[Bug classpath/42551] New: MetaMessage.java does not write out stored data correctly

[pdfernhout at kurtz-fernhout dot com]

After many hours of frustration, :-) it turns out that in MetaMessage.java
[classpath] / classpath / javax / sound / midi / MetaMessage.java 
http://cvs.savannah.gnu.org/viewvc/classpath/javax/sound/midi/MetaMessage.java?root=classpath&view=log

there is a function:

  public void setMessage(int type, byte[] data, int length)

that includes the code snippet:

   while ((length >>= 7) > 0)
    {
      buffer <<= 8;
      buffer |= ((length & 0x7F) | 0x80);
    }

This code section has a side effect of setting length to zero. But the function
later uses length in copying a data array:

    // Now copy the real data.
    System.arraycopy(data, 0, this.data, index, length);

Thus the passed in data is not copied because length is by then zero, leaving
the instance's own data array with zeros in those end positions. 

As a consequence, MIDI events like Tempo events and TimeSignature events will
be written incorrectly to files, which can even lead to subsequent problems in
applications that read the generated MIDI files if they are not expecting zeros
there. This even caused a crash in one application I tried to use to analyze
the resulting problematical files, thus leading me to guess that the files were
malformed somehow.

A possible fix that is working for me:

    // BUG FIX -- Need the original length later
    int lengthCopy = length;
    while ((lengthCopy >>= 7) > 0)
    {
      buffer <<= 8;
      buffer |= ((lengthCopy & 0x7F) | 0x80);
    }

This bug has apparently been in all versions of the code I looked at from the
latest 0.98 to the earliest code for that file. It would be easy to miss that
assign operator in what might just look like a test.

I hereby put those changes in the public domain.


-- 
           Summary: MetaMessage.java does not write out stored data
                    correctly
           Product: classpath
           Version: 0.98
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: classpath
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pdfernhout at kurtz-fernhout dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42551