[MAPREDUCE-6238] MR2 can't run local jobs with -libjars command options which is a regression from MR1

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Fix Version/s: 2.6.1, 2.8.0, 2.7.1, 3.0.0-alpha1
• Component/s: mrv2
• Labels: 2.6.1-candidate
• Hadoop Flags: Reviewed
• Assignee: Zhihai Xu
• Reporter: Zhihai Xu
• Created: 31/Jan/15 08:51
• Updated: 06/Jan/17 00:44
• Resolved: 20/Apr/15 21:15

Description

MR2 can't run local jobs with -libjars command options which is a regression from MR1.
When run MR2 job with -jt local and -libjars, the job fails with java.io.FileNotFoundException: File does not exist: hdfs://XXXXXXXXXXXXXXX.jar.
But the same command is working in MR1.
I find the problem is
1.
because when MR2 run local job using LocalJobRunner
from JobSubmitter, the JobSubmitter#jtFs is local filesystem,
So copyRemoteFiles will return from the middle of the function
because source and destination file system are same.

    if (compareFs(remoteFs, jtFs)) {
      return originalPath;
    }

The following code at JobSubmitter.java
try to add the destination file to DistributedCache which introduce a bug for local job.

        Path newPath = copyRemoteFiles(libjarsDir, tmp, conf, replication);
        DistributedCache.addFileToClassPath(
            new Path(newPath.toUri().getPath()), conf);

Because new Path(newPath.toUri().getPath()) will lose the filesystem information from newPath, the file added to DistributedCache will use the default Uri filesystem hdfs based on the following code. This causes the
FileNotFoundException when we access the file later at
determineTimestampsAndCacheVisibilities

  public static void addFileToClassPath(Path file, Configuration conf)
    throws IOException {
	  addFileToClassPath(file, conf, file.getFileSystem(conf));
  }
  public static void addFileToClassPath
           (Path file, Configuration conf, FileSystem fs)
        throws IOException {
    String classpath = conf.get(MRJobConfig.CLASSPATH_FILES);
    conf.set(MRJobConfig.CLASSPATH_FILES, classpath == null ? file.toString()
             : classpath + "," + file.toString());
    URI uri = fs.makeQualified(file).toUri();
    addCacheFile(uri, conf);
  }

Compare to the following MR1 code:

        Path newPath = copyRemoteFiles(fs, libjarsDir, tmp, job, replication);
        DistributedCache.addFileToClassPath(
          new Path(newPath.toUri().getPath()), job, fs);

You will see why MR1 doesn't have this issue.
because it passes the local filesystem into DistributedCache#addFileToClassPath instead of using the default Uri filesystem hdfs.
2.
Another incompatible change in MR2 is in LocalDistributedCacheManager#setup

    // Find which resources are to be put on the local classpath
    Map<String, Path> classpaths = new HashMap<String, Path>();
    Path[] archiveClassPaths = DistributedCache.getArchiveClassPaths(conf);
    if (archiveClassPaths != null) {
      for (Path p : archiveClassPaths) {
        FileSystem remoteFS = p.getFileSystem(conf);
        p = remoteFS.resolvePath(p.makeQualified(remoteFS.getUri(),
            remoteFS.getWorkingDirectory()));
        classpaths.put(p.toUri().getPath().toString(), p);
      }
    }
    Path[] fileClassPaths = DistributedCache.getFileClassPaths(conf);
    if (fileClassPaths != null) {
      for (Path p : fileClassPaths) {
        FileSystem remoteFS = p.getFileSystem(conf);
        p = remoteFS.resolvePath(p.makeQualified(remoteFS.getUri(),
            remoteFS.getWorkingDirectory()));
        classpaths.put(p.toUri().getPath().toString(), p);
      }
    }

Similar code from MR1 is at TaskDistributedCacheManager#makeCacheFiles

        Map<String, Path> classPaths = new HashMap<String, Path>();
        if (paths != null) {
          for (Path p : paths) {
            classPaths.put(p.toUri().getPath().toString(), p);
            }
        }

I think we don't need call remoteFS.resolvePath to get the class path and
We can use the class path from DistributedCache.getFileClassPaths directly.
Also p.toUri().getPath().toString() will remove the filesystem information(scheme) and only keySet of classpaths is used(ValueSet of classpaths is not used).
It is better to do the same in MR2 to maintain backward compatible with MR1.

Issue Links

• is depended upon by: HIVE-5016 Local mode FileNotFoundException: File does not exist
• is duplicated by: MAPREDUCE-6289 libjars are assumed to be in the DistributedCache but are never added in pseudo distributed mode