-run read-only transactions in proper read-only scope (at least some of them)

src/exchange/taler-exchange-httpd_common_deposit.c

@@ -240,7 +240,7 @@ TEH_common_deposit_check_purse_deposit (
     GNUNET_break_op (0);
     return TALER_MHD_reply_with_error (connection,
                                        MHD_HTTP_BAD_REQUEST,
-                                       /* FIXME: other error code? */
+                                       /* FIXME-Oec: other error code? */
                                        TALER_EC_EXCHANGE_PURSE_DEPOSIT_COIN_AGE_ATTESTATION_FAILURE,
                                        "invalid attest for minimum age");
   }

src/exchange/taler-exchange-httpd_responses.c

@@ -568,10 +568,9 @@ TEH_RESPONSE_reply_coin_insufficient_funds (
   json_t *history;
 
   TEH_plugin->rollback (TEH_plugin->cls);
-  // FIXME: maybe start read-only transaction here?
   if (GNUNET_OK !=
-      TEH_plugin->start_read_committed (TEH_plugin->cls,
+      TEH_plugin->start_read_only (TEH_plugin->cls,
-                                        "get_coin_transactions"))
+                                   "get_coin_transactions"))
   {
     return TALER_MHD_reply_with_error (
       connection,
@@ -900,10 +899,9 @@ TEH_RESPONSE_reply_reserve_insufficient_balance (
   enum GNUNET_DB_QueryStatus qs;
   MHD_RESULT mhd_ret;
 
-  // FIXME: maybe start read-committed here?
   if (GNUNET_OK !=
-      TEH_plugin->start (TEH_plugin->cls,
+      TEH_plugin->start_read_only (TEH_plugin->cls,
-                         "get_reserve_history on insufficient balance"))
+                                   "get_reserve_history on insufficient balance"))
   {
     GNUNET_break (0);
     return TALER_MHD_reply_with_error (connection,

src/exchangedb/plugin_exchangedb_postgres.c

@@ -4450,6 +4450,45 @@ postgres_start_read_committed (void *cls,
 }
 
 
+/**
+ * Start a READ ONLY serializable transaction.
+ *
+ * @param cls the `struct PostgresClosure` with the plugin-specific state
+ * @param name unique name identifying the transaction (for debugging)
+ *             must point to a constant
+ * @return #GNUNET_OK on success
+ */
+static enum GNUNET_GenericReturnValue
+postgres_start_read_only (void *cls,
+                          const char *name)
+{
+  struct PostgresClosure *pg = cls;
+  struct GNUNET_PQ_ExecuteStatement es[] = {
+    GNUNET_PQ_make_execute (
+      "START TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY"),
+    GNUNET_PQ_EXECUTE_STATEMENT_END
+  };
+
+  GNUNET_assert (NULL != name);
+  if (GNUNET_SYSERR ==
+      postgres_preflight (pg))
+    return GNUNET_SYSERR;
+  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+              "Starting READ ONLY transaction `%s`\n",
+              name);
+  if (GNUNET_OK !=
+      GNUNET_PQ_exec_statements (pg->conn,
+                                 es))
+  {
+    TALER_LOG_ERROR ("Failed to start transaction\n");
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  pg->transaction_name = name;
+  return GNUNET_OK;
+}
+
+
 /**
  * Roll back the current transaction of a database connection.
  *
@@ -16021,6 +16060,7 @@ libtaler_plugin_exchangedb_postgres_init (void *cls)
   plugin->setup_foreign_servers = &postgres_setup_foreign_servers;
   plugin->start = &postgres_start;
   plugin->start_read_committed = &postgres_start_read_committed;
+  plugin->start_read_only = &postgres_start_read_only;
   plugin->commit = &postgres_commit;
   plugin->preflight = &postgres_preflight;
   plugin->rollback = &postgres_rollback;

src/include/taler_exchangedb_plugin.h

@@ -2844,6 +2844,18 @@ struct TALER_EXCHANGEDB_Plugin
   (*start_read_committed)(void *cls,
                           const char *name);
 
+  /**
+   * Start a READ ONLY serializable transaction.
+   *
+   * @param cls the `struct PostgresClosure` with the plugin-specific state
+   * @param name unique name identifying the transaction (for debugging)
+   *             must point to a constant
+   * @return #GNUNET_OK on success
+   */
+  enum GNUNET_GenericReturnValue
+  (*start_read_only)(void *cls,
+                     const char *name);
+
 
   /**
    * Commit a transaction.