/** @file
 * @brief Replication support for Xapian databases.
 */
/* Copyright (C) 2008 Lemur Consulting Ltd
 * Copyright (C) 2008,2009,2010,2011,2012,2013,2014,2015,2016,2017 Olly Betts
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program 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 this program; if not, see
 * <https://www.gnu.org/licenses/>.
 */

#include <config.h>

#include "replication.h"

#include "xapian/intrusive_ptr.h"
#include "xapian/constants.h"
#include "xapian/dbfactory.h"
#include "xapian/error.h"
#include "xapian/version.h"

#include "backends/databaseinternal.h"
#include "backends/databasereplicator.h"
#include "debuglog.h"
#include "filetests.h"
#include "fileutils.h"
#include "io_utils.h"
#include "omassert.h"
#include "pack.h"
#include "realtime.h"
#include "net/remoteconnection.h"
#include "replicationprotocol.h"
#include "safesysstat.h"
#include "safeunistd.h"
#include "str.h"
#include "unicode/description_append.h"

#include <cerrno>
#include <fstream>
#include <memory>
#include <string>

using namespace std;
using namespace Xapian;

// The banner comment used at the top of the replica's stub database file.
#define REPLICA_STUB_BANNER \
"# Automatically generated by Xapian::DatabaseReplica v" XAPIAN_VERSION ".\n" \
"# Do not manually edit - replication operations may regenerate this file.\n"

[[noreturn]]
static void
throw_connection_closed_unexpectedly()
{
    throw Xapian::NetworkError("Connection closed unexpectedly");
}

void
DatabaseMaster::write_changesets_to_fd(int fd,
                                       const string & start_revision,
                                       ReplicationInfo * info) const
{
    LOGCALL_VOID(REPLICA, "DatabaseMaster::write_changesets_to_fd", fd | start_revision | info);
    if (info != NULL)
        info->clear();
    Database db;
    try {
        db = Database(path);
    } catch (const Xapian::DatabaseError & e) {
        RemoteConnection conn(-1, fd);
        conn.send_message(REPL_REPLY_FAIL,
                          "Can't open database: " + e.get_msg(),
                          0.0);
        return;
    }
    if (db.internal->size() != 1) {
        throw Xapian::InvalidOperationError("DatabaseMaster needs to be pointed at exactly one subdatabase");
    }

    // Extract the UUID from start_revision and compare it to the database.
    bool need_whole_db = false;
    string revision;
    if (start_revision.empty()) {
        need_whole_db = true;
    } else {
        const char * ptr = start_revision.data();
        const char * end = ptr + start_revision.size();
        string request_uuid;
        if (!unpack_string(&ptr, end, request_uuid)) {
            unpack_throw_serialisation_error(ptr);
        }
        string db_uuid = db.internal->get_uuid();
        if (request_uuid != db_uuid) {
            need_whole_db = true;
        }
        revision.assign(ptr, end - ptr);
    }

    db.internal->write_changesets_to_fd(fd, revision, need_whole_db, info);
}

string
DatabaseMaster::get_description() const
{
    string desc = "DatabaseMaster(";
    description_append(desc, path);
    desc += ")";
    return desc;
}

/// Internal implementation of DatabaseReplica
class DatabaseReplica::Internal : public Xapian::Internal::intrusive_base {
    /// Don't allow assignment.
    void operator=(const Internal &);

    /// Don't allow copying.
    Internal(const Internal &);

    /// The path to the replica directory.
    string path;

    /// The id of the currently live database in the replica (0 or 1).
    int live_id;

    /** The live database being replicated.
     *
     *  This needs to be mutable because it is sometimes lazily opened.
     */
    mutable WritableDatabase live_db;

    /// Do we need to heal the replica?
    bool live_db_corrupt = false;

    /** Do we have an offline database currently?
     *
     *  The offline database is a new copy of the database we're bringing up
     *  to the required revision, which can't yet be made live.
     */
    bool have_offline_db;

    /** Flag to indicate that the only valid operation next is a full copy.
     */
    bool need_copy_next;

    /** The revision that the secondary database has been updated to.
     */
    string offline_revision;

    /** The UUID of the secondary database.
     */
    string offline_uuid;

    /** The revision that the secondary database must reach before it can be
     *  made live.
     */
    string offline_needed_revision;

    /** The time at which a changeset was last applied to the live database.
     *
     *  Set to 0 if no changeset applied to the live database so far.
     */
    double last_live_changeset_time;

    /// The remote connection we're using.
    RemoteConnection * conn;

    /** Update the stub database which points to a single database.
     *
     *  The stub database file is created at a separate path, and then
     *  atomically moved into place to replace the old stub database.  This
     *  should allow searches to continue uninterrupted.
     */
    void update_stub_database() const;

    /** Delete the offline database. */
    void remove_offline_db();

    /** Apply a set of DB copy messages from the connection.
     */
    void apply_db_copy(double end_time);

    /** Check that a message type is as expected.
     *
     *  Throws a NetworkError if the type is not the expected one.
     */
    void check_message_type(int type, int expected) const;

    /** Check if the offline database has reached the required version.
     *
     *  If so, make it live, and remove the old live database.
     *
     *  @return true iff the offline database is made live
     */
    bool possibly_make_offline_live();

    string get_replica_path(int id) const {
        string p = path;
        p += "/replica_";
        p += char('0' + id);
        return p;
    }

  public:
    /// Open a new DatabaseReplica::Internal for the specified path.
    explicit Internal(const string & path_);

    /// Destructor.
    ~Internal() { delete conn; }

    /// Get a string describing the current revision of the replica.
    string get_revision_info() const;

    /// Set the file descriptor to read changesets from.
    void set_read_fd(int fd);

    /// Read and apply the next changeset.
    bool apply_next_changeset(ReplicationInfo * info,
                              double reader_close_time);

    /// Return a string describing this object.
    string get_description() const { return path; }
};

// Methods of DatabaseReplica

DatabaseReplica::DatabaseReplica(const string & path)
        : internal(new DatabaseReplica::Internal(path))
{
    LOGCALL_CTOR(REPLICA, "DatabaseReplica", path);
}

DatabaseReplica::~DatabaseReplica()
{
    LOGCALL_DTOR(REPLICA, "DatabaseReplica");
    delete internal;
}

string
DatabaseReplica::get_revision_info() const
{
    LOGCALL(REPLICA, string, "DatabaseReplica::get_revision_info", NO_ARGS);
    RETURN(internal->get_revision_info());
}

void
DatabaseReplica::set_read_fd(int fd)
{
    LOGCALL_VOID(REPLICA, "DatabaseReplica::set_read_fd", fd);
    internal->set_read_fd(fd);
}

bool
DatabaseReplica::apply_next_changeset(ReplicationInfo * info,
                                      double reader_close_time)
{
    LOGCALL(REPLICA, bool, "DatabaseReplica::apply_next_changeset", info | reader_close_time);
    if (info != NULL)
        info->clear();
    RETURN(internal->apply_next_changeset(info, reader_close_time));
}

string
DatabaseReplica::get_description() const
{
    string desc("DatabaseReplica(");
    desc += internal->get_description();
    desc += ')';
    return desc;
}

// Methods of DatabaseReplica::Internal

void
DatabaseReplica::Internal::update_stub_database() const
{
    string stub_path = path;
    stub_path += "/XAPIANDB";
    string tmp_path = stub_path;
    tmp_path += ".tmp";
    {
        ofstream stub(tmp_path.c_str());
        stub << REPLICA_STUB_BANNER
                "auto replica_" << live_id << endl;
    }
    if (!io_tmp_rename(tmp_path, stub_path)) {
        string msg("Failed to update stub db file for replica: ");
        msg += path;
        throw Xapian::DatabaseOpeningError(msg, errno);
    }
}

DatabaseReplica::Internal::Internal(const string & path_)
        : path(path_), live_id(0), live_db(), have_offline_db(false),
          need_copy_next(false), offline_revision(), offline_needed_revision(),
          last_live_changeset_time(), conn(NULL)
{
    LOGCALL_CTOR(REPLICA, "DatabaseReplica::Internal", path_);
#ifndef XAPIAN_HAS_GLASS_BACKEND
    throw FeatureUnavailableError("Replication requires the glass backend to be enabled");
#else
    if (mkdir(path.c_str(), 0777) == 0) {
        // The database doesn't already exist - make a directory, containing a
        // stub database, and point it to a new database.
        //
        // Create an empty database - the backend doesn't matter as if the
        // master is a different type, then the replica will become that type
        // automatically.
        live_db = WritableDatabase(get_replica_path(live_id),
                                   Xapian::DB_CREATE);
        update_stub_database();
    } else {
        if (errno != EEXIST) {
            throw DatabaseOpeningError("Couldn't create directory '" + path + "'", errno);
        }
        if (!dir_exists(path)) {
            throw DatabaseOpeningError("Replica path must be a directory");
        }
        string stub_path = path;
        stub_path += "/XAPIANDB";
        try {
            live_db = WritableDatabase(stub_path,
                                       Xapian::DB_OPEN|Xapian::DB_BACKEND_STUB);
        } catch (const Xapian::DatabaseCorruptError &) {
            // If the database is too corrupt to open, force a full copy so we
            // auto-heal from this condition.  Instance seen in the wild was
            // that the replica had all files truncated to size 0.
            live_db_corrupt = true;
        }
        // FIXME: simplify all this?
        ifstream stub(stub_path.c_str());
        string line;
        while (getline(stub, line)) {
            if (!line.empty() && line[0] != '#') {
                live_id = line[line.size() - 1] - '0';
                break;
            }
        }
    }
#endif
}

string
DatabaseReplica::Internal::get_revision_info() const
{
    LOGCALL(REPLICA, string, "DatabaseReplica::Internal::get_revision_info", NO_ARGS);
    if (live_db_corrupt) {
        RETURN(string());
    }

    switch (live_db.internal->size()) {
        case 0:
            live_db = WritableDatabase(get_replica_path(live_id), Xapian::DB_OPEN);
            break;
        case 1:
            // OK
            break;
        default:
            throw Xapian::InvalidOperationError("DatabaseReplica needs to be "
                                                "pointed at exactly one "
                                                "subdatabase");
    }

    string buf;
    pack_string(buf, live_db.get_uuid());
    pack_uint(buf, live_db.get_revision());
    RETURN(buf);
}

void
DatabaseReplica::Internal::remove_offline_db()
{
    // Delete the offline database.
    removedir(get_replica_path(live_id ^ 1));
    have_offline_db = false;
}

void
DatabaseReplica::Internal::apply_db_copy(double end_time)
{
    have_offline_db = true;
    last_live_changeset_time = 0;
    string offline_path = get_replica_path(live_id ^ 1);
    // If there's already an offline database, discard it.  This happens if one
    // copy of the database was sent, but further updates were needed before it
    // could be made live, and the remote end was then unable to send those
    // updates (probably due to not having changesets available, or the remote
    // database being replaced by a new database).
    removedir(offline_path);
    if (mkdir(offline_path.c_str(), 0777)) {
        throw Xapian::DatabaseError("Cannot make directory '" +
                                    offline_path + "'", errno);
    }

    {
        string buf;
        int type = conn->get_message(buf, end_time);
        check_message_type(type, REPL_REPLY_DB_HEADER);
        const char * ptr = buf.data();
        const char * end = ptr + buf.size();
        if (!unpack_string(&ptr, end, offline_uuid)) {
            unpack_throw_serialisation_error(ptr);
        }
        offline_revision.assign(ptr, end - ptr);
    }

    // Now, read the files for the database from the connection and create it.
    while (true) {
        string filename;
        int type = conn->sniff_next_message_type(end_time);
        if (type < 0 || type == REPL_REPLY_FAIL)
            return;
        if (type == REPL_REPLY_DB_FOOTER)
            break;

        type = conn->get_message(filename, end_time);
        check_message_type(type, REPL_REPLY_DB_FILENAME);

        // Check that the filename doesn't contain '..'.  No valid database
        // file contains .., so we don't need to check that the .. is a path.
        if (filename.find("..") != string::npos) {
            throw NetworkError("Filename in database contains '..'");
        }

        type = conn->sniff_next_message_type(end_time);
        if (type < 0 || type == REPL_REPLY_FAIL)
            return;

        string filepath = offline_path + "/" + filename;
        type = conn->receive_file(filepath, end_time);
        if (type < 0)
            throw_connection_closed_unexpectedly();
        check_message_type(type, REPL_REPLY_DB_FILEDATA);
    }
    int type = conn->get_message(offline_needed_revision, end_time);
    check_message_type(type, REPL_REPLY_DB_FOOTER);
    need_copy_next = false;
}

void
DatabaseReplica::Internal::check_message_type(int type, int expected) const
{
    if (type != expected) {
        if (type < 0)
            throw_connection_closed_unexpectedly();
        string m = "Expected replication protocol message type #";
        m += str(expected);
        m += ", got #";
        m += str(type);
        throw NetworkError(m);
    }
}

bool
DatabaseReplica::Internal::possibly_make_offline_live()
{
    string replica_path(get_replica_path(live_id ^ 1));
    unique_ptr<DatabaseReplicator> replicator;
    try {
        replicator.reset(DatabaseReplicator::open(replica_path));
    } catch (const Xapian::DatabaseError &) {
        return false;
    }
    if (offline_needed_revision.empty()) {
        return false;
    }
    if (!replicator->check_revision_at_least(offline_revision,
                                             offline_needed_revision)) {
        return false;
    }

    string replicated_uuid = replicator->get_uuid();
    if (replicated_uuid.empty()) {
        return false;
    }

    if (replicated_uuid != offline_uuid) {
        return false;
    }

    live_id ^= 1;
    // Open the database first, so that if there's a problem, an exception
    // will be thrown before we make the new database live.
    live_db = WritableDatabase(replica_path, Xapian::DB_OPEN);
    live_db_corrupt = false;
    update_stub_database();
    remove_offline_db();
    return true;
}

void
DatabaseReplica::Internal::set_read_fd(int fd)
{
    delete conn;
    conn = NULL;
    conn = new RemoteConnection(fd, -1);
}

bool
DatabaseReplica::Internal::apply_next_changeset(ReplicationInfo * info,
                                                double reader_close_time)
{
    LOGCALL(REPLICA, bool, "DatabaseReplica::Internal::apply_next_changeset", info | reader_close_time);
    while (true) {
        int type = conn->sniff_next_message_type(0.0);
        switch (type) {
            case REPL_REPLY_END_OF_CHANGES: {
                string buf;
                type = conn->get_message(buf, 0.0);
                check_message_type(type, REPL_REPLY_END_OF_CHANGES);
                RETURN(false);
            }
            case REPL_REPLY_DB_HEADER:
                // Apply the copy - remove offline db in case of any error.
                try {
                    apply_db_copy(0.0);
                    if (info != NULL)
                        ++(info->fullcopy_count);
                    string replica_uuid;
                    {
                        unique_ptr<DatabaseReplicator> replicator(
                                DatabaseReplicator::open(get_replica_path(live_id ^ 1)));
                        replica_uuid = replicator->get_uuid();
                    }
                    if (replica_uuid != offline_uuid) {
                        remove_offline_db();
                        // We've been sent an database with the wrong uuid,
                        // which only happens if the database at the server
                        // got changed during the copy, so the only safe
                        // action next is a new copy.  Set a flag to ensure
                        // that this happens, or we're at risk of database
                        // corruption.
                        need_copy_next = true;
                    }
                } catch (...) {
                    remove_offline_db();
                    throw;
                }
                if (possibly_make_offline_live()) {
                    if (info != NULL)
                        info->changed = true;
                }
                break;
            case REPL_REPLY_CHANGESET:
                if (need_copy_next) {
                    throw NetworkError("Needed a database copy next");
                }
                if (!have_offline_db) {
                    // Close the live db.
                    string replica_path(get_replica_path(live_id));
                    live_db = WritableDatabase();

                    if (last_live_changeset_time != 0.0) {
                        // Wait until at least "reader_close_time" seconds have
                        // passed since the last changeset was applied, to
                        // allow any active readers to finish and be reopened.
                        double until;
                        until = last_live_changeset_time + reader_close_time;
                        RealTime::sleep(until);
                    }

                    // Open a replicator for the live path, and apply the
                    // changeset.
                    {
                        unique_ptr<DatabaseReplicator> replicator(
                                DatabaseReplicator::open(replica_path));

                        // Ignore the returned revision number, since we are
                        // live so the changeset must be safe to apply to a
                        // live DB.
                        replicator->apply_changeset_from_conn(*conn, 0.0, true);
                    }
                    last_live_changeset_time = RealTime::now();

                    if (info != NULL) {
                        ++(info->changeset_count);
                        info->changed = true;
                    }
                    // Now the replicator is closed, open the live db again.
                    live_db = WritableDatabase(replica_path, Xapian::DB_OPEN);
                    live_db_corrupt = false;
                    RETURN(true);
                }

                {
                    unique_ptr<DatabaseReplicator> replicator(
                            DatabaseReplicator::open(get_replica_path(live_id ^ 1)));

                    offline_revision = replicator->
                            apply_changeset_from_conn(*conn, 0.0, false);

                    if (info != NULL) {
                        ++(info->changeset_count);
                    }
                }
                if (possibly_make_offline_live()) {
                    if (info != NULL)
                        info->changed = true;
                }
                RETURN(true);
            case REPL_REPLY_FAIL: {
                string buf;
                if (conn->get_message(buf, 0.0) < 0)
                    throw_connection_closed_unexpectedly();
                throw NetworkError("Unable to fully synchronise: " + buf);
            }
            case -1:
                throw_connection_closed_unexpectedly();
            default:
                throw NetworkError("Unknown replication protocol message (" +
                                   str(type) + ")");
        }
    }
}
