen (str),
+                                     &rctx->wtid,
+                                     sizeof (rctx->wtid)))
+  {
+    return TALER_MHD_reply_with_error (connection,
+                                       MHD_HTTP_BAD_REQUEST,
+                                       TALER_EC_PARAMETER_MALFORMED,
+                                       "wtid");
+  }
+
+  /* Check if reply is already in database! */
+  db->preflight (db->cls);
+  qs = db->find_proof_by_wtid (db->cls,
+                               rctx->url,
+                               &rctx->wtid,
+                               &proof_cb,
+                               rctx);
+  if (0 > qs)
+  {
+    /* Simple select queries should not cause serialization issues */
+    GNUNET_break (GNUNET_DB_STATUS_SOFT_ERROR != qs);
+    /* Always report on hard error as well to enable diagnostics */
+    GNUNET_break (GNUNET_DB_STATUS_HARD_ERROR == qs);
+    return TALER_MHD_reply_with_error (connection,
+                                       MHD_HTTP_INTERNAL_SERVER_ERROR,
+                                       TALER_EC_TRACK_TRANSFER_DB_FETCH_DEPOSIT_ERROR,
+                                       "Fail to query database about proofs");
+  }
+  if (0 != rctx->response_code)
+  {
+    ret = MHD_queue_response (connection,
+                              rctx->response_code,
+                              rctx->response);
+    if (NULL != rctx->response)
+    {
+      MHD_destroy_response (rctx->response);
+      rctx->response = NULL;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Queueing response (%u) for /track/transfer (%s).\n",
+                (unsigned int) rctx->response_code,
+                ret ? "OK" : "FAILED");
+    return ret;
+  }
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Suspending /track/transfer handling while working with the exchange\n");
+  MHD_suspend_connection (connection);
+  rctx->fo = TMH_EXCHANGES_find_exchange (url,
+                                          NULL,
+                                          GNUNET_NO,
+                                          &process_track_transfer_with_exchange,
+                                          rctx);
+  rctx->timeout_task
+    = GNUNET_SCHEDULER_add_delayed (TRACK_TIMEOUT,
+                                    &handle_track_transfer_timeout,
+                                    rctx);
+  return MHD_YES;
+}
+
+
+/* end of taler-merchant-httpd_track-transfer.c */