src/main/java/com/google/devtools/build/lib/remote/common/LazyFileOutputStream.java

// Copyright 2021 The Bazel Authors. All rights reserved.
//
// 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.google.devtools.build.lib.remote.common;

import com.google.devtools.build.lib.vfs.Path;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

/**
 * Creates an {@link OutputStream} backed by a file that isn't actually opened until the first data
 * is written. This is useful to only have as many open file descriptors as necessary at a time to
 * avoid running into system limits.
 */
public class LazyFileOutputStream extends OutputStream {

  private final Path path;
  private OutputStream out;

  public LazyFileOutputStream(Path path) {
    this.path = path;
  }

  @Override
  public void write(byte[] b) throws IOException {
    ensureOpen();
    out.write(b);
  }

  @Override
  public void write(byte[] b, int off, int len) throws IOException {
    ensureOpen();
    out.write(b, off, len);
  }

  @Override
  public void write(int b) throws IOException {
    ensureOpen();
    out.write(b);
  }

  @Override
  public void flush() throws IOException {
    ensureOpen();
    out.flush();
  }

  @Override
  public void close() throws IOException {
    ensureOpen();
    out.close();
  }

  /**
   * If the output stream is a {@link FileOutputStream}, call {@link FileDescriptor#sync} on it.
   * Otherwise, do nothing.
   */
  public void syncIfPossible() throws IOException {
    ensureOpen();
    if (out instanceof FileOutputStream) {
      ((FileOutputStream) out).getFD().sync();
    }
  }

  private void ensureOpen() throws IOException {
    if (out == null) {
      out = path.getOutputStream();
    }
  }
}