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Rate limiting for Apache James email processing
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package org.apache.james.transport.mailets

import java.time.Duration
import java.util
import com.google.common.annotations.VisibleForTesting
import com.google.common.collect.ImmutableList

import javax.inject.Inject
import org.apache.james.core.MailAddress
import org.apache.james.rate.limiter.api.{AcceptableRate, RateExceeded, RateLimiter, RateLimiterFactory, RateLimitingKey, RateLimitingResult}
import org.apache.james.transport.mailets.ConfigurationOps.DurationOps
import org.apache.mailet.base.GenericMailet
import org.apache.mailet.{Mail, ProcessingState}
import org.reactivestreams.Publisher
import reactor.core.scala.publisher.{SFlux, SMono}

case class PerSenderRateLimiter(rateLimiter: Option[RateLimiter], keyPrefix: Option[KeyPrefix], entityType: EntityType) {
  def rateLimit(sender: MailAddress, mail: Mail): Publisher[RateLimitingResult] = {
    val rateLimitingKey = SenderKey(keyPrefix, entityType, sender)

    rateLimiter.map(limiter =>
      EntityType.extractQuantity(entityType, mail)
        .map(increment => limiter.rateLimit(rateLimitingKey, increment))
        .getOrElse(SMono.just[RateLimitingResult](RateExceeded)))
      .getOrElse(SMono.just[RateLimitingResult](AcceptableRate))
  }
}

case class SenderKey(keyPrefix: Option[KeyPrefix], entityType: EntityType, mailAddress: MailAddress) extends RateLimitingKey {
  override def asString(): String = keyPrefix.map(prefix => s"${prefix.value}_${entityType.asString}_${mailAddress.asString()}")
    .getOrElse(s"${entityType.asString}_${mailAddress.asString()}")
}

/**
 * PerSenderRateLimit allows defining and enforcing rate limits for the sender of matching emails.
 *
 * This allows writing rules like:
 *   A sender can send 10 emails per hour
 *   A sender can send email to a total of 20 recipients per hour
 *   A sender can send 100 MB of emails (total computed taking only the email size into account) per hour
 *   A sender can send 200 MB of emails (total computed taking each recipient copies into account) per hour
 * 
 *
 * Depending on its position and the matcher it is being combined with, those rate limiting rules could be applied to
 * submitted emails, received emails or emitted email being relayed to third parties.
 *
 *  Here are supported configuration parameters:
 *    keyPrefix: An optional key prefix to apply to rate limiting. Choose distinct values if you specify
 *    this mailet twice within your mailetcontainer.xml file. Defaults to none.
 *    exceededProcessor: Processor to which emails whose rate is exceeded should be redirected to. Defaults to error.
 *    Use this to customize the behaviour upon exceeded rate.
 *    duration: Duration during which the rate limiting shall be applied. Compulsory, must be a valid duration of at least one second. Supported units includes s (second), m (minute), h (hour), d (day).
 *    precision: Defines the time precision that will be used to approximate the sliding window. A lower duration increases precision but requests more computing power. The precision must be greater than 1 second. Optional, default to duration means fixed window counters. Supported units includes s (second), m (minute), h (hour), d (day).
 *    count: Count of emails allowed for a given sender during duration. Optional, if unspecified this rate limit is not applied.
 *    recipients: Count of recipients allowed for a given sender during duration. Optional, if unspecified this rate limit is not applied.
 *    size: Size of emails allowed for a given sender during duration (each email count one time, regardless of recipient count). Optional, if unspecified this rate limit is not applied. Supported units : B ( 2^0 ), K ( 2^10 ), M ( 2^20 ), G ( 2^30 ), defaults to B.
 *    totalSize: Size of emails allowed for a given sender during duration (each recipient of the email email count one time). Optional, if unspecified this rate limit is not applied. Supported units : B ( 2^0 ), K ( 2^10 ), M ( 2^20 ), G ( 2^30 ), defaults to B. Note that
 *    totalSize is limited in increments of 2exp(31) - ~2 billions: sending a 10MB file to more than 205 recipients will be rejected if this parameter is enabled.
 *  
 *
 *  For instance, to apply all the examples given above:
 *
 *   
 * <mailet matcher="All" class="PerSenderRateLimit">
 *     <keyPrefix>myPrefix</keyPrefix>
 *     <duration>1h</duration>
 *     <precision>1h</precision>
 *     <count>10</count>
 *     <recipients>20</recipients>
 *     <size>100M</size>
 *     <totalSize>200M</totalSize>
 *     <exceededProcessor>tooMuchMails</exceededProcessor>
 * </mailet>
 *   
 *
 *  Note that to use this extension you need to place the rate-limiter JAR in the extensions-jars folder
 *  and need to configure a viable option to invoke RateLimiterFactory which can be done by
 *  loading org.apache.james.rate.limiter.memory.MemoryRateLimiterModule Guice module within the
 *  guice.extension.module in extensions.properties configuration file. Note that other Rate
 *  limiter implementation might require extra configuration parameters within your mailet.
 *
 * @param rateLimiterFactory Allows instantiations of the underlying rate limiters.
 */
class PerSenderRateLimit @Inject()(rateLimiterFactory: RateLimiterFactory) extends GenericMailet {
  private var countRateLimiter: PerSenderRateLimiter = _
  private var recipientsRateLimiter: PerSenderRateLimiter = _
  private var sizeRateLimiter: PerSenderRateLimiter = _
  private var totalSizeRateLimiter: PerSenderRateLimiter = _
  private var exceededProcessor: String = _
  private var keyPrefix: Option[KeyPrefix] = _

  override def init(): Unit = {
    val duration: Duration = parseDuration()
    val precision: Option[Duration] = getMailetConfig.getDuration("precision")

    keyPrefix = Option(getInitParameter("keyPrefix")).map(KeyPrefix)
    exceededProcessor = getInitParameter("exceededProcessor", Mail.ERROR)

    def perSenderRateLimiter(entityType: EntityType): PerSenderRateLimiter = createRateLimiter(rateLimiterFactory, entityType, keyPrefix, duration, precision)

    countRateLimiter = perSenderRateLimiter(Count)
    recipientsRateLimiter = perSenderRateLimiter(Recipients)
    sizeRateLimiter = perSenderRateLimiter(Size)
    totalSizeRateLimiter = perSenderRateLimiter(TotalSize)
  }

  @VisibleForTesting
  def parseDuration(): Duration = getMailetConfig.getDuration("duration")
    .getOrElse(throw new IllegalArgumentException("'duration' is compulsory"))

  override def service(mail: Mail): Unit = mail.getMaybeSender
      .asOptional()
      .ifPresent(sender => {
        applyRateLimiter(mail, sender)
      })

  private def applyRateLimiter(mail: Mail, sender: MailAddress): Unit = {
    val pivot: RateLimitingResult = AcceptableRate
    val result = SFlux.merge(Seq(
        countRateLimiter.rateLimit(sender, mail),
        recipientsRateLimiter.rateLimit(sender, mail),
        sizeRateLimiter.rateLimit(sender, mail),
        totalSizeRateLimiter.rateLimit(sender, mail)))
      .fold(pivot)((a, b) => a.merge(b))
      .block()

    if (result.equals(RateExceeded)) {
      mail.setState(exceededProcessor)
    }
  }

  private def createRateLimiter(rateLimiterFactory: RateLimiterFactory, entityType: EntityType, keyPrefix: Option[KeyPrefix],
                                duration: Duration, precision: Option[Duration]): PerSenderRateLimiter =
    PerSenderRateLimiter(rateLimiter = EntityType.extractRules(entityType, duration, getMailetConfig)
      .map(rateLimiterFactory.withSpecification(_, precision)),
      keyPrefix = keyPrefix,
      entityType = entityType)

  override def requiredProcessingState(): util.Collection[ProcessingState] = ImmutableList.of(new ProcessingState(exceededProcessor))
}