// Copyright (C) 2023 The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package com.googlesource.gerrit.plugins.spannerrefdb;

import com.gerritforge.gerrit.globalrefdb.GlobalRefDbLockException;
import com.google.cloud.Timestamp;
import com.google.cloud.spanner.DatabaseClient;
import com.google.cloud.spanner.ErrorCode;
import com.google.cloud.spanner.Key;
import com.google.cloud.spanner.KeySet;
import com.google.cloud.spanner.Mutation;
import com.google.cloud.spanner.ResultSet;
import com.google.cloud.spanner.SpannerException;
import com.google.cloud.spanner.Statement;
import com.google.cloud.spanner.Struct;
import com.google.cloud.spanner.TransactionRunner;
import com.google.cloud.spanner.Value;
import com.google.common.flogger.FluentLogger;
import com.google.gerrit.server.config.GerritInstanceId;
import com.google.inject.Inject;
import com.google.inject.assistedinject.Assisted;
import java.util.Arrays;
import java.util.Collections;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

/***
 * Locks are a mechanism to ensure exclusive usage of the locked resource. Here we
 * provide a Spanner-specific implementation of locks which are used from the
 * global-refdb module, which is used to ensure refs are updated consistently
 * across multiple Gerrit primary sites (see README.md).
 *
 * Persistent distributed Locks are used to guard distributed transactions
 * spanning RefUpdates in a git repo of the involved Gerrit primary sites and
 * corresponding updates of those refs in the global-refdb.
 */
public class Lock implements AutoCloseable {
  private static final FluentLogger logger = FluentLogger.forEnclosingClass();

  public interface LockFactory {
    Lock create(@Assisted("projectName") String projectName, @Assisted("refName") String refName);
  }

  private static final Long SECONDS_FOR_STALE_HEARTBEAT = 30L;
  private static final int HEARTBEAT_INTERVAL = 2;
  private static final String RECLAIM_LOCK_PREFIX = "RECLAIM";
  private final DatabaseClient dbClient;
  private final String projectName;
  private final String gerritInstanceId;
  private final String refName;
  private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);

  private Timestamp token;

  @Inject
  Lock(
      DatabaseClient dbClient,
      @GerritInstanceId String gerritInstanceId,
      @Assisted("projectName") String projectName,
      @Assisted("refName") String refName) {
    this.projectName = projectName;
    this.refName = refName;
    this.dbClient = dbClient;
    this.gerritInstanceId = gerritInstanceId;
  }

  /**
   * Attempt to lock a project/ref by creating a lock in the locks table. If the lock already
   * exists, attempt to reclaim it.
   *
   * <p>Once the lock is successfully claimed, saves the token information and schedules a
   * heartbeat.
   *
   * @throws GlobalRefDbLockException if there is a failure to claim the lock
   */
  public void tryLock() throws GlobalRefDbLockException {
    try {
      TransactionRunner transactionRunner = dbClient.readWriteTransaction();
      transactionRunner.run(
          transaction -> {
            transaction.buffer(createLock());
            return true;
          });
      token = transactionRunner.getCommitTimestamp();
      scheduler.scheduleAtFixedRate(this::heartbeat, 0, HEARTBEAT_INTERVAL, TimeUnit.SECONDS);
      logger.atFine().log("Locked %s %s.", projectName, refName);
    } catch (Exception e) {
      Throwable cause = e.getCause();
      if (cause instanceof SpannerException) {
        ErrorCode code = ((SpannerException) cause).getErrorCode();
        if (code == ErrorCode.ALREADY_EXISTS) {
          logger.atFine().log(
              "Lock for %s %s already exists, attempting to reclaim.", projectName, refName);
          if (reclaim()) {
            return;
          }
        }
      }
      throw new GlobalRefDbLockException(projectName, refName, e);
    }
  }

  /**
   * Attempt to reclaim a lock that is already claimed by another process.
   *
   * <p>Checks for heartbeat staleness; if the heartbeat is not stale, does not proceed further. If
   * the heartbeat is stale, ensure the lock still exists and has the expected token. Then insert a
   * reclaim-lock to assert the reclamation process is in progress and update the old lock. This is
   * all done in one transaction so that if any one step fails, the transaction is aborted. Clean up
   * the reclaim-lock on success.
   */
  private boolean reclaim() {
    TransactionRunner transactionRunner = dbClient.readWriteTransaction();

    boolean success =
        transactionRunner.run(
            transaction -> {
              Struct row =
                  transaction.readRow(
                      "locks", Key.of(projectName, refName), Arrays.asList("token", "heartbeat"));

              if (row == null) {
                // If the old lock doesn't exist any more, we create a new one
                transaction.buffer(createLock());
                return true;
              }
              String timestampDifferenceQuery =
                  "SELECT TIMESTAMP_DIFF(CURRENT_TIMESTAMP(), token, SECOND) AS seconds FROM locks"
                      + " WHERE project = @project AND ref = @ref;";
              ResultSet resultSet =
                  transaction.executeQuery(
                      Statement.newBuilder(timestampDifferenceQuery)
                          .bind("project")
                          .to(projectName)
                          .bind("ref")
                          .to(refName)
                          .build());
              if (resultSet.getLong("seconds") <= SECONDS_FOR_STALE_HEARTBEAT) {
                // Check if the old lock is stale; if not, do nothing
                return false;
              }
              Timestamp oldToken = row.getTimestamp("token");
              transaction.buffer(createReclaimLock(oldToken));
              transaction.buffer(updateReclaimedLock());
              transaction.buffer(deleteReclaimLockForCleanup(oldToken));
              return true;
            });

    if (success) {
      token = transactionRunner.getCommitTimestamp();
      scheduler.scheduleAtFixedRate(this::heartbeat, 0, HEARTBEAT_INTERVAL, TimeUnit.SECONDS);
      logger.atFine().log("Reclaimed lock for %s %s.", projectName, refName);
    }
    return success;
  }

  private Mutation deleteReclaimLockForCleanup(Timestamp reclaimToken) {
    return Mutation.delete(
        "locks", KeySet.prefixRange(Key.of(reclaimLockName(reclaimToken), refName)));
  }

  /**
   * Update the heartbeat timestamp of the current lock if it matches the expected token. If the
   * tokens do not match, the lock has been reclaimed and a new heartbeat started, so this heartbeat
   * ends.
   *
   * @throws GlobalRefDbLockException if the lock unexpectedly no longer exists but the heartbeat is
   *     still ongoing
   */
  private void heartbeat() {
    try {
      boolean success =
          dbClient
              .readWriteTransaction()
              .run(
                  transaction -> {
                    Struct row =
                        transaction.readRow(
                            "locks", Key.of(projectName, refName), Collections.singleton("token"));
                    if (row == null) {
                      logger.atWarning().log(
                          "Lock %s %s (%s) not found", projectName, refName, token);
                      return false;
                    }
                    if (!token.equals(row.getTimestamp("token"))) {
                      logger.atWarning().log(
                          "Lock for %s %s has unexpected token (%s), expecting (%s)",
                          projectName, refName, row.getTimestamp("token"), token);
                      return false;
                    }
                    transaction.buffer(updateHeartbeat());
                    return true;
                  });
      if (!success) {
        logger.atFine().log("Heartbeat stopping for lock %s %s (%s)", projectName, refName, token);
        scheduler.shutdown();
      }
    } catch (Exception e) {
      logger.atWarning().withCause(e).log(
          "Heartbeat failed to update for lock %s %s (%s)", projectName, refName, token);
    }
  }

  /** Close the lock, shutting down the heartbeat and deleting the lock from the locks table. */
  @Override
  public void close() {
    scheduler.shutdownNow();

    dbClient
        .readWriteTransaction()
        .run(
            transaction -> {
              transaction.executeUpdate(deleteLockStatement());
              return true;
            });
  }

  private String reclaimLockName(Timestamp reclaimToken) {
    return String.format(
        "%s/%s/%012d%09d",
        RECLAIM_LOCK_PREFIX, projectName, reclaimToken.getSeconds(), reclaimToken.getNanos());
  }

  private Mutation createLock() {
    return Mutation.newInsertBuilder("locks")
        .set("project")
        .to(projectName)
        .set("ref")
        .to(refName)
        .set("heartbeat")
        .to(Value.COMMIT_TIMESTAMP)
        .set("token")
        .to(Value.COMMIT_TIMESTAMP)
        .set("owner")
        .to(gerritInstanceId)
        .build();
  }

  private Mutation createReclaimLock(Timestamp reclaimToken) {
    return Mutation.newInsertBuilder("locks")
        .set("project")
        .to(reclaimLockName(reclaimToken))
        .set("ref")
        .to(refName)
        .set("heartbeat")
        .to(Value.COMMIT_TIMESTAMP)
        .set("token")
        .to(Value.COMMIT_TIMESTAMP)
        .set("owner")
        .to(gerritInstanceId)
        .build();
  }

  private Mutation updateReclaimedLock() {
    return Mutation.newUpdateBuilder("locks")
        .set("project")
        .to(projectName)
        .set("ref")
        .to(refName)
        .set("heartbeat")
        .to(Value.COMMIT_TIMESTAMP)
        .set("token")
        .to(Value.COMMIT_TIMESTAMP)
        .set("owner")
        .to(gerritInstanceId)
        .build();
  }

  private Mutation updateHeartbeat() {
    return Mutation.newUpdateBuilder("locks")
        .set("project")
        .to(projectName)
        .set("ref")
        .to(refName)
        .set("heartbeat")
        .to(Value.COMMIT_TIMESTAMP)
        .build();
  }

  private Statement deleteLockStatement() {
    return Statement.newBuilder(
            "DELETE FROM locks WHERE project = @project AND ref = @ref AND token = @token")
        .bind("project")
        .to(projectName)
        .bind("ref")
        .to(refName)
        .bind("token")
        .to(token)
        .build();
  }
}