package net.corda.node.utilities.registration

import net.corda.core.crypto.Crypto
import net.corda.core.crypto.internal.AliasPrivateKey
import net.corda.core.identity.CordaX500Name
import net.corda.core.internal.CertRole
import net.corda.core.internal.isEquivalentTo
import net.corda.core.internal.safeSymbolicRead
import net.corda.core.internal.toX500Name
import net.corda.core.utilities.contextLogger
import net.corda.node.NodeRegistrationOption
import net.corda.node.services.config.NodeConfiguration
import net.corda.nodeapi.internal.config.CertificateStore
import net.corda.nodeapi.internal.config.MutualSslConfiguration
import net.corda.nodeapi.internal.crypto.CertificateType
import net.corda.nodeapi.internal.crypto.NOT_YET_REGISTERED_MARKER_KEYS_AND_CERTS
import net.corda.nodeapi.internal.crypto.X509KeyStore
import net.corda.nodeapi.internal.crypto.X509Utilities
import net.corda.nodeapi.internal.crypto.X509Utilities.CORDA_CLIENT_CA
import net.corda.nodeapi.internal.crypto.X509Utilities.CORDA_CLIENT_TLS
import net.corda.nodeapi.internal.crypto.X509Utilities.CORDA_ROOT_CA
import net.corda.nodeapi.internal.crypto.X509Utilities.DEFAULT_VALIDITY_WINDOW
import net.corda.nodeapi.internal.crypto.x509
import net.corda.nodeapi.internal.cryptoservice.CryptoService
import net.corda.nodeapi.internal.cryptoservice.DefaultCryptoService
import org.bouncycastle.asn1.x500.X500Name
import org.bouncycastle.openssl.jcajce.JcaPEMWriter
import org.bouncycastle.operator.ContentSigner
import org.bouncycastle.util.io.pem.PemObject
import java.io.IOException
import java.io.StringWriter
import java.net.ConnectException
import java.net.URL
import java.nio.file.Path
import java.security.PublicKey
import java.security.cert.X509Certificate
import java.time.Duration
import javax.naming.ServiceUnavailableException
import javax.security.auth.x500.X500Principal
import kotlin.io.path.createDirectories
import kotlin.io.path.deleteIfExists
import kotlin.io.path.div
import kotlin.io.path.exists
import kotlin.io.path.useLines
import kotlin.io.path.writeLines

/**
 * Helper for managing the node registration process, which checks for any existing certificates and requests them if
 * needed.
 */
open class NetworkRegistrationHelper(
        config: NodeRegistrationConfiguration,
        private val certService: NetworkRegistrationService,
        private val networkRootTrustStorePath: Path,
        networkRootTrustStorePassword: String,
        private val nodeCaKeyAlias: String,
        private val certRole: CertRole,
        private val nextIdleDuration: (Duration?) -> Duration? = FixedPeriodLimitedRetrialStrategy(10, Duration.ofMinutes(1)),
        protected val logProgress: (String) -> Unit = ::println,
        protected val logError: (String) -> Unit = System.err::println
) {

    companion object {
        const val SELF_SIGNED_PRIVATE_KEY = "SelfSignedPrivateKey"
        val logger = contextLogger()
    }
    private val certificatesDirectory: Path = config.certificatesDirectory
    private val myLegalName: CordaX500Name = config.myLegalName
    private val emailAddress: String = config.emailAddress
    private val cryptoService = config.cryptoService
    private val certificateStore = config.certificateStore
    private val requestIdStore = certificatesDirectory / "certificate-request-id.txt"
    protected val rootTrustStore: X509KeyStore
    protected val rootCerts: Set<X509Certificate>
    private val notaryServiceConfig: NotaryServiceConfig? = config.notaryServiceConfig

    init {
        require(networkRootTrustStorePath.exists()) {
            "$networkRootTrustStorePath does not exist. This file must contain the root CA cert of your compatibility zone. " +
                    "Please contact your CZ operator."
        }
        rootTrustStore = X509KeyStore.fromFile(networkRootTrustStorePath, networkRootTrustStorePassword)
        val rootAliases = rootTrustStore.aliases().asSequence().filter { it.startsWith(CORDA_ROOT_CA) }
        rootCerts = rootAliases.map { rootTrustStore.getCertificate(it) }.toSet()
    }

    /**
     * Ensure the initial keys and certificates for a node are set up.
     *
     * This checks the "config.certificatesDirectory" field for certificates required to connect to a Corda network.
     * If the certificates are not found, a PKCS #10 certification request will be submitted to the
     * Corda network permissioning server via [NetworkRegistrationService]. This process will enter a polling loop until
     * the request has been approved, and then the certificate chain will be downloaded and stored in [certificateStore].
     *
     * @throws CertificateRequestException if the certificate retrieved by doorman is invalid.
     */
    fun generateKeysAndRegister() {
        certificatesDirectory.safeSymbolicRead().createDirectories()
        // We need this in case cryptoService and certificateStore share the same KeyStore (for backwards compatibility purposes).
        // If we didn't, then an update to cryptoService wouldn't be reflected to certificateStore that is already loaded in memory.
        val certStore: CertificateStore = if (cryptoService is DefaultCryptoService) cryptoService.certificateStore else certificateStore

        // SELF_SIGNED_PRIVATE_KEY is used as progress indicator.
        if (certStore.contains(nodeCaKeyAlias) && !certStore.contains(SELF_SIGNED_PRIVATE_KEY)) {
            logProgress("Certificate already exists, Corda node will now terminate...")
            return
        }

        notaryServiceConfig?.let { validateNotaryServiceKeyAndCert(certStore, it.notaryServiceKeyAlias, it.notaryServiceLegalName) }

        val tlsCrlIssuerCert = getTlsCrlIssuerCert()

        // We use SELF_SIGNED_PRIVATE_KEY as progress indicator so we just store a dummy key and cert.
        // When registration succeeds, this entry should be deleted.
        certStore.query { setPrivateKey(SELF_SIGNED_PRIVATE_KEY, AliasPrivateKey(SELF_SIGNED_PRIVATE_KEY), listOf(NOT_YET_REGISTERED_MARKER_KEYS_AND_CERTS.ECDSAR1_CERT), certificateStore.entryPassword) }

        val (entityPublicKey, receivedCertificates) = generateKeyPairAndCertificate(nodeCaKeyAlias, myLegalName, certRole, certStore)

        onSuccess(entityPublicKey, cryptoService.getSigner(nodeCaKeyAlias), receivedCertificates, tlsCrlIssuerCert?.subjectX500Principal?.toX500Name())
        // All done, clean up temp files.
        requestIdStore.deleteIfExists()
    }

    private fun generateKeyPairAndCertificate(keyAlias: String,
                                              legalName: CordaX500Name,
                                              certificateRole: CertRole,
                                              certStore: CertificateStore): Pair<PublicKey, List<X509Certificate>> {
        val entityPublicKey = loadOrGenerateKeyPair(keyAlias, certificateRole)

        val requestId = submitOrResumeCertificateSigningRequest(entityPublicKey, legalName, certificateRole, cryptoService.getSigner(keyAlias))

        val receivedCertificates = pollServerForCertificates(requestId)
        validateCertificates(entityPublicKey, legalName, certificateRole, receivedCertificates)

        certStore.setCertPathOnly(keyAlias, receivedCertificates)
        certStore.value.internal.deleteEntry(SELF_SIGNED_PRIVATE_KEY)
        certStore.value.save()
        logProgress("Private key '$keyAlias' and its certificate-chain stored successfully.")
        return Pair(entityPublicKey, receivedCertificates)
    }

    /**
     * Used when registering a notary to validate that the shared notary service key and certificate can be accessed.
     *
     * In the case that the notary service certificate and key is not available, a new key key is generated and a separate CSR is
     * submitted to the Identity Manager.
     *
     * If this method successfully completes then the [cryptoService] will contain the notary service key and the [certStore] will contain
     * the notary service certificate chain.
     *
     * @throws IllegalStateException If the notary service certificate already exists but the private key is not available.
     */
    private fun validateNotaryServiceKeyAndCert(certStore: CertificateStore, notaryServiceKeyAlias: String, notaryServiceLegalName: CordaX500Name) {
        if (certStore.contains(notaryServiceKeyAlias) && !cryptoService.containsKey(notaryServiceKeyAlias)) {
            throw IllegalStateException("Notary service identity certificate exists but key pair missing. " +
                    "Please check no old certificates exist in the certificate store.")
        }

        if (certStore.contains(notaryServiceKeyAlias)) {
            logProgress("Notary service certificate already exists. Continuing with node registration...")
            return
        }

        logProgress("Generating notary service identity for $notaryServiceLegalName...")
        generateKeyPairAndCertificate(notaryServiceKeyAlias, notaryServiceLegalName, CertRole.SERVICE_IDENTITY, certStore)
        // The request id store is reused for the next step - registering the node identity.
        // Therefore we can remove this to enable it to be reused.
        requestIdStore.deleteIfExists()
    }

    fun generateNodeIdentity() {
        certificatesDirectory.safeSymbolicRead().createDirectories()
        // We need this in case cryptoService and certificateStore share the same KeyStore (for backwards compatibility purposes).
        // If we didn't, then an update to cryptoService wouldn't be reflected to certificateStore that is already loaded in memory.
        val certStore: CertificateStore = if (cryptoService is DefaultCryptoService) cryptoService.certificateStore else certificateStore

        if (!certStore.contains(nodeCaKeyAlias)) {
            logProgress("Node CA key doesn't exist, program will now terminate...")
            throw IllegalStateException("Node CA not found")
        }

        val nodeIdentityAlias = X509Utilities.NODE_IDENTITY_KEY_ALIAS
        if (certStore.contains(nodeIdentityAlias)) {
            logProgress("Node identity already exists, Corda node will now terminate...")
            return
        }

        certStore.update {
            logProgress("Generating node identity certificate.")
            val nodeIdentityPublicKey = cryptoService.generateKeyPair(nodeIdentityAlias, cryptoService.defaultIdentitySignatureScheme())
            val nodeCaCertChain = getCertificateChain(nodeCaKeyAlias)
            val nodeCaCertificate = nodeCaCertChain.first()
            val validityWindow = X509Utilities.getCertificateValidityWindow(DEFAULT_VALIDITY_WINDOW.first, DEFAULT_VALIDITY_WINDOW.second, nodeCaCertificate)

            val nodeIdentityCert = X509Utilities.createCertificate(
                    CertificateType.LEGAL_IDENTITY,
                    nodeCaCertificate.subjectX500Principal,
                    nodeCaCertificate.x509.publicKey,
                    cryptoService.getSigner(nodeCaKeyAlias),
                    nodeCaCertificate.subjectX500Principal,
                    nodeIdentityPublicKey,
                    validityWindow,
                    crlDistPoint = null,
                    crlIssuer = null)

            logger.info("Generated Node Identity certificate: $nodeIdentityCert")

            val nodeIdentityCertificateChain: List<X509Certificate> = listOf(nodeIdentityCert) + nodeCaCertChain
            X509Utilities.validateCertificateChain(rootCerts, nodeIdentityCertificateChain)
            certStore.setCertPathOnly(nodeIdentityAlias, nodeIdentityCertificateChain)
        }
        logProgress("Node identity private key and certificate chain stored in $nodeIdentityAlias.")
    }

    private fun loadOrGenerateKeyPair(keyAlias: String, certificateRole: CertRole): PublicKey {
        return if (cryptoService.containsKey(keyAlias)) {
            cryptoService.getPublicKey(keyAlias)!!
        } else {
            val signatureScheme = if (certificateRole == CertRole.SERVICE_IDENTITY) {
                cryptoService.defaultIdentitySignatureScheme()
            } else {
                cryptoService.defaultTLSSignatureScheme()
            }
            cryptoService.generateKeyPair(keyAlias, signatureScheme)
        }
    }

    private fun getTlsCrlIssuerCert(): X509Certificate? {
        val tlsCrlIssuerCert = validateAndGetTlsCrlIssuerCert()
        if (tlsCrlIssuerCert == null && isTlsCrlIssuerCertRequired()) {
            logError("""tlsCrlIssuerCert config does not match the root certificate issuer and nor is there any other certificate in the trust store with a matching issuer.
                    | Please make sure the config is correct or that the correct certificate for the CRL issuer is added to the node's trust store.
                    | The node registration will now terminate.""".trimMargin())
            throw IllegalArgumentException("TLS CRL issuer certificate not found in the trust store.")
        }
        return tlsCrlIssuerCert
    }

    private fun validateCertificates(
            registeringPublicKey: PublicKey,
            registeringLegalName: CordaX500Name,
            expectedCertRole: CertRole,
            certificates: List<X509Certificate>
    ) {
        val receivedCertificate = certificates.first()

        val certificateSubject = try {
            CordaX500Name.build(receivedCertificate.subjectX500Principal)
        } catch (e: IllegalArgumentException) {
            throw CertificateRequestException("Received cert has invalid subject name: ${e.message}")
        }
        if (certificateSubject != registeringLegalName) {
            throw CertificateRequestException("Subject of received cert doesn't match with legal name: $certificateSubject")
        }

        val receivedCertRole = try {
            CertRole.extract(receivedCertificate)
        } catch (e: IllegalArgumentException) {
            throw CertificateRequestException("Unable to extract cert role from received cert: ${e.message}")
        }

        if (expectedCertRole != receivedCertRole) {
            throw CertificateRequestException("Received certificate contains invalid cert role, expected '$expectedCertRole', got '$receivedCertRole'.")
        }

        // Validate returned certificate is for the correct public key.
        if (Crypto.toSupportedPublicKey(certificates.first().publicKey) != Crypto.toSupportedPublicKey(registeringPublicKey)) {
            throw CertificateRequestException("Received certificate contains incorrect public key, expected '$registeringPublicKey', got '${certificates.first().publicKey}'.")
        }

        // Validate certificate chain returned from the doorman with the root cert obtained via out-of-band process, to prevent MITM attack on doorman server.
        X509Utilities.validateCertificateChain(rootCerts, certificates)
        logProgress("Certificate signing request approved, storing private key with the certificate chain.")
    }

    /**
     * Poll Certificate Signing Server for approved certificate,
     * enter a slow polling loop if server return null.
     * @param requestId Certificate signing request ID.
     * @return List of certificate chain.
     */
    private fun pollServerForCertificates(requestId: String): List<X509Certificate> {
        try {
            logProgress("Start polling server for certificate signing approval.")
            // Poll server to download the signed certificate once request has been approved.
            var idlePeriodDuration: Duration? = null
            while (true) {
                try {
                    val (pollInterval, certificates) = certService.retrieveCertificates(requestId)
                    if (certificates != null) {
                        return certificates
                    }
                    Thread.sleep(pollInterval.toMillis())
                } catch (e: ServiceUnavailableException) {
                    idlePeriodDuration = nextIdleDuration(idlePeriodDuration)
                    if (idlePeriodDuration != null) {
                        Thread.sleep(idlePeriodDuration.toMillis())
                    } else {
                        throw NodeRegistrationException("Compatibility Zone registration service is currently unavailable, "
                                + "try again later!.", e)
                    }
                }
            }
        } catch (certificateRequestException: CertificateRequestException) {
            certificateRequestException.message?.let { logError(it) }
            logError("Please make sure the details in configuration file are correct and try again.")
            logError("Corda node registration will now terminate.")
            requestIdStore.deleteIfExists()
            throw certificateRequestException
        }
    }

    /**
     * Submit Certificate Signing Request to Certificate signing service if request ID not found in file system.
     * New request ID will be stored in requestId.txt
     * @param publicKey public key for which we need a certificate.
     * @param legalName legal name of the entity for which we need a certificate.
     * @param certRole desired role of the entities certificate.
     * @param contentSigner the [ContentSigner] that will sign the CSR.
     * @return Request ID return from the server.
     */
    private fun submitOrResumeCertificateSigningRequest(
            publicKey: PublicKey,
            legalName: CordaX500Name,
            certRole: CertRole,
            contentSigner: ContentSigner
    ): String {
        try {
            // Retrieve request id from file if exists, else post a request to server.
            return if (!requestIdStore.exists()) {
                val request = X509Utilities.createCertificateSigningRequest(legalName.x500Principal, emailAddress, publicKey, contentSigner, certRole)
                val writer = StringWriter()
                JcaPEMWriter(writer).use {
                    it.writeObject(PemObject("CERTIFICATE REQUEST", request.encoded))
                }
                logProgress("Certificate signing request with the following information will be submitted to the Corda certificate signing server.")
                logProgress("Legal Name: $legalName")
                logProgress("Email: $emailAddress")
                logProgress("Public Key: $publicKey")
                logProgress("$writer")
                // Post request to signing server via http.
                logProgress("Submitting certificate signing request to Corda certificate signing server.")
                val requestId = certService.submitRequest(request)
                // Persists request ID to file in case of node shutdown.
                requestIdStore.writeLines(listOf(requestId))
                logProgress("Successfully submitted request to Corda certificate signing server, request ID: $requestId.")
                requestId
            } else {
                val requestId = requestIdStore.useLines { it.first() }
                logProgress("Resuming from previous certificate signing request, request ID: $requestId.")
                requestId
            }
        } catch (e: Exception) {
            throw if (e is ConnectException || e is ServiceUnavailableException || e is IOException) {
                NodeRegistrationException(e.message, e)
            } else e
        }
    }

    protected open fun onSuccess(publicKey: PublicKey, contentSigner: ContentSigner, certificates: List<X509Certificate>, tlsCrlCertificateIssuer: X500Name?) {}

    protected open fun validateAndGetTlsCrlIssuerCert(): X509Certificate? = null

    protected open fun isTlsCrlIssuerCertRequired(): Boolean = false
}

class NodeRegistrationConfiguration(
        val p2pSslOptions: MutualSslConfiguration,
        val myLegalName: CordaX500Name,
        val tlsCertCrlIssuer: X500Principal?,
        val tlsCertCrlDistPoint: URL?,
        val certificatesDirectory: Path,
        val emailAddress: String,
        val cryptoService: CryptoService,
        val certificateStore: CertificateStore,
        val notaryServiceConfig: NotaryServiceConfig? = null) {

    constructor(config: NodeConfiguration) : this(
            p2pSslOptions = config.p2pSslOptions,
            myLegalName = config.myLegalName,
            tlsCertCrlIssuer = config.tlsCertCrlIssuer,
            tlsCertCrlDistPoint = config.tlsCertCrlDistPoint,
            certificatesDirectory = config.certificatesDirectory,
            emailAddress = config.emailAddress,
            cryptoService = DefaultCryptoService(config.myLegalName.x500Principal, config.signingCertificateStore),
            certificateStore = config.signingCertificateStore.get(true),
            notaryServiceConfig = config.notary?.let {
                // Validation of the presence of the notary service legal name is only done here and not in the top level configuration
                // file. This is to maintain backwards compatibility with older notaries using the legacy identity structure. Older
                // notaries will be signing requests using the nodes legal identity key and therefore no separate notary service entity
                // exists. Just having the validation here prevents any new notaries from being created with the legacy identity scheme
                // but still allows drop in JAR replacements for old notaries.
                requireNotNull(it.serviceLegalName) {
                    "The notary service legal name must be provided via the 'notary.serviceLegalName' configuration parameter"
                }
                require(it.serviceLegalName != config.myLegalName) {
                    "The notary service legal name must be different from the node legal name"
                }
                NotaryServiceConfig(X509Utilities.DISTRIBUTED_NOTARY_KEY_ALIAS, it.serviceLegalName)
            }
    )
}

data class NotaryServiceConfig(
        val notaryServiceKeyAlias: String,
        val notaryServiceLegalName: CordaX500Name
)

class NodeRegistrationException(
        message: String?,
        cause: Throwable?
) : IOException(message ?: "Unable to contact node registration service", cause)

class NodeRegistrationHelper(
        private val config: NodeRegistrationConfiguration,
        certService: NetworkRegistrationService,
        regConfig: NodeRegistrationOption,
        computeNextIdleDoormanConnectionPollInterval: (Duration?) -> Duration? = FixedPeriodLimitedRetrialStrategy(10, Duration.ofMinutes(1)),
        logProgress: (String) -> Unit = ::println,
        logError: (String) -> Unit = System.err::println) :
            NetworkRegistrationHelper(
                    config,
                    certService,
                    regConfig.networkRootTrustStorePath,
                    regConfig.networkRootTrustStorePassword,
                    CORDA_CLIENT_CA,
                    CertRole.NODE_CA,
                    computeNextIdleDoormanConnectionPollInterval, logProgress, logError) {

    @Deprecated("Prefer to use NodeRegistrationConfiguration instead of NodeConfiguration")
    constructor(
            config: NodeConfiguration,
            certService: NetworkRegistrationService,
            regConfig: NodeRegistrationOption
    ) : this(NodeRegistrationConfiguration(config), certService, regConfig)

    companion object {
        val logger = contextLogger()
    }

    override fun onSuccess(publicKey: PublicKey, contentSigner: ContentSigner, certificates: List<X509Certificate>, tlsCrlCertificateIssuer: X500Name?) {
        createSSLKeystore(publicKey, contentSigner, certificates, tlsCrlCertificateIssuer)
        createTruststore()
    }

    private fun createSSLKeystore(nodeCaPublicKey: PublicKey, nodeCaContentSigner: ContentSigner, nodeCaCertificateChain: List<X509Certificate>, tlsCertCrlIssuer: X500Name?) {
        val keyStore = config.p2pSslOptions.keyStore
        val certificateStore = keyStore.get(createNew = true)
        certificateStore.update {
            logProgress("Generating SSL certificate for node messaging service.")
            val sslKeyPair = Crypto.generateKeyPair(X509Utilities.DEFAULT_TLS_SIGNATURE_SCHEME)
            val issuerCertificate = nodeCaCertificateChain.first()
            val validityWindow = X509Utilities.getCertificateValidityWindow(DEFAULT_VALIDITY_WINDOW.first, DEFAULT_VALIDITY_WINDOW.second, issuerCertificate)

            val sslCert = X509Utilities.createCertificate(
                    CertificateType.TLS,
                    issuerCertificate.subjectX500Principal,
                    nodeCaPublicKey,
                    nodeCaContentSigner,
                    config.myLegalName.x500Principal,
                    sslKeyPair.public,
                    validityWindow,
                    crlDistPoint = config.tlsCertCrlDistPoint?.toString(),
                    crlIssuer = tlsCertCrlIssuer)

            logger.info("Generated TLS certificate: $sslCert")

            val sslCertificateChain: List<X509Certificate> = listOf(sslCert) + nodeCaCertificateChain
            X509Utilities.validateCertificateChain(rootCerts, sslCertificateChain)
            setPrivateKey(CORDA_CLIENT_TLS, sslKeyPair.private, sslCertificateChain, keyStore.entryPassword)
        }
        logProgress("SSL private key and certificate chain stored in ${keyStore.path}.")
    }

    private fun createTruststore() {
        // Save root certificates to trust store.
        config.p2pSslOptions.trustStore.get(createNew = true).update {
            if (this.aliases().hasNext()) {
                logger.warn("The node's trust store already exists. The following certificates will be overridden: ${this.aliases().asSequence()}")
            }
            logProgress("Generating trust store for corda node.")
            // Copy all certificates from the network-trust-store.
            rootTrustStore.aliases().forEach {
                val certificate = rootTrustStore.getCertificate(it)
                logger.info("Copying trusted certificate to the node's trust store: Alias: $it, Certificate: $certificate")
                setCertificate(it, certificate)
            }
        }
        logProgress("Node trust store stored in ${config.p2pSslOptions.trustStore.path}.")
    }

    override fun validateAndGetTlsCrlIssuerCert(): X509Certificate? {
        val tlsCertCrlIssuer = config.tlsCertCrlIssuer
        tlsCertCrlIssuer ?: return null
        for (rootCert in rootCerts) {
            if (principalMatchesCertificatePrincipal(tlsCertCrlIssuer, rootCert)) {
                return rootCert
            }
        }
        return findMatchingCertificate(tlsCertCrlIssuer, rootTrustStore)
    }

    override fun isTlsCrlIssuerCertRequired(): Boolean {
        return config.tlsCertCrlIssuer != null
    }

    private fun findMatchingCertificate(principal: X500Principal, trustStore: X509KeyStore): X509Certificate? {
        trustStore.aliases().forEach {
            val certificate = trustStore.getCertificate(it)
            if (principalMatchesCertificatePrincipal(principal, certificate)) {
                return certificate
            }
        }
        return null
    }

    private fun principalMatchesCertificatePrincipal(principal: X500Principal, certificate: X509Certificate): Boolean {
        return certificate.subjectX500Principal.isEquivalentTo(principal)
    }
}

private class FixedPeriodLimitedRetrialStrategy(times: Int, private val period: Duration) : (Duration?) -> Duration? {

    init {
        require(times > 0){"Retry attempts must be larger than zero"}
    }

    private var counter = times

    override fun invoke(previousPeriod: Duration?): Duration? {
        synchronized(this) {
            return if (counter-- > 0) period else null
        }
    }
}
