InstantiatingGrpcChannelProvider.java

/*
 * Copyright 2016 Google LLC
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google LLC nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package com.google.api.gax.grpc;

import com.google.api.core.ApiFunction;
import com.google.api.core.BetaApi;
import com.google.api.core.InternalApi;
import com.google.api.core.InternalExtensionOnly;
import com.google.api.gax.core.ExecutorProvider;
import com.google.api.gax.rpc.FixedHeaderProvider;
import com.google.api.gax.rpc.HeaderProvider;
import com.google.api.gax.rpc.TransportChannel;
import com.google.api.gax.rpc.TransportChannelProvider;
import com.google.auth.Credentials;
import com.google.auth.oauth2.ComputeEngineCredentials;
import com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.io.CharStreams;
import io.grpc.ManagedChannel;
import io.grpc.ManagedChannelBuilder;
import io.grpc.alts.ComputeEngineChannelBuilder;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Map;
import java.util.concurrent.Executor;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import javax.annotation.Nullable;
import org.threeten.bp.Duration;

/**
 * InstantiatingGrpcChannelProvider is a TransportChannelProvider which constructs a gRPC
 * ManagedChannel with a number of configured inputs every time getChannel(...) is called. These
 * inputs include a port, a service address, and credentials.
 *
 * <p>The credentials can either be supplied directly (by providing a FixedCredentialsProvider to
 * Builder.setCredentialsProvider()) or acquired implicitly from Application Default Credentials (by
 * providing a GoogleCredentialsProvider to Builder.setCredentialsProvider()).
 *
 * <p>The client lib header and generator header values are used to form a value that goes into the
 * http header of requests to the service.
 */
@InternalExtensionOnly
public final class InstantiatingGrpcChannelProvider implements TransportChannelProvider {
  static final String DIRECT_PATH_ENV_VAR = "GOOGLE_CLOUD_ENABLE_DIRECT_PATH";
  static final long DIRECT_PATH_KEEP_ALIVE_TIME_SECONDS = 3600;
  static final long DIRECT_PATH_KEEP_ALIVE_TIMEOUT_SECONDS = 20;
  // reduce the thundering herd problem of too many channels trying to (re)connect at the same time
  static final int MAX_POOL_SIZE = 1000;
  static final String GCE_PRODUCTION_NAME_PRIOR_2016 = "Google";
  static final String GCE_PRODUCTION_NAME_AFTER_2016 = "Google Compute Engine";

  private final int processorCount;
  private final Executor executor;
  private final HeaderProvider headerProvider;
  private final String endpoint;
  private final EnvironmentProvider envProvider;
  @Nullable private final GrpcInterceptorProvider interceptorProvider;
  @Nullable private final Integer maxInboundMessageSize;
  @Nullable private final Integer maxInboundMetadataSize;
  @Nullable private final Duration keepAliveTime;
  @Nullable private final Duration keepAliveTimeout;
  @Nullable private final Boolean keepAliveWithoutCalls;
  @Nullable private final Integer poolSize;
  @Nullable private final Credentials credentials;
  @Nullable private final ChannelPrimer channelPrimer;
  @Nullable private final Boolean attemptDirectPath;

  @Nullable
  private final ApiFunction<ManagedChannelBuilder, ManagedChannelBuilder> channelConfigurator;

  private InstantiatingGrpcChannelProvider(Builder builder) {
    this.processorCount = builder.processorCount;
    this.executor = builder.executor;
    this.headerProvider = builder.headerProvider;
    this.endpoint = builder.endpoint;
    this.envProvider = builder.envProvider;
    this.interceptorProvider = builder.interceptorProvider;
    this.maxInboundMessageSize = builder.maxInboundMessageSize;
    this.maxInboundMetadataSize = builder.maxInboundMetadataSize;
    this.keepAliveTime = builder.keepAliveTime;
    this.keepAliveTimeout = builder.keepAliveTimeout;
    this.keepAliveWithoutCalls = builder.keepAliveWithoutCalls;
    this.poolSize = builder.poolSize;
    this.channelConfigurator = builder.channelConfigurator;
    this.credentials = builder.credentials;
    this.channelPrimer = builder.channelPrimer;
    this.attemptDirectPath = builder.attemptDirectPath;
  }

  @Override
  public boolean needsExecutor() {
    return executor == null;
  }

  @Deprecated
  @Override
  public TransportChannelProvider withExecutor(ScheduledExecutorService executor) {
    return withExecutor((Executor) executor);
  }

  @Override
  public TransportChannelProvider withExecutor(Executor executor) {
    return toBuilder().setExecutor(executor).build();
  }

  @Override
  @BetaApi("The surface for customizing headers is not stable yet and may change in the future.")
  public boolean needsHeaders() {
    return headerProvider == null;
  }

  @Override
  @BetaApi("The surface for customizing headers is not stable yet and may change in the future.")
  public TransportChannelProvider withHeaders(Map<String, String> headers) {
    return toBuilder().setHeaderProvider(FixedHeaderProvider.create(headers)).build();
  }

  @Override
  public String getTransportName() {
    return GrpcTransportChannel.getGrpcTransportName();
  }

  @Override
  public boolean needsEndpoint() {
    return endpoint == null;
  }

  /**
   * Specify the endpoint the channel should connect to.
   *
   * <p>The value of {@code endpoint} must be of the form {@code host:port}.
   *
   * @param endpoint The endpoint to connect to
   * @return A new {@link InstantiatingGrpcChannelProvider} with the specified endpoint configured
   */
  @Override
  public TransportChannelProvider withEndpoint(String endpoint) {
    validateEndpoint(endpoint);
    return toBuilder().setEndpoint(endpoint).build();
  }

  @Override
  @BetaApi("The surface for customizing pool size is not stable yet and may change in the future.")
  public boolean acceptsPoolSize() {
    return poolSize == null;
  }

  @Override
  @BetaApi("The surface for customizing pool size is not stable yet and may change in the future.")
  public TransportChannelProvider withPoolSize(int size) {
    Preconditions.checkState(acceptsPoolSize(), "pool size already set to %s", poolSize);
    return toBuilder().setPoolSize(size).build();
  }

  @Override
  public boolean needsCredentials() {
    return credentials == null;
  }

  @Override
  public TransportChannelProvider withCredentials(Credentials credentials) {
    return toBuilder().setCredentials(credentials).build();
  }

  @Override
  public TransportChannel getTransportChannel() throws IOException {
    if (needsExecutor()) {
      throw new IllegalStateException("getTransportChannel() called when needsExecutor() is true");
    } else if (needsHeaders()) {
      throw new IllegalStateException("getTransportChannel() called when needsHeaders() is true");
    } else if (needsEndpoint()) {
      throw new IllegalStateException("getTransportChannel() called when needsEndpoint() is true");
    } else {
      return createChannel();
    }
  }

  private TransportChannel createChannel() throws IOException {

    int realPoolSize = MoreObjects.firstNonNull(poolSize, 1);
    ChannelFactory channelFactory =
        new ChannelFactory() {
          public ManagedChannel createSingleChannel() throws IOException {
            return InstantiatingGrpcChannelProvider.this.createSingleChannel();
          }
        };
    ManagedChannel outerChannel;
    if (channelPrimer != null) {
      outerChannel = ChannelPool.createRefreshing(realPoolSize, channelFactory);
    } else {
      outerChannel = ChannelPool.create(realPoolSize, channelFactory);
    }
    return GrpcTransportChannel.create(outerChannel);
  }

  // TODO(weiranf): Use attemptDirectPath as the only indicator once setAttemptDirectPath is adapted
  //                and the env var is removed from client environment.
  private boolean isDirectPathEnabled(String serviceAddress) {
    if (attemptDirectPath != null) {
      return attemptDirectPath;
    }
    // Only check DIRECT_PATH_ENV_VAR when attemptDirectPath is not set.
    String whiteList = envProvider.getenv(DIRECT_PATH_ENV_VAR);
    if (whiteList == null) return false;
    for (String service : whiteList.split(",")) {
      if (!service.isEmpty() && serviceAddress.contains(service)) return true;
    }
    return false;
  }

  // DirectPath should only be used on Compute Engine.
  // Notice Windows is supported for now.
  static boolean isOnComputeEngine() {
    String osName = System.getProperty("os.name");
    if ("Linux".equals(osName)) {
      String cmd = "cat /sys/class/dmi/id/product_name";
      try {
        Process process = Runtime.getRuntime().exec(new String[] {"/bin/sh", "-c", cmd});
        process.waitFor();
        String result =
            CharStreams.toString(new InputStreamReader(process.getInputStream(), "UTF-8"));
        return result.contains(GCE_PRODUCTION_NAME_PRIOR_2016)
            || result.contains(GCE_PRODUCTION_NAME_AFTER_2016);
      } catch (IOException | InterruptedException e) {
        return false;
      }
    }
    return false;
  }

  private ManagedChannel createSingleChannel() throws IOException {
    GrpcHeaderInterceptor headerInterceptor =
        new GrpcHeaderInterceptor(headerProvider.getHeaders());
    GrpcMetadataHandlerInterceptor metadataHandlerInterceptor =
        new GrpcMetadataHandlerInterceptor();

    int colon = endpoint.lastIndexOf(':');
    if (colon < 0) {
      throw new IllegalStateException("invalid endpoint - should have been validated: " + endpoint);
    }
    int port = Integer.parseInt(endpoint.substring(colon + 1));
    String serviceAddress = endpoint.substring(0, colon);

    ManagedChannelBuilder builder;

    // TODO(weiranf): Add API in ComputeEngineCredentials to check default service account.
    if (isDirectPathEnabled(serviceAddress)
        && credentials instanceof ComputeEngineCredentials
        && isOnComputeEngine()) {
      builder = ComputeEngineChannelBuilder.forAddress(serviceAddress, port);
      // Set default keepAliveTime and keepAliveTimeout when directpath environment is enabled.
      // Will be overridden by user defined values if any.
      builder.keepAliveTime(DIRECT_PATH_KEEP_ALIVE_TIME_SECONDS, TimeUnit.SECONDS);
      builder.keepAliveTimeout(DIRECT_PATH_KEEP_ALIVE_TIMEOUT_SECONDS, TimeUnit.SECONDS);

      // When channel pooling is enabled, force the pick_first grpclb strategy.
      // This is necessary to avoid the multiplicative effect of creating channel pool with
      // `poolSize` number of `ManagedChannel`s, each with a `subSetting` number of number of
      // subchannels.
      // See the service config proto definition for more details:
      // https://github.com/grpc/grpc-proto/blob/master/grpc/service_config/service_config.proto#L182
      ImmutableMap<String, Object> pickFirstStrategy =
          ImmutableMap.<String, Object>of("pick_first", ImmutableMap.of());

      ImmutableMap<String, Object> childPolicy =
          ImmutableMap.<String, Object>of("childPolicy", ImmutableList.of(pickFirstStrategy));

      ImmutableMap<String, Object> grpcLbPolicy =
          ImmutableMap.<String, Object>of("grpclb", childPolicy);

      ImmutableMap<String, Object> loadBalancingConfig =
          ImmutableMap.<String, Object>of("loadBalancingConfig", ImmutableList.of(grpcLbPolicy));

      builder.defaultServiceConfig(loadBalancingConfig);
    } else {
      builder = ManagedChannelBuilder.forAddress(serviceAddress, port);
    }
    builder =
        builder
            // See https://github.com/googleapis/gapic-generator/issues/2816
            .disableServiceConfigLookUp()
            .intercept(new GrpcChannelUUIDInterceptor())
            .intercept(headerInterceptor)
            .intercept(metadataHandlerInterceptor)
            .userAgent(headerInterceptor.getUserAgentHeader())
            .executor(executor);

    if (maxInboundMetadataSize != null) {
      builder.maxInboundMetadataSize(maxInboundMetadataSize);
    }
    if (maxInboundMessageSize != null) {
      builder.maxInboundMessageSize(maxInboundMessageSize);
    }
    if (keepAliveTime != null) {
      builder.keepAliveTime(keepAliveTime.toMillis(), TimeUnit.MILLISECONDS);
    }
    if (keepAliveTimeout != null) {
      builder.keepAliveTimeout(keepAliveTimeout.toMillis(), TimeUnit.MILLISECONDS);
    }
    if (keepAliveWithoutCalls != null) {
      builder.keepAliveWithoutCalls(keepAliveWithoutCalls);
    }
    if (interceptorProvider != null) {
      builder.intercept(interceptorProvider.getInterceptors());
    }
    if (channelConfigurator != null) {
      builder = channelConfigurator.apply(builder);
    }

    ManagedChannel managedChannel = builder.build();
    if (channelPrimer != null) {
      channelPrimer.primeChannel(managedChannel);
    }
    return managedChannel;
  }

  /** The endpoint to be used for the channel. */
  public String getEndpoint() {
    return endpoint;
  }

  /** The time without read activity before sending a keepalive ping. */
  public Duration getKeepAliveTime() {
    return keepAliveTime;
  }

  /** The time without read activity after sending a keepalive ping. */
  public Duration getKeepAliveTimeout() {
    return keepAliveTimeout;
  }

  /** Whether keepalive will be performed when there are no outstanding RPCs. */
  public Boolean getKeepAliveWithoutCalls() {
    return keepAliveWithoutCalls;
  }

  /** The maximum metadata size allowed to be received on the channel. */
  @BetaApi("The surface for maximum metadata size is not stable yet and may change in the future.")
  public Integer getMaxInboundMetadataSize() {
    return maxInboundMetadataSize;
  }

  @Override
  public boolean shouldAutoClose() {
    return true;
  }

  public Builder toBuilder() {
    return new Builder(this);
  }

  public static Builder newBuilder() {
    return new Builder();
  }

  public static final class Builder {
    private int processorCount;
    private Executor executor;
    private HeaderProvider headerProvider;
    private String endpoint;
    private EnvironmentProvider envProvider;
    @Nullable private GrpcInterceptorProvider interceptorProvider;
    @Nullable private Integer maxInboundMessageSize;
    @Nullable private Integer maxInboundMetadataSize;
    @Nullable private Duration keepAliveTime;
    @Nullable private Duration keepAliveTimeout;
    @Nullable private Boolean keepAliveWithoutCalls;
    @Nullable private Integer poolSize;
    @Nullable private ApiFunction<ManagedChannelBuilder, ManagedChannelBuilder> channelConfigurator;
    @Nullable private Credentials credentials;
    @Nullable private ChannelPrimer channelPrimer;
    @Nullable private Boolean attemptDirectPath;

    private Builder() {
      processorCount = Runtime.getRuntime().availableProcessors();
      envProvider = DirectPathEnvironmentProvider.getInstance();
    }

    private Builder(InstantiatingGrpcChannelProvider provider) {
      this.processorCount = provider.processorCount;
      this.executor = provider.executor;
      this.headerProvider = provider.headerProvider;
      this.endpoint = provider.endpoint;
      this.envProvider = provider.envProvider;
      this.interceptorProvider = provider.interceptorProvider;
      this.maxInboundMessageSize = provider.maxInboundMessageSize;
      this.maxInboundMetadataSize = provider.maxInboundMetadataSize;
      this.keepAliveTime = provider.keepAliveTime;
      this.keepAliveTimeout = provider.keepAliveTimeout;
      this.keepAliveWithoutCalls = provider.keepAliveWithoutCalls;
      this.poolSize = provider.poolSize;
      this.channelConfigurator = provider.channelConfigurator;
      this.credentials = provider.credentials;
      this.channelPrimer = provider.channelPrimer;
      this.attemptDirectPath = provider.attemptDirectPath;
    }

    /** Sets the number of available CPUs, used internally for testing. */
    Builder setProcessorCount(int processorCount) {
      this.processorCount = processorCount;
      return this;
    }

    /**
     * Sets the Executor for this TransportChannelProvider.
     *
     * <p>This is optional; if it is not provided, needsExecutor() will return true, meaning that an
     * Executor must be provided when getChannel is called on the constructed
     * TransportChannelProvider instance. Note: GrpcTransportProvider will automatically provide its
     * own Executor in this circumstance when it calls getChannel.
     */
    public Builder setExecutor(Executor executor) {
      this.executor = executor;
      return this;
    }

    /** @deprecated Please use {@link #setExecutor(Executor)}. */
    @Deprecated
    public Builder setExecutorProvider(ExecutorProvider executorProvider) {
      return setExecutor((Executor) executorProvider.getExecutor());
    }

    /**
     * Sets the HeaderProvider for this TransportChannelProvider.
     *
     * <p>This is optional; if it is not provided, needsHeaders() will return true, meaning that
     * headers must be provided when getChannel is called on the constructed
     * TransportChannelProvider instance.
     */
    public Builder setHeaderProvider(HeaderProvider headerProvider) {
      this.headerProvider = headerProvider;
      return this;
    }

    /** Sets the endpoint used to reach the service, eg "localhost:8080". */
    public Builder setEndpoint(String endpoint) {
      validateEndpoint(endpoint);
      this.endpoint = endpoint;
      return this;
    }

    /**
     * Sets the GrpcInterceptorProvider for this TransportChannelProvider.
     *
     * <p>The provider will be called once for each underlying gRPC ManagedChannel that is created.
     * It is recommended to return a new list of new interceptors on each call so that interceptors
     * are not shared among channels, but this is not required.
     */
    public Builder setInterceptorProvider(GrpcInterceptorProvider interceptorProvider) {
      this.interceptorProvider = interceptorProvider;
      return this;
    }

    public String getEndpoint() {
      return endpoint;
    }

    /** The maximum message size allowed to be received on the channel. */
    public Builder setMaxInboundMessageSize(Integer max) {
      this.maxInboundMessageSize = max;
      return this;
    }

    /** The maximum message size allowed to be received on the channel. */
    public Integer getMaxInboundMessageSize() {
      return maxInboundMessageSize;
    }

    /** The maximum metadata size allowed to be received on the channel. */
    @BetaApi(
        "The surface for maximum metadata size is not stable yet and may change in the future.")
    public Builder setMaxInboundMetadataSize(Integer max) {
      this.maxInboundMetadataSize = max;
      return this;
    }

    /** The maximum metadata size allowed to be received on the channel. */
    @BetaApi(
        "The surface for maximum metadata size is not stable yet and may change in the future.")
    public Integer getMaxInboundMetadataSize() {
      return maxInboundMetadataSize;
    }

    /** The time without read activity before sending a keepalive ping. */
    public Builder setKeepAliveTime(Duration duration) {
      this.keepAliveTime = duration;
      return this;
    }

    /** The time without read activity before sending a keepalive ping. */
    public Duration getKeepAliveTime() {
      return keepAliveTime;
    }

    /** The time without read activity after sending a keepalive ping. */
    public Builder setKeepAliveTimeout(Duration duration) {
      this.keepAliveTimeout = duration;
      return this;
    }

    /** The time without read activity after sending a keepalive ping. */
    public Duration getKeepAliveTimeout() {
      return keepAliveTimeout;
    }

    /** Whether keepalive will be performed when there are no outstanding RPCs. */
    public Builder setKeepAliveWithoutCalls(Boolean keepalive) {
      this.keepAliveWithoutCalls = keepalive;
      return this;
    }

    /** Whether keepalive will be performed when there are no outstanding RPCs. */
    public Boolean getKeepAliveWithoutCalls() {
      return keepAliveWithoutCalls;
    }

    /**
     * Number of underlying grpc channels to open. Calls will be load balanced round robin across
     * them.
     */
    public int getPoolSize() {
      if (poolSize == null) {
        return 1;
      }
      return poolSize;
    }

    /**
     * Number of underlying grpc channels to open. Calls will be load balanced round robin across
     * them
     */
    public Builder setPoolSize(int poolSize) {
      Preconditions.checkArgument(poolSize > 0, "Pool size must be positive");
      Preconditions.checkArgument(
          poolSize <= MAX_POOL_SIZE, "Pool size must be less than %s", MAX_POOL_SIZE);
      this.poolSize = poolSize;
      return this;
    }

    /** Sets the number of channels relative to the available CPUs. */
    public Builder setChannelsPerCpu(double multiplier) {
      return setChannelsPerCpu(multiplier, 100);
    }

    public Builder setChannelsPerCpu(double multiplier, int maxChannels) {
      Preconditions.checkArgument(multiplier > 0, "multiplier must be positive");
      Preconditions.checkArgument(maxChannels > 0, "maxChannels must be positive");

      int channelCount = (int) Math.ceil(processorCount * multiplier);
      if (channelCount > maxChannels) {
        channelCount = maxChannels;
      }
      return setPoolSize(channelCount);
    }

    public Builder setCredentials(Credentials credentials) {
      this.credentials = credentials;
      return this;
    }

    /**
     * By setting a channelPrimer, the ChannelPool created by the provider will be refreshing
     * ChannelPool. channelPrimer will be invoked periodically when the channels are refreshed
     *
     * <p>This is public only for technical reasons, for advanced usage.
     *
     * @param channelPrimer invoked when the channels are refreshed
     * @return builder for the provider
     */
    @InternalApi("For internal use by google-cloud-java clients only")
    public Builder setChannelPrimer(ChannelPrimer channelPrimer) {
      this.channelPrimer = channelPrimer;
      return this;
    }

    /** Whether attempt DirectPath. */
    @InternalApi("For internal use by google-cloud-java clients only")
    public Builder setAttemptDirectPath(boolean attemptDirectPath) {
      this.attemptDirectPath = attemptDirectPath;
      return this;
    }

    public InstantiatingGrpcChannelProvider build() {
      return new InstantiatingGrpcChannelProvider(this);
    }

    /**
     * Add a callback that can intercept channel creation.
     *
     * <p>This can be used for advanced configuration like setting the netty event loop. The
     * callback will be invoked with a fully configured channel builder, which the callback can
     * augment or replace.
     */
    @BetaApi("Surface for advanced channel configuration is not yet stable")
    public Builder setChannelConfigurator(
        @Nullable ApiFunction<ManagedChannelBuilder, ManagedChannelBuilder> channelConfigurator) {
      this.channelConfigurator = channelConfigurator;
      return this;
    }

    @Nullable
    public ApiFunction<ManagedChannelBuilder, ManagedChannelBuilder> getChannelConfigurator() {
      return channelConfigurator;
    }
  }

  private static void validateEndpoint(String endpoint) {
    int colon = endpoint.lastIndexOf(':');
    if (colon < 0) {
      throw new IllegalArgumentException(
          String.format("invalid endpoint, expecting \"<host>:<port>\""));
    }
    Integer.parseInt(endpoint.substring(colon + 1));
  }

  /**
   * EnvironmentProvider currently provides DirectPath environment variable, and is only used during
   * initial rollout for DirectPath. This interface will be removed once the DirectPath environment
   * is not used.
   */
  interface EnvironmentProvider {
    @Nullable
    String getenv(String env);
  }

  static class DirectPathEnvironmentProvider implements EnvironmentProvider {
    private static DirectPathEnvironmentProvider provider;

    private DirectPathEnvironmentProvider() {}

    public static DirectPathEnvironmentProvider getInstance() {
      if (provider == null) {
        provider = new DirectPathEnvironmentProvider();
      }
      return provider;
    }

    @Override
    public String getenv(String env) {
      return System.getenv(env);
    }
  }
}