Define clear semantics for Directory.fileLength
-----------------------------------------------

                 Key: LUCENE-2316
                 URL: https://issues.apache.org/jira/browse/LUCENE-2316
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Index
            Reporter: Shai Erera
            Priority: Minor
             Fix For: 3.1


On this thread: 
http://mail-archives.apache.org/mod_mbox/lucene-java-dev/201003.mbox/%3c126142c1003121525v24499625u1589bbef4c079...@mail.gmail.com%3e
 it was mentioned that Directory's fileLength behavior is not consistent 
between Directory implementations if the given file name does not exist. 
FSDirectory returns a 0 length while RAMDirectory throws FNFE.

The problem is that the semantics of fileLength() are not defined. As proposed 
in the thread, we'll define the following semantics:

* Returns the length of the file denoted by <code>name</code> if the file 
exists. The return value may be anything between 0 and Long.MAX_VALUE.
* Throws FileNotFoundException if the file does not exist. Note that you can 
call dir.fileExists(name) if you are not sure whether the file exists or not.

For backwards we'll create a new method w/ clear semantics. Something like:

{code}
/**
 * @deprecated the method will become abstract when #fileLength(name) has been 
removed.
 */
public long getFileLength(String name) throws IOException {
  long len = fileLength(name);
  if (len == 0 && !fileExists(name)) {
    throw new FileNotFoundException(name);
  }
  return len;
}
{code}

The first line just calls the current impl. If it throws exception for a 
non-existing file, we're ok. The second line verifies whether a 0 length is for 
an existing file or not and throws an exception appropriately.

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