The following code in TFile.KeyReigster.close:
byte lastKey = lastKeyBufferOS.getBuffer();
int lastLen = lastKeyBufferOS.size();
if (tfileMeta.getComparator().compare(key, 0, len, lastKey, 0,
lastLen) < 0)
throw new IOException("Keys are not added in sorted order");
compares the initial key (passed in via TFile.Writer.append) against a technically NULL lastKey. lastKey is not initialized until after the first call to TFile.Writer.append. The underlying RawComparator interface used for comparisons does not stipulate the proper behavior when either length 1 or length 2 is zero. In the case of LongWritable, its WritableComparator implementation does an unsafe read on the passed in byte arrays b1 and b2. Since TFile pre-allocates the buffer used for storing lastKey, this passes a valid buffer with zero count to LongWritable's comparator, which ignores length and thus produces incorrect results.