pgsql_connection.cc

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// Copyright (C) 2016-2025 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
#include <config.h>
 
#include <asiolink/io_service.h>
#include <asiolink/process_spawn.h>
#include <cc/default_credentials.h>
#include <database/database_connection.h>
#include <database/db_exceptions.h>
#include <database/db_log.h>
#include <pgsql/pgsql_connection.h>
#include <util/filesystem.h>
 
#include <exception>
#include <sstream>
#include <unordered_map>
 
using namespace isc::asiolink;
using namespace isc::data;
using namespace std;
 
namespace isc {
namespace db {
 
std::string PgSqlConnection::KEA_ADMIN_ = KEA_ADMIN;
 
// Default connection timeout

const int PGSQL_DEFAULT_CONNECTION_TIMEOUT = 5; // seconds
 
// Length of error codes
constexpr size_t PGSQL_STATECODE_LEN = 5;
 
// Error codes from https://www.postgresql.org/docs/current/errcodes-appendix.html or utils/errcodes.h
const char PgSqlConnection::DUPLICATE_KEY[] = "23505";
const char PgSqlConnection::NULL_KEY[] = "23502";
 
bool PgSqlConnection::warned_about_tls = false;
 
PgSqlResult::PgSqlResult(PGresult *result)
    : result_(result), rows_(0), cols_(0) {
    if (!result) {
        // Certain failures, like a loss of connectivity, can return a
        // null PGresult and we still need to be able to create a PgSqlResult.
        // We'll set row and col counts to -1 to prevent anyone going off the
        // rails.
        rows_ = -1;
        cols_ = -1;
    } else {
        rows_ = PQntuples(result);
        cols_ = PQnfields(result);
    }
}
 
void
PgSqlResult::rowCheck(int row) const {
    if (row < 0 || row >= rows_) {
        isc_throw (db::DbOperationError, "row: " << row
                   << ", out of range: 0.." << rows_);
    }
}
 
PgSqlResult::~PgSqlResult() {
    if (result_)  {
        PQclear(result_);
    }
}
 
void
PgSqlResult::colCheck(int col) const {
    if (col < 0 || col >= cols_) {
        isc_throw (DbOperationError, "col: " << col
                   << ", out of range: 0.." << cols_);
    }
}
 
void
PgSqlResult::rowColCheck(int row, int col) const {
    rowCheck(row);
    colCheck(col);
}
 
std::string
PgSqlResult::getColumnLabel(const int col) const {
    const char* label = NULL;
    try {
        colCheck(col);
        label = PQfname(result_, col);
    } catch (...) {
        std::ostringstream os;
        os << "Unknown column:" << col;
        return (os.str());
    }
 
    return (label);
}
 
PgSqlTransaction::PgSqlTransaction(PgSqlConnection& conn)
    : conn_(conn), committed_(false) {
    conn_.startTransaction();
}
 
PgSqlTransaction::~PgSqlTransaction() {
    // If commit() wasn't explicitly called, rollback.
    if (!committed_) {
        conn_.rollback();
    }
}
 
void
PgSqlTransaction::commit() {
    conn_.commit();
    committed_ = true;
}
 
PgSqlConnection::~PgSqlConnection() {
    if (conn_ && !isUnusable()) {
        // Deallocate the prepared queries.
        if (PQstatus(conn_) == CONNECTION_OK) {
            PgSqlResult r(PQexec(conn_, "DEALLOCATE all"));
            if (PQresultStatus(r) != PGRES_COMMAND_OK) {
                // Highly unlikely but we'll log it and go on.
                DB_LOG_ERROR(PGSQL_DEALLOC_ERROR)
                    .arg(PQerrorMessage(conn_));
            }
        }
    }
}
 
std::pair<uint32_t, uint32_t>
PgSqlConnection::getVersion(const ParameterMap& parameters,
                            const IOServiceAccessorPtr& ac,
                            const DbCallback& cb,
                            const string& timer_name,
                            unsigned int id) {
    // Get a connection.
    PgSqlConnection conn(parameters, ac, cb);
 
    if (!timer_name.empty()) {
        conn.makeReconnectCtl(timer_name, id);
    }
 
    // Open the database.
    conn.openDatabaseInternal(false);
 
    const char* version_sql = "SELECT version, minor FROM schema_version;";
    PgSqlResult r(PQexec(conn.conn_, version_sql));
    if (PQresultStatus(r) != PGRES_TUPLES_OK) {
        isc_throw(DbOperationError, "unable to execute PostgreSQL statement <"
                  << version_sql << ", reason: " << PQerrorMessage(conn.conn_));
    }
 
    uint32_t version;
    PgSqlExchange::getColumnValue(r, 0, 0, version);
 
    uint32_t minor;
    PgSqlExchange::getColumnValue(r, 0, 1, minor);
 
    return (make_pair(version, minor));
}
 
void
PgSqlConnection::ensureSchemaVersion(const ParameterMap& parameters,
                                     const DbCallback& cb,
                                     const string& timer_name) {
    // retry-on-startup?
    bool const retry(parameters.count("retry-on-startup") &&
                     parameters.at("retry-on-startup") == "true");
 
    IOServiceAccessorPtr ac(new IOServiceAccessor(&DatabaseConnection::getIOService));
    pair<uint32_t, uint32_t> schema_version;
    try {
        schema_version = getVersion(parameters, ac, cb, retry ? timer_name : string());
    } catch (DbOpenError const& exception) {
        throw;
    } catch (DbOpenErrorWithRetry const& exception) {
        throw;
    } catch (exception const& exception) {
        // Disable the recovery mechanism in test mode.
        if (DatabaseConnection::test_mode_) {
            throw;
        }
        // This failure may occur for a variety of reasons. We are looking at
        // initializing schema as the only potential mitigation. We could narrow
        // down on the error that would suggest an uninitialized schema
        // which would sound something along the lines of
        // "table schema_version does not exist", but we do not necessarily have
        // to. If the error had another cause, it will fail again during
        // initialization or during the subsequent version retrieval and that is
        // fine, and the error should still be relevant.
        initializeSchema(parameters);
 
        // Retrieve again because the initial retrieval failed.
        schema_version = getVersion(parameters, ac, cb, retry ? timer_name : string());
    }
 
    // Check that the versions match.
    pair<uint32_t, uint32_t> const expected_version(PGSQL_SCHEMA_VERSION_MAJOR,
                                                    PGSQL_SCHEMA_VERSION_MINOR);
    if (schema_version != expected_version) {
        isc_throw(DbOpenError, "PostgreSQL schema version mismatch: expected version: "
                                   << expected_version.first << "." << expected_version.second
                                   << ", found version: " << schema_version.first << "."
                                   << schema_version.second);
    }
}
 
void
PgSqlConnection::initializeSchema(const ParameterMap& parameters) {
    if (parameters.count("readonly") && parameters.at("readonly") == "true") {
        // The readonly flag is historically used for host backends. Still, if
        // enabled, it is a strong indication that we should not meDDLe with it.
        return;
    }
 
    if (!isc::util::file::isFile(KEA_ADMIN_)) {
        // It can happen for kea-admin to not exist, especially with
        // packages that install it in a separate package.
        return;
    }
 
    // Convert parameters.
    auto const tupl(toKeaAdminParameters(parameters));
    vector<string> kea_admin_parameters(get<0>(tupl));
    ProcessEnvVars const vars(get<1>(tupl));
    kea_admin_parameters.insert(kea_admin_parameters.begin(), "db-init");
 
    // Run.
    ProcessSpawn kea_admin(ProcessSpawn::SYNC, KEA_ADMIN_, kea_admin_parameters, vars,
                           /* inherit_env = */ true);
    DB_LOG_INFO(PGSQL_INITIALIZE_SCHEMA).arg(kea_admin.getCommandLine());
    pid_t const pid(kea_admin.spawn());
    if (kea_admin.isRunning(pid)) {
        isc_throw(SchemaInitializationFailed, "kea-admin still running");
    }
    int const exit_code(kea_admin.getExitStatus(pid));
    if (exit_code != 0) {
        isc_throw(SchemaInitializationFailed, "Expected exit code 0 for kea-admin. Got " << exit_code);
    }
}
 
tuple<vector<string>, vector<string>>
PgSqlConnection::toKeaAdminParameters(ParameterMap const& params) {
    vector<string> result{"pgsql"};
    ProcessEnvVars vars;
    for (auto const& p : params) {
        string const& keyword(p.first);
        string const& value(p.second);
 
        // These Kea parameters are the same as the kea-admin parameters.
        if (keyword == "user" ||
            keyword == "password" ||
            keyword == "host" ||
            keyword == "port" ||
            keyword == "name") {
            result.push_back("--" + keyword);
            result.push_back(value);
            continue;
        }
 
        // These Kea parameters do not have a direct kea-admin equivalent.
        // But they do have a psql client flag equivalent.
        // We pass them to kea-admin using the --extra flag.
        static unordered_map<string, string> conversions{
            {"cert-file", "sslcert"},
            {"key-file", "sslkey"},
            {"trust-anchor", "sslrootcert"},
            {"ssl-mode", "sslmode"},
        };
        if (conversions.count(keyword)) {
            result.push_back("--extra");
            result.push_back(conversions.at(keyword) + "=" + value);
        }
 
        // These Kea parameters do not have a direct kea-admin equivalent.
        // But they do have a psql client environment variable equivalent.
        // We pass them to kea-admin.
        static unordered_map<string, string> env_conversions{
            {"connect-timeout", "PGCONNECT_TIMEOUT"},
            // {"tcp-user-timeout", "N/A"},
        };
        if (env_conversions.count(keyword)) {
            vars.push_back(env_conversions.at(keyword) + "=" + value);
        }
    }
    return make_tuple(result, vars);
}
 
void
PgSqlConnection::prepareStatement(const PgSqlTaggedStatement& statement) {
    // Prepare all statements queries with all known fields datatype
    PgSqlResult r(PQprepare(conn_, statement.name, statement.text,
                            statement.nbparams, statement.types));
    if (PQresultStatus(r) != PGRES_COMMAND_OK) {
        isc_throw(DbOperationError, "unable to prepare PostgreSQL statement: "
                  << " name: " << statement.name
                  << ", reason: " << PQerrorMessage(conn_)
                  << ", text: " << statement.text);
    }
}
 
void
PgSqlConnection::prepareStatements(const PgSqlTaggedStatement* start_statement,
                                   const PgSqlTaggedStatement* end_statement) {
    // Created the PostgreSQL prepared statements.
    for (const PgSqlTaggedStatement* tagged_statement = start_statement;
         tagged_statement != end_statement; ++tagged_statement) {
        prepareStatement(*tagged_statement);
    }
}
 
std::string
PgSqlConnection::getConnParameters() {
    return (getConnParametersInternal(false));
}
 
std::string
PgSqlConnection::getConnParametersInternal(bool logging) {
    string dbconnparameters;
    string shost = "localhost";
    try {
        shost = getParameter("host");
    } catch(...) {
        // No host. Fine, we'll use "localhost"
    }
 
    dbconnparameters += "host = '" + shost + "'" ;
 
    unsigned int port = 0;
    try {
        setIntParameterValue("port", 0, numeric_limits<uint16_t>::max(), port);
 
    } catch (const std::exception& ex) {
        isc_throw(DbInvalidPort, ex.what());
    }
 
    // Add port to connection parameters when not default.
    if (port > 0) {
        std::ostringstream oss;
        oss << port;
        dbconnparameters += " port = " + oss.str();
    }
 
    string suser;
    try {
        suser = getParameter("user");
        dbconnparameters += " user = '" + suser + "'";
    } catch(...) {
        // No user. Fine, we'll use NULL
    }
 
    string spassword;
    try {
        spassword = getParameter("password");
        dbconnparameters += " password = '" + spassword + "'";
    } catch(...) {
        // No password. Fine, we'll use NULL
    }
    if (!spassword.empty()) {
        // Refuse default password.
        DefaultCredentials::check(spassword);
    }
 
    string sname;
    try {
        sname = getParameter("name");
        dbconnparameters += " dbname = '" + sname + "'";
    } catch(...) {
        // No database name.  Throw a "NoDatabaseName" exception
        isc_throw(NoDatabaseName, "must specify a name for the database");
    }
 
    unsigned int connect_timeout = PGSQL_DEFAULT_CONNECTION_TIMEOUT;
    unsigned int tcp_user_timeout = 0;
    try {
        // The timeout is only valid if greater than zero, as depending on the
        // database, a zero timeout might signify something like "wait
        // indefinitely".
        setIntParameterValue("connect-timeout", 1, numeric_limits<int>::max(), connect_timeout);
        // This timeout value can be 0, meaning that the database client will
        // follow a default behavior. Earlier Postgres versions didn't have
        // this parameter, so we allow 0 to skip setting them for these
        // earlier versions.
        setIntParameterValue("tcp-user-timeout", 0, numeric_limits<int>::max(), tcp_user_timeout);
 
    } catch (const std::exception& ex) {
        isc_throw(DbInvalidTimeout, ex.what());
    }
 
    // Append connection timeout.
    std::ostringstream oss;
    oss << " connect_timeout = " << connect_timeout;
 
    if (tcp_user_timeout > 0) {
// tcp_user_timeout parameter is a PostgreSQL 12+ feature.
#ifdef HAVE_PGSQL_TCP_USER_TIMEOUT
        oss << " tcp_user_timeout = " << tcp_user_timeout * 1000;
        static_cast<void>(logging);
#else
        if (logging) {
            DB_LOG_WARN(PGSQL_TCP_USER_TIMEOUT_UNSUPPORTED).arg();
        }
#endif
    }
    dbconnparameters += oss.str();
 
    bool tls = false;
 
    string ssslmode;
    try {
        ssslmode = getParameter("ssl-mode");
        tls = true;
    } catch (...) {
        // No strict ssl mode
    }
 
    string sca;
    try {
        sca = getParameter("trust-anchor");
        tls = true;
        if (ssslmode.empty()) {
            ssslmode = "verify-ca";
        }
        dbconnparameters += " sslrootcert = " + sca;
    } catch (...) {
        // No trust anchor
    }
 
    string scert;
    try {
        scert = getParameter("cert-file");
        tls = true;
        dbconnparameters += " sslcert = " + scert;
    } catch (...) {
        // No client certificate file
    }
 
    string skey;
    try {
        skey = getParameter("key-file");
        tls = true;
        dbconnparameters += " sslkey = " + skey;
    } catch (...) {
        // No private key file
    }
 
    if (tls) {
        if (ssslmode.empty()) {
            ssslmode = "require";
        }
        dbconnparameters += " gssencmode = disable";
    }
 
    if (!ssslmode.empty()) {
        dbconnparameters += " sslmode = " + ssslmode;
    }
 
    return (dbconnparameters);
}
 
void
PgSqlConnection::openDatabase() {
    openDatabaseInternal(true);
}
 
void
PgSqlConnection::openDatabaseInternal(bool logging) {
    std::string dbconnparameters = getConnParametersInternal(logging);
    // Connect to PostgreSQL, saving the low level connection pointer
    // in the holder object
    PGconn* new_conn = PQconnectdb(dbconnparameters.c_str());
    if (!new_conn) {
        isc_throw(DbOpenError, "could not allocate connection object");
    }
 
    if (PQstatus(new_conn) != CONNECTION_OK) {
        // Mark this connection as no longer usable.
        markUnusable();
 
        // If we have a connection object, we have to call finish
        // to release it, but grab the error message first.
        std::string error_message = PQerrorMessage(new_conn);
        PQfinish(new_conn);
 
        auto const& rec = reconnectCtl();
        if (rec && DatabaseConnection::retry_) {
 
            // Start the connection recovery.
            startRecoverDbConnection();
 
            std::ostringstream s;
 
            s << " (scheduling retry " << rec->retryIndex() + 1 << " of " << rec->maxRetries() << " in " << rec->retryInterval() << " milliseconds)";
 
            error_message += s.str();
 
            isc_throw(DbOpenErrorWithRetry, error_message);
        }
 
        isc_throw(DbOpenError, error_message);
    }
 
    // We have a valid connection, so let's save it to our holder
    conn_.setConnection(new_conn);
}
 
bool
PgSqlConnection::compareError(const PgSqlResult& r, const char* error_state) {
    const char* sqlstate = PQresultErrorField(r, PG_DIAG_SQLSTATE);
    // PostgreSQL guarantees it will always be 5 characters long
    return ((sqlstate != NULL) &&
            (memcmp(sqlstate, error_state, PGSQL_STATECODE_LEN) == 0));
}
 
void
PgSqlConnection::checkStatementError(const PgSqlResult& r,
                                     PgSqlTaggedStatement& statement) {
    int s = PQresultStatus(r);
    if (s != PGRES_COMMAND_OK && s != PGRES_TUPLES_OK) {
        // We're testing the first two chars of SQLSTATE, as this is the
        // error class. Note, there is a severity field, but it can be
        // misleadingly returned as fatal. However, a loss of connectivity
        // can lead to a NULL sqlstate with a status of PGRES_FATAL_ERROR.
        const char* sqlstate = PQresultErrorField(r, PG_DIAG_SQLSTATE);
        if ((sqlstate == NULL) ||
            ((memcmp(sqlstate, "08", 2) == 0) ||  // Connection Exception
             (memcmp(sqlstate, "53", 2) == 0) ||  // Insufficient resources
             (memcmp(sqlstate, "54", 2) == 0) ||  // Program Limit exceeded
             (memcmp(sqlstate, "57", 2) == 0) ||  // Operator intervention
             (memcmp(sqlstate, "58", 2) == 0))) { // System error
            DB_LOG_ERROR(PGSQL_FATAL_ERROR)
                .arg(statement.name)
                .arg(PQerrorMessage(conn_))
                .arg(sqlstate ? sqlstate : "<sqlstate null>");
 
            // Mark this connection as no longer usable.
            markUnusable();
 
            // Start the connection recovery.
            startRecoverDbConnection();
 
            // We still need to throw so caller can error out of the current
            // processing.
            isc_throw(DbConnectionUnusable,
                      "fatal database error or connectivity lost");
        }
 
        // Failure: check for the special case of duplicate entry.
        if (compareError(r, PgSqlConnection::DUPLICATE_KEY)) {
            isc_throw(DuplicateEntry, "statement: " << statement.name
                      << ", reason: " << PQerrorMessage(conn_));
        }
 
        // Failure: check for the special case of null key violation.
        if (compareError(r, PgSqlConnection::NULL_KEY)) {
            isc_throw(NullKeyError, "statement: " << statement.name
                      << ", reason: " << PQerrorMessage(conn_));
        }
 
        // Apparently it wasn't fatal, so we throw with a helpful message.
        const char* error_message = PQerrorMessage(conn_);
        isc_throw(DbOperationError, "Statement exec failed for: "
                  << statement.name << ", status: " << s
                  << "sqlstate:[ " << (sqlstate ? sqlstate : "<null>")
                  << " ], reason: " << error_message);
    }
}
 
void
PgSqlConnection::startTransaction() {
    // If it is nested transaction, do nothing.
    if (++transaction_ref_count_ > 1) {
        return;
    }
 
    DB_LOG_DEBUG(DB_DBG_TRACE_DETAIL, PGSQL_START_TRANSACTION);
    checkUnusable();
    PgSqlResult r(PQexec(conn_, "START TRANSACTION"));
    if (PQresultStatus(r) != PGRES_COMMAND_OK) {
        const char* error_message = PQerrorMessage(conn_);
        isc_throw(DbOperationError, "unable to start transaction"
                  << error_message);
    }
}
 
bool
PgSqlConnection::isTransactionStarted() const {
    return (transaction_ref_count_ > 0);
}
 
void
PgSqlConnection::commit() {
    if (transaction_ref_count_ <= 0) {
        isc_throw(Unexpected, "commit called for not started transaction - coding error");
    }
 
    // When committing nested transaction, do nothing.
    if (--transaction_ref_count_ > 0) {
        return;
    }
 
    DB_LOG_DEBUG(DB_DBG_TRACE_DETAIL, PGSQL_COMMIT);
    checkUnusable();
    PgSqlResult r(PQexec(conn_, "COMMIT"));
    if (PQresultStatus(r) != PGRES_COMMAND_OK) {
        const char* error_message = PQerrorMessage(conn_);
        isc_throw(DbOperationError, "commit failed: " << error_message);
    }
}
 
void
PgSqlConnection::rollback() {
    if (transaction_ref_count_ <= 0) {
        isc_throw(Unexpected, "rollback called for not started transaction - coding error");
    }
 
    // When rolling back nested transaction, do nothing.
    if (--transaction_ref_count_ > 0) {
        return;
    }
 
    DB_LOG_DEBUG(DB_DBG_TRACE_DETAIL, PGSQL_ROLLBACK);
    checkUnusable();
    PgSqlResult r(PQexec(conn_, "ROLLBACK"));
    if (PQresultStatus(r) != PGRES_COMMAND_OK) {
        const char* error_message = PQerrorMessage(conn_);
        isc_throw(DbOperationError, "rollback failed: " << error_message);
    }
}
 
void
PgSqlConnection::createSavepoint(const std::string& name) {
    if (transaction_ref_count_ <= 0) {
        isc_throw(InvalidOperation, "no transaction, cannot create savepoint: " << name);
    }
 
    DB_LOG_DEBUG(DB_DBG_TRACE_DETAIL, PGSQL_CREATE_SAVEPOINT).arg(name);
    std::string sql("SAVEPOINT " + name);
    executeSQL(sql);
}
 
void
PgSqlConnection::rollbackToSavepoint(const std::string& name) {
    if (transaction_ref_count_ <= 0) {
        isc_throw(InvalidOperation, "no transaction, cannot rollback to savepoint: " << name);
    }
 
    std::string sql("ROLLBACK TO SAVEPOINT " + name);
    executeSQL(sql);
}
 
void
PgSqlConnection::executeSQL(const std::string& sql) {
    // Use a TaggedStatement so we can call checkStatementError and ensure
    // we detect connectivity issues properly.
    PgSqlTaggedStatement statement({0, {OID_NONE}, "run-statement", sql.c_str()});
    checkUnusable();
    PgSqlResult r(PQexec(conn_, statement.text));
    checkStatementError(r, statement);
}
 
PgSqlResultPtr
PgSqlConnection::executePreparedStatement(PgSqlTaggedStatement& statement,
                                          const PsqlBindArray& in_bindings) {
    checkUnusable();
 
    if (static_cast<size_t>(statement.nbparams) != in_bindings.size()) {
        isc_throw (InvalidOperation, "executePreparedStatement:"
                   << " expected: " << statement.nbparams
                   << " parameters, given: " << in_bindings.size()
                   << ", statement: " << statement.name
                   << ", SQL: " << statement.text);
    }
 
    const char* const* values = 0;
    const int* lengths = 0;
    const int* formats = 0;
    if (statement.nbparams > 0) {
        values = static_cast<const char* const*>(&in_bindings.values_[0]);
        lengths = static_cast<const int *>(&in_bindings.lengths_[0]);
        formats = static_cast<const int *>(&in_bindings.formats_[0]);
    }
 
    PgSqlResultPtr result_set;
    result_set.reset(new PgSqlResult(PQexecPrepared(conn_, statement.name, statement.nbparams,
                                                    values, lengths, formats, 0)));
 
    checkStatementError(*result_set, statement);
    return (result_set);
}
 
void
PgSqlConnection::selectQuery(PgSqlTaggedStatement& statement,
                             const PsqlBindArray& in_bindings,
                             ConsumeResultRowFun process_result_row) {
    // Execute the prepared statement.
    PgSqlResultPtr result_set = executePreparedStatement(statement, in_bindings);
 
    // Iterate over the returned rows and invoke the row consumption
    // function on each one.
    int rows = result_set->getRows();
    for (int row = 0; row < rows; ++row) {
        try {
            process_result_row(*result_set, row);
        } catch (const std::exception& ex) {
            // Rethrow the exception with a bit more data.
            isc_throw(BadValue, ex.what() << ". Statement is <" <<
                      statement.text << ">");
        }
    }
}
 
void
PgSqlConnection::insertQuery(PgSqlTaggedStatement& statement,
                             const PsqlBindArray& in_bindings) {
    // Execute the prepared statement.
    PgSqlResultPtr result_set = executePreparedStatement(statement, in_bindings);
}
 
uint64_t
PgSqlConnection::updateDeleteQuery(PgSqlTaggedStatement& statement,
                                   const PsqlBindArray& in_bindings) {
    // Execute the prepared statement.
    PgSqlResultPtr result_set = executePreparedStatement(statement, in_bindings);
 
    return (boost::lexical_cast<int>(PQcmdTuples(*result_set)));
}
 
template<typename T>
void
PgSqlConnection::setIntParameterValue(const std::string& name, int64_t min, int64_t max, T& value) {
    string svalue;
    try {
        svalue = getParameter(name);
    } catch (...) {
        // Do nothing if the parameter is not present.
    }
    if (svalue.empty()) {
        return;
    }
    try {
        // Try to convert the value.
        auto parsed_value = boost::lexical_cast<T>(svalue);
        // Check if the value is within the specified range.
        if ((parsed_value < min) || (parsed_value > max)) {
            isc_throw(BadValue, "bad " << svalue << " value");
        }
        // Everything is fine. Return the parsed value.
        value = parsed_value;
 
    } catch (...) {
        // We may end up here when lexical_cast fails or when the
        // parsed value is not within the desired range. In both
        // cases let's throw the same general error.
        isc_throw(BadValue, name << " parameter (" <<
                  svalue << ") must be an integer between "
                  << min << " and " << max);
    }
}
 
 
} // end of isc::db namespace
} // end of isc namespace