[SPARK-4323] Utils#fetchFile method should close lock file certainly

[sarutak]

In Utils#fetchFile method, lock file is created like as follows.

    val raf = new RandomAccessFile(lockFile, "rw")                                                                                                          
    // Only one executor entry.                                                                                                                             
    // The FileLock is only used to control synchronization for executors download file,                                                                    
    // it's always safe regardless of lock type (mandatory or advisory).                                                                                    
    val lock = raf.getChannel().lock()                                                                                                                      
    val cachedFile = new File(localDir, cachedFileName)                                                                                                     
    try {                                                                                                                                                   
      if (!cachedFile.exists()) {                                                                                                                           
        doFetchFile(url, localDir, cachedFileName, conf, securityMgr, hadoopConf)                                                                           
      }                                                                                                                                                     
    } finally {                                                                                                                                             
      lock.release()                                                                                                                                        
    }  

If some error occurs between opening RandomAccessFile and getting lock, lock file can be not closed.

[vanzin]

This is the only statement after the lock acquisition. It doesn't throw anything (unless you want to be pedantic and say it can throw an OutOfMemoryError), so it shouldn't cause problems. But if you want to be pedantic, you could achieve the exact same behavior by moving this statement one line up (before the lock acquisition). Which is much simpler than your patch.

[SparkQA]

Test build #23155 has started for PR 3186 at commit a2b30bc.

• This patch merges cleanly.

[SparkQA]

Test build #23155 has finished for PR 3186 at commit a2b30bc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/23155/
Test PASSed.

[pwendell]

The construction of a File cannot cause an exception, so I think the existing code is safe, right?

[sarutak]

As @vanzin said, I considered some corner cases like OOM. If we don't need to worry about that, I'll close this PR.