/*
  This file is part of TALER
  Copyright (C) 2023 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 Affero General Public License for more details.

  You should have received a copy of the GNU Affero General Public License along with
  TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
*/
/**
 * @file taler-merchant-exchange.c
 * @brief Process that reconciles information about incoming bank transfers with orders by asking the exchange
 * @author Christian Grothoff
 */
#include "platform.h"
#include <gnunet/gnunet_util_lib.h>
#include <jansson.h>
#include <pthread.h>
#include <taler/taler_dbevents.h>
#include "taler_merchant_bank_lib.h"
#include "taler_merchantdb_lib.h"
#include "taler_merchantdb_plugin.h"

/**
 * Timeout for the exchange interaction.  Rather long as we should do
 * long-polling and do not want to wake up too often.
 */
#define EXCHANGE_TIMEOUT GNUNET_TIME_relative_multiply ( \
    GNUNET_TIME_UNIT_MINUTES, \
    30)

/**
 * How many inquiries do we process concurrently at most.
 */
#define OPEN_INQUIRY_LIMIT 1024

/**
 * How many inquiries do we process concurrently per exchange at most.
 */
#define EXCHANGE_INQUIRY_LIMIT 16


/**
 * Information about an inquiry job.
 */
struct Inquiry;


/**
 * Information about an exchange.
 */
struct Exchange
{
  /**
   * Kept in a DLL.
   */
  struct Exchange *next;

  /**
   * Kept in a DLL.
   */
  struct Exchange *prev;

  /**
   * Head of active inquiries.
   */
  struct Inquiry *w_head;

  /**
   * Tail of active inquiries.
   */
  struct Inquiry *w_tail;

  /**
   * Which exchange are we tracking here.
   */
  char *exchange_url;

  /**
   * A connection to this exchange
   */
  struct TALER_EXCHANGE_GetKeysHandle *conn;

  /**
   * The keys of this exchange
   */
  struct TALER_EXCHANGE_Keys *keys;

  /**
   * Task where we retry fetching /keys from the exchange.
   */
  struct GNUNET_SCHEDULER_Task *retry_task;

  /**
   * How many active inquiries do we have right now with this exchange.
   */
  unsigned int exchange_inquiries;

  /**
   * How soon can may we, at the earliest, re-download /keys?
   */
  struct GNUNET_TIME_Absolute first_retry;

  /**
   * How long should we wait between the next retry?
   */
  struct GNUNET_TIME_Relative retry_delay;

  /**
   * How long should we wait between requests
   * for transfer details?
   */
  struct GNUNET_TIME_Relative transfer_delay;

  /**
   * False to indicate that there is an ongoing
   * /keys transfer we are waiting for;
   * true to indicate that /keys data is up-to-date.
   */
  bool ready;

};


/**
 * Information about an inquiry job.
 */
struct Inquiry
{
  /**
   * Kept in a DLL.
   */
  struct Inquiry *next;

  /**
   * Kept in a DLL.
   */
  struct Inquiry *prev;

  /**
   * Handle to the exchange that made the transfer.
   */
  struct Exchange *exchange;

  /**
   * Task where we retry fetching transfer details from the exchange.
   */
  struct GNUNET_SCHEDULER_Task *task;

  /**
   * For which merchant instance is this tracking request?
   */
  char *instance_id;

  /**
   * payto:// URI used for the transfer.
   */
  char *payto_uri;

  /**
   * Handle for the /wire/transfers request.
   */
  struct TALER_EXCHANGE_TransfersGetHandle *wdh;

  /**
   * When did the transfer happen?
   */
  struct GNUNET_TIME_Timestamp execution_time;

  /**
   * Argument for the /wire/transfers request.
   */
  struct TALER_WireTransferIdentifierRawP wtid;

  /**
   * Amount of the wire transfer.
   */
  struct TALER_Amount total;

  /**
   * Row of the wire transfer in our database.
   */
  uint64_t rowid;

};


/**
 * Head of known exchanges.
 */
static struct Exchange *e_head;

/**
 * Tail of known exchanges.
 */
static struct Exchange *e_tail;

/**
 * The merchant's configuration.
 */
static const struct GNUNET_CONFIGURATION_Handle *cfg;

/**
 * Our database plugin.
 */
static struct TALER_MERCHANTDB_Plugin *db_plugin;

/**
 * Handle to the context for interacting with the bank.
 */
static struct GNUNET_CURL_Context *ctx;

/**
 * Scheduler context for running the @e ctx.
 */
static struct GNUNET_CURL_RescheduleContext *rc;

/**
 * Main task for #find_work().
 */
static struct GNUNET_SCHEDULER_Task *task;

/**
 * Event handler to learn that there are new transfers
 * to check.
 */
static struct GNUNET_DB_EventHandler *eh;

/**
 * How many active inquiries do we have right now.
 */
static unsigned int active_inquiries;

/**
 * Value to return from main(). 0 on success, non-zero on errors.
 */
static int global_ret;

/**
 * #GNUNET_YES if we are in test mode and should exit when idle.
 */
static int test_mode;

/**
 * True if the last DB query was limited by the
 * #OPEN_INQUIRY_LIMIT and we thus should check again
 * as soon as we are substantially below that limit,
 * and not only when we get a DB notification.
 */
static bool at_limit;


/**
 * Initiate download from exchange.
 *
 * @param cls a `struct Inquiry *`
 */
static void
exchange_request (void *cls);


/**
 * The exchange @a e is ready to handle more inquiries,
 * prepare to launch them.
 *
 * @param[in,out] e exchange to potentially launch inquiries on
 */
static void
launch_inquiries_at_exchange (struct Exchange *e)
{
  for (struct Inquiry *w = e->w_head;
       NULL != w;
       w = w->next)
  {
    if (e->exchange_inquiries > EXCHANGE_INQUIRY_LIMIT)
      break;
    if ( (NULL == w->task) &&
         (NULL == w->wdh) )
    {
      e->exchange_inquiries++;
      w->task = GNUNET_SCHEDULER_add_now (&exchange_request,
                                          w);
    }
  }
}


/**
 * Function that initiates a /keys download.
 *
 * @param cls a `struct Exchange *`
 */
static void
download_keys (void *cls);


/**
 * Function called with information about who is auditing
 * a particular exchange and what keys the exchange is using.
 *
 * @param cls closure with a `struct Exchange *`
 * @param kr response data
 * @param[in] keys the keys of the exchange
 */
static void
cert_cb (
  void *cls,
  const struct TALER_EXCHANGE_KeysResponse *kr,
  struct TALER_EXCHANGE_Keys *keys)
{
  struct Exchange *e = cls;
  struct GNUNET_TIME_Absolute n;

  e->conn = NULL;
  switch (kr->hr.http_status)
  {
  case MHD_HTTP_OK:
    e->ready = true;
    TALER_EXCHANGE_keys_decref (e->keys);
    e->keys = keys;
    launch_inquiries_at_exchange (e);
    /* Reset back-off */
    e->retry_delay = GNUNET_TIME_UNIT_ZERO;
    /* Success: rate limit at once per minute */
    e->first_retry = GNUNET_TIME_relative_to_absolute (
      GNUNET_TIME_UNIT_MINUTES);
    n = GNUNET_TIME_absolute_max (e->first_retry,
                                  keys->key_data_expiration.abs_time);
    if (NULL != e->retry_task)
      GNUNET_SCHEDULER_cancel (e->retry_task);
    e->retry_task = GNUNET_SCHEDULER_add_at (n,
                                             &download_keys,
                                             e);
    break;
  default:
    e->retry_delay
      = GNUNET_TIME_STD_BACKOFF (e->retry_delay);
    e->first_retry
      = GNUNET_TIME_relative_to_absolute (e->retry_delay);
    if (NULL != e->retry_task)
      GNUNET_SCHEDULER_cancel (e->retry_task);
    e->retry_task = GNUNET_SCHEDULER_add_delayed (e->retry_delay,
                                                  &download_keys,
                                                  e);
    break;
  }
}


static void
download_keys (void *cls)
{
  struct Exchange *e = cls;
  struct GNUNET_TIME_Relative n;

  /* If we do not hear back again soon, try again automatically */
  n = GNUNET_TIME_STD_BACKOFF (e->retry_delay);
  n = GNUNET_TIME_relative_max (n,
                                GNUNET_TIME_UNIT_MINUTES);
  e->retry_task = GNUNET_SCHEDULER_add_delayed (n,
                                                &download_keys,
                                                e);
  if ( (NULL == e->keys) ||
       (GNUNET_TIME_absolute_is_past (e->keys->key_data_expiration.abs_time)) )
    e->conn = TALER_EXCHANGE_get_keys (ctx,
                                       e->exchange_url,
                                       e->keys,
                                       &cert_cb,
                                       e);
}


/**
 * Updates the transaction status for inquiry @a w to the given values.
 *
 * @param w inquiry to update status for
 * @param next_attempt when should we retry @a w (if ever)
 * @param ec error code to use (if any)
 * @param failed failure status (if ultimately failed)
 * @param verified success status (if ultimately successful)
 */
static void
update_transaction_status (const struct Inquiry *w,
                           struct GNUNET_TIME_Absolute next_attempt,
                           enum TALER_ErrorCode ec,
                           bool failed,
                           bool verified)
{
  enum GNUNET_DB_QueryStatus qs;

  qs = db_plugin->update_transfer_status (db_plugin->cls,
                                          w->exchange->exchange_url,
                                          &w->wtid,
                                          next_attempt,
                                          ec,
                                          failed,
                                          verified);
  if (qs < 0)
  {
    GNUNET_break (0);
    global_ret = 1;
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
}


/**
 * Lookup our internal data structure for the given
 * @a exchange_url or create one if we do not yet have
 * one.
 *
 * @param exchange_url base URL of the exchange
 * @return our state for this exchange
 */
static struct Exchange *
find_exchange (const char *exchange_url)
{
  struct Exchange *e;

  for (e = e_head; NULL != e; e = e->next)
    if (0 == strcmp (exchange_url,
                     e->exchange_url))
      return e;
  e = GNUNET_new (struct Exchange);
  e->exchange_url = GNUNET_strdup (exchange_url);
  GNUNET_CONTAINER_DLL_insert (e_head,
                               e_tail,
                               e);
  e->retry_task = GNUNET_SCHEDULER_add_now (&download_keys,
                                            e);
  return e;
}


/**
 * Finds new transfers that require work in the merchant database.
 *
 * @param cls NULL
 */
static void
find_work (void *cls);


/**
 * Free resources of @a w.
 *
 * @param[in] w inquiry job to terminate
 */
static void
end_inquiry (struct Inquiry *w)
{
  struct Exchange *e = w->exchange;

  GNUNET_assert (active_inquiries > 0);
  active_inquiries--;
  if (NULL != w->wdh)
  {
    TALER_EXCHANGE_transfers_get_cancel (w->wdh);
    w->wdh = NULL;
  }
  GNUNET_free (w->instance_id);
  GNUNET_free (w->payto_uri);
  GNUNET_CONTAINER_DLL_remove (e->w_head,
                               e->w_tail,
                               w);
  GNUNET_free (w);
  if ( (active_inquiries < OPEN_INQUIRY_LIMIT / 2) &&
       (NULL == task) &&
       (at_limit) )
  {
    at_limit = false;
    task = GNUNET_SCHEDULER_add_now (&find_work,
                                     NULL);
  }
  if ( (NULL == task) &&
       (! at_limit) &&
       (0 == active_inquiries) &&
       (test_mode) )
  {
    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
                "No more open inquiries and in test mode. Existing.\n");
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
}


/**
 * We're being aborted with CTRL-C (or SIGTERM). Shut down.
 *
 * @param cls closure (NULL)
 */
static void
shutdown_task (void *cls)
{
  (void) cls;
  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
              "Running shutdown\n");
  while (NULL != e_head)
  {
    struct Exchange *e = e_head;

    while (NULL != e->w_head)
    {
      struct Inquiry *w = e->w_head;

      end_inquiry (w);
    }
    GNUNET_free (e->exchange_url);
    if (NULL != e->conn)
    {
      TALER_EXCHANGE_get_keys_cancel (e->conn);
      e->conn = NULL;
    }
    if (NULL != e->retry_task)
    {
      GNUNET_SCHEDULER_cancel (e->retry_task);
      e->retry_task = NULL;
    }
    GNUNET_CONTAINER_DLL_remove (e_head,
                                 e_tail,
                                 e);
    GNUNET_free (e);
  }
  if (NULL != eh)
  {
    db_plugin->event_listen_cancel (eh);
    eh = NULL;
  }
  TALER_MERCHANTDB_plugin_unload (db_plugin);
  db_plugin = NULL;
  cfg = NULL;
  if (NULL != ctx)
  {
    GNUNET_CURL_fini (ctx);
    ctx = NULL;
  }
  if (NULL != rc)
  {
    GNUNET_CURL_gnunet_rc_destroy (rc);
    rc = NULL;
  }
}


/**
 * Check that the given @a wire_fee is what the @a e 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 w inquiry to check fees of
 * @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 enum GNUNET_GenericReturnValue
check_wire_fee (struct Inquiry *w,
                struct GNUNET_TIME_Timestamp execution_time,
                const struct TALER_Amount *wire_fee)
{
  struct Exchange *e = w->exchange;
  const struct TALER_EXCHANGE_Keys *keys = e->keys;
  struct TALER_WireFeeSet fees;
  struct TALER_MasterSignatureP master_sig;
  struct GNUNET_TIME_Timestamp start_date;
  struct GNUNET_TIME_Timestamp end_date;
  enum GNUNET_DB_QueryStatus qs;
  char *wire_method;

  if (NULL == keys)
  {
    GNUNET_break (0);
    return GNUNET_NO;
  }
  wire_method = TALER_payto_get_method (w->payto_uri);
  qs = db_plugin->lookup_wire_fee (db_plugin->cls,
                                   &keys->master_pub,
                                   wire_method,
                                   execution_time,
                                   &fees,
                                   &start_date,
                                   &end_date,
                                   &master_sig);
  switch (qs)
  {
  case GNUNET_DB_STATUS_HARD_ERROR:
    GNUNET_break (0);
    GNUNET_free (wire_method);
    return GNUNET_SYSERR;
  case GNUNET_DB_STATUS_SOFT_ERROR:
    GNUNET_free (wire_method);
    return GNUNET_NO;
  case GNUNET_DB_STATUS_SUCCESS_NO_RESULTS:
    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 (&keys->master_pub),
                wire_method,
                GNUNET_TIME_timestamp2s (execution_time),
                TALER_amount2s (wire_fee));
    GNUNET_free (wire_method);
    return GNUNET_OK;
  case GNUNET_DB_STATUS_SUCCESS_ONE_RESULT:
    break;
  }
  if ( (GNUNET_OK !=
        TALER_amount_cmp_currency (&fees.wire,
                                   wire_fee)) ||
       (0 > TALER_amount_cmp (&fees.wire,
                               wire_fee)) )
  {
    GNUNET_break_op (0);
    GNUNET_free (wire_method);
    return GNUNET_SYSERR;   /* expected_fee >= wire_fee */
  }
  GNUNET_free (wire_method);
  return GNUNET_OK;
}


/**
 * Closure for #check_transfer()
 */
struct CheckTransferContext
{

  /**
   * Pointer to the detail that we are currently
   * checking in #check_transfer().
   */
  const struct TALER_TrackTransferDetails *current_detail;

  /**
   * Which transaction detail are we currently looking at?
   */
  unsigned int current_offset;

  /**
   * #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.
   */
  enum GNUNET_GenericReturnValue check_transfer_result;

  /**
   * Set to error code, if any.
   */
  enum TALER_ErrorCode ec;

  /**
   * Set to true if @e ec indicates a permanent failure.
   */
  bool failure;
};


/**
 * 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 CheckTransferContext  *`
 * @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 wire_fee paid wire fee
 * @param h_wire hash of merchant's wire details
 * @param deposit_timestamp when did the exchange receive the deposit
 * @param refund_deadline until when are refunds allowed
 * @param exchange_sig signature by the exchange
 * @param exchange_pub exchange signing key used for @a exchange_sig
 */
static void
check_transfer (void *cls,
                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 struct TALER_MerchantWireHashP *h_wire,
                struct GNUNET_TIME_Timestamp deposit_timestamp,
                struct GNUNET_TIME_Timestamp refund_deadline,
                const struct TALER_ExchangeSignatureP *exchange_sig,
                const struct TALER_ExchangePublicKeyP *exchange_pub)
{
  struct CheckTransferContext *ctc = cls;
  const struct TALER_TrackTransferDetails *ttd = ctc->current_detail;

  if (GNUNET_SYSERR == ctc->check_transfer_result)
  {
    GNUNET_break (0);
    return;   /* already had a serious issue; odd that we're called more than once as well... */
  }
  if ( (GNUNET_OK !=
        TALER_amount_cmp_currency (amount_with_fee,
                                   &ttd->coin_value)) ||
       (0 != TALER_amount_cmp (amount_with_fee,
                               &ttd->coin_value)) ||
       (GNUNET_OK !=
        TALER_amount_cmp_currency (deposit_fee,
                                   &ttd->coin_fee)) ||
       (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);
    ctc->check_transfer_result = GNUNET_SYSERR;
    /* Build the `TrackTransferConflictDetails` */
    ctc->ec = TALER_EC_MERCHANT_PRIVATE_POST_TRANSFERS_CONFLICTING_REPORTS;
    ctc->failure = true;
    /* TODO: this should be reported to the auditor! */
    return;
  }
  ctc->check_transfer_result = GNUNET_OK;
}


/**
 * Function called with detailed wire transfer data, including all
 * of the coin transactions that were combined into the wire transfer.
 *
 * @param cls closure a `struct Inquiry *`
 * @param tgr response details
 */
static void
wire_transfer_cb (void *cls,
                  const struct TALER_EXCHANGE_TransfersGetResponse *tgr)
{
  struct Inquiry *w = cls;
  struct Exchange *e = w->exchange;
  enum GNUNET_DB_QueryStatus qs;
  const struct TALER_EXCHANGE_TransferData *td = NULL;

  e->exchange_inquiries--;
  w->wdh = NULL;
  if (EXCHANGE_INQUIRY_LIMIT - 1 == e->exchange_inquiries)
    launch_inquiries_at_exchange (e);
  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
              "Got response code %u from exchange for GET /transfers/$WTID\n",
              tgr->hr.http_status);
  switch (tgr->hr.http_status)
  {
  case MHD_HTTP_OK:
    td = &tgr->details.ok.td;
    w->execution_time = td->execution_time;
    e->transfer_delay = GNUNET_TIME_UNIT_ZERO;
    break;
  case MHD_HTTP_BAD_REQUEST:
  case MHD_HTTP_FORBIDDEN:
    update_transaction_status (w,
                               GNUNET_TIME_UNIT_FOREVER_ABS,
                               TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_HARD_FAILURE,
                               true,
                               false);
    end_inquiry (w);
    return;
  case MHD_HTTP_NOT_FOUND:
    update_transaction_status (w,
                               GNUNET_TIME_UNIT_FOREVER_ABS,
                               TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_FATAL_NOT_FOUND,
                               true,
                               false);
    end_inquiry (w);
    return;
  case MHD_HTTP_INTERNAL_SERVER_ERROR:
  case MHD_HTTP_BAD_GATEWAY:
  case MHD_HTTP_GATEWAY_TIMEOUT:
    e->transfer_delay = GNUNET_TIME_STD_BACKOFF (e->transfer_delay);
    update_transaction_status (w,
                               GNUNET_TIME_relative_to_absolute (
                                 e->transfer_delay),
                               TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_TRANSIENT_FAILURE,
                               false,
                               false);
    end_inquiry (w);
    return;
  default:
    e->transfer_delay = GNUNET_TIME_STD_BACKOFF (e->transfer_delay);
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Unexpected HTTP status %u\n",
                tgr->hr.http_status);
    update_transaction_status (w,
                               GNUNET_TIME_relative_to_absolute (
                                 e->transfer_delay),
                               TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_TRANSIENT_FAILURE,
                               false,
                               false);
    end_inquiry (w);
    return;
  }
  db_plugin->preflight (db_plugin->cls);
  qs = db_plugin->insert_transfer_details (db_plugin->cls,
                                           w->instance_id,
                                           w->exchange->exchange_url,
                                           w->payto_uri,
                                           &w->wtid,
                                           td);
  if (0 > qs)
  {
    /* Always report on DB error as well to enable diagnostics */
    GNUNET_break (GNUNET_DB_STATUS_HARD_ERROR == qs);
    global_ret = 1;
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  if (0 == qs)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
                "Transfer already known. Ignoring duplicate.\n");
    return;
  }

  {
    struct CheckTransferContext ctc = {
      .ec = TALER_EC_NONE,
      .failure = false
    };

    for (unsigned int i = 0; i<td->details_length; i++)
    {
      const struct TALER_TrackTransferDetails *ttd = &td->details[i];
      enum GNUNET_DB_QueryStatus qs;

      if (TALER_EC_NONE != ctc.ec)
        break; /* already encountered an error */
      ctc.current_offset = i;
      ctc.current_detail = ttd;
      /* Set the coin as "never seen" before. */
      ctc.check_transfer_result = GNUNET_NO;
      qs = db_plugin->lookup_deposits_by_contract_and_coin (
        db_plugin->cls,
        w->instance_id,
        &ttd->h_contract_terms,
        &ttd->coin_pub,
        &check_transfer,
        &ctc);
      switch (qs)
      {
      case GNUNET_DB_STATUS_SOFT_ERROR:
        GNUNET_break (0);
        ctc.ec = TALER_EC_GENERIC_DB_FETCH_FAILED;
        break;
      case GNUNET_DB_STATUS_HARD_ERROR:
        GNUNET_break (0);
        ctc.ec = TALER_EC_GENERIC_DB_FETCH_FAILED;
        break;
      case GNUNET_DB_STATUS_SUCCESS_NO_RESULTS:
        /* 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");
        ctc.check_transfer_result = GNUNET_OK;
        break;
      case GNUNET_DB_STATUS_SUCCESS_ONE_RESULT:
        break;
      }
      switch (ctc.check_transfer_result)
      {
      case GNUNET_NO:
        /* Internal error: how can we have called #check_transfer()
           but still have no result? */
        GNUNET_break (0);
        ctc.ec = TALER_EC_GENERIC_INTERNAL_INVARIANT_FAILURE;
        return;
      case GNUNET_SYSERR:
        /* #check_transfer() failed, report conflict! */
        GNUNET_break_op (0);
        GNUNET_assert (TALER_EC_NONE != ctc.ec);
        break;
      case GNUNET_OK:
        break;
      }
    }
    if (TALER_EC_NONE != ctc.ec)
    {
      update_transaction_status (
        w,
        ctc.failure
        ? GNUNET_TIME_UNIT_FOREVER_ABS
        : GNUNET_TIME_relative_to_absolute (
          GNUNET_TIME_UNIT_MINUTES),
        ctc.ec,
        ctc.failure,
        false);
      end_inquiry (w);
      return;
    }
  }

  if (GNUNET_SYSERR ==
      check_wire_fee (w,
                      td->execution_time,
                      &td->wire_fee))
  {
    GNUNET_break_op (0);
    update_transaction_status (w,
                               GNUNET_TIME_UNIT_FOREVER_ABS,
                               TALER_EC_MERCHANT_PRIVATE_POST_TRANSFERS_BAD_WIRE_FEE,
                               true,
                               false);
    end_inquiry (w);
    return;
  }

  if ( (GNUNET_OK !=
        TALER_amount_cmp_currency (&td->total_amount,
                                   &w->total)) ||
       (0 !=
        TALER_amount_cmp (&td->total_amount,
                          &w->total)) )
  {
    GNUNET_break_op (0);
    update_transaction_status (w,
                               GNUNET_TIME_UNIT_FOREVER_ABS,
                               TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_CONFLICTING_TRANSFERS,
                               true,
                               false);
    end_inquiry (w);
    return;
  }
  /* set transaction to successful */
  update_transaction_status (w,
                             GNUNET_TIME_UNIT_FOREVER_ABS,
                             TALER_EC_NONE,
                             false,
                             true);
  end_inquiry (w);
}


/**
 * Initiate download from an exchange for a given inquiry.
 *
 * @param cls a `struct Inquiry *`
 */
static void
exchange_request (void *cls)
{
  struct Inquiry *w = cls;
  struct Exchange *e = w->exchange;

  w->task = NULL;
  GNUNET_assert (e->ready);
  w->wdh = TALER_EXCHANGE_transfers_get (
    ctx,
    e->exchange_url,
    e->keys,
    &w->wtid,
    &wire_transfer_cb,
    w);
  if (NULL == w->wdh)
  {
    GNUNET_break (0);
    e->exchange_inquiries--;
    e->transfer_delay = GNUNET_TIME_STD_BACKOFF (e->transfer_delay);
    update_transaction_status (w,
                               GNUNET_TIME_relative_to_absolute (
                                 e->transfer_delay),
                               TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_TRANSIENT_FAILURE,
                               false,
                               false);
    end_inquiry (w);
    return;
  }
  /* Wait at least 1m for the network transfer */
  update_transaction_status (w,
                             GNUNET_TIME_relative_to_absolute (
                               GNUNET_TIME_UNIT_MINUTES),
                             TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_AWAITING_LIST,
                             false,
                             false);
}


/**
 * Function called with information about a transfer we
 * should ask the exchange about.
 *
 * @param cls closure (NULL)
 * @param rowid row of the transfer in the merchant database
 * @param instance_id instance that received the transfer
 * @param exchange_url base URL of the exchange that initiated the transfer
 * @param payto_uri account of the merchant that received the transfer
 * @param wtid wire transfer subject identifying the aggregation
 * @param total total amount that was wired
 * @param next_attempt when should we next try to interact with the exchange
 */
static void
start_inquiry (
  void *cls,
  uint64_t rowid,
  const char *instance_id,
  const char *exchange_url,
  const char *payto_uri,
  const struct TALER_WireTransferIdentifierRawP *wtid,
  const struct TALER_Amount *total,
  struct GNUNET_TIME_Absolute next_attempt)
{
  struct Exchange *e;
  struct Inquiry *w;

  (void) cls;
  if (GNUNET_TIME_absolute_is_future (next_attempt))
  {
    if (NULL == task)
      task = GNUNET_SCHEDULER_add_at (next_attempt,
                                      &find_work,
                                      NULL);
    return;
  }
  e = find_exchange (exchange_url);
  for (w = e->w_head; NULL != w; w = w->next)
  {
    if (0 == GNUNET_memcmp (&w->wtid,
                            wtid))
    {
      GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                  "Already processing inquiry. Aborting ongoing inquiry\n");
      end_inquiry (w);
      break;
    }
  }

  active_inquiries++;
  w = GNUNET_new (struct Inquiry);
  w->payto_uri = GNUNET_strdup (payto_uri);
  w->instance_id = GNUNET_strdup (instance_id);
  w->rowid = rowid;
  w->wtid = *wtid;
  w->total = *total;
  GNUNET_CONTAINER_DLL_insert (e->w_head,
                               e->w_tail,
                               w);
  w->exchange = e;
  if (w->exchange->ready)
    w->task = GNUNET_SCHEDULER_add_now (&exchange_request,
                                        w);
  /* Wait at least 1 minute for /keys */
  update_transaction_status (w,
                             GNUNET_TIME_relative_to_absolute (
                               GNUNET_TIME_UNIT_MINUTES),
                             TALER_EC_MERCHANT_EXCHANGE_TRANSFERS_AWAITING_KEYS,
                             false,
                             false);
}


static void
find_work (void *cls)
{
  enum GNUNET_DB_QueryStatus qs;
  int limit;

  (void) cls;
  task = NULL;
  GNUNET_assert (OPEN_INQUIRY_LIMIT >= active_inquiries);
  limit = OPEN_INQUIRY_LIMIT - active_inquiries;
  if (0 == limit)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
                "Not looking for work: at limit\n");
    at_limit = true;
    return;
  }
  at_limit = false;
  qs = db_plugin->select_open_transfers (db_plugin->cls,
                                         limit,
                                         &start_inquiry,
                                         NULL);
  if (qs < 0)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to obtain open transfers from database\n");
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  if (qs == limit)
  {
    /* DB limited response, re-trigger DB interaction
       the moment we significantly fall below the
       limit */
    at_limit = true;
  }
  if (0 == active_inquiries)
  {
    if (test_mode)
    {
      GNUNET_log (GNUNET_ERROR_TYPE_INFO,
                  "No more open inquiries and in test mode. Existing.\n");
      GNUNET_SCHEDULER_shutdown ();
      return;
    }
    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
                "No open inquiries found, waiting for notification to resume\n");
  }
}


/**
 * Function called when transfers are added to the merchant database.  We look
 * for more work.
 *
 * @param cls closure (NULL)
 * @param extra additional event data provided
 * @param extra_size number of bytes in @a extra
 */
static void
transfer_added (void *cls,
                const void *extra,
                size_t extra_size)
{
  (void) cls;
  (void) extra;
  (void) extra_size;
  if (active_inquiries > OPEN_INQUIRY_LIMIT / 2)
  {
    /* Trigger DB only once we are substantially below the limit */
    at_limit = true;
    return;
  }
  if (NULL != task)
    return;
  task = GNUNET_SCHEDULER_add_now (&find_work,
                                   NULL);
}


/**
 * First task.
 *
 * @param cls closure, NULL
 * @param args remaining command-line arguments
 * @param cfgfile name of the configuration file used (for saving, can be NULL!)
 * @param c configuration
 */
static void
run (void *cls,
     char *const *args,
     const char *cfgfile,
     const struct GNUNET_CONFIGURATION_Handle *c)
{
  (void) args;
  (void) cfgfile;

  cfg = c;
  GNUNET_SCHEDULER_add_shutdown (&shutdown_task,
                                 NULL);
  ctx = GNUNET_CURL_init (&GNUNET_CURL_gnunet_scheduler_reschedule,
                          &rc);
  rc = GNUNET_CURL_gnunet_rc_create (ctx);
  if (NULL == ctx)
  {
    GNUNET_break (0);
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  if (NULL ==
      (db_plugin = TALER_MERCHANTDB_plugin_load (cfg)))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to initialize DB subsystem\n");
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  if (GNUNET_OK !=
      db_plugin->connect (db_plugin->cls))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to connect to database\n");
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  {
    struct GNUNET_DB_EventHeaderP es = {
      .size = htons (sizeof (es)),
      .type = htons (TALER_DBEVENT_MERCHANT_WIRE_TRANSFER_CONFIRMED)
    };

    eh = db_plugin->event_listen (db_plugin->cls,
                                  &es,
                                  GNUNET_TIME_UNIT_FOREVER_REL,
                                  &transfer_added,
                                  NULL);
  }
  task = GNUNET_SCHEDULER_add_now (&find_work,
                                   NULL);
}


/**
 * The main function of taler-merchant-exchange
 *
 * @param argc number of arguments from the command line
 * @param argv command line arguments
 * @return 0 ok, 1 on error
 */
int
main (int argc,
      char *const *argv)
{
  struct GNUNET_GETOPT_CommandLineOption options[] = {
    GNUNET_GETOPT_option_flag ('t',
                               "test",
                               "run in test mode and exit when idle",
                               &test_mode),
    GNUNET_GETOPT_option_version (VERSION "-" VCS_VERSION),
    GNUNET_GETOPT_OPTION_END
  };
  enum GNUNET_GenericReturnValue ret;

  if (GNUNET_OK !=
      GNUNET_STRINGS_get_utf8_args (argc, argv,
                                    &argc, &argv))
    return EXIT_INVALIDARGUMENT;
  TALER_OS_init ();
  ret = GNUNET_PROGRAM_run (
    argc, argv,
    "taler-merchant-exchange",
    gettext_noop (
      "background process that reconciles bank transfers with orders by asking the exchange"),
    options,
    &run, NULL);
  GNUNET_free_nz ((void *) argv);
  if (GNUNET_SYSERR == ret)
    return EXIT_INVALIDARGUMENT;
  if (GNUNET_NO == ret)
    return EXIT_SUCCESS;
  return global_ret;
}


/* end of taler-merchant-exchange.c */