Code that previously worked correctly on JBoss 4 now returns the incorrect number of bytes. We have a servlet that uses the 3 argument version of output.write as follows:

output.write(bytes, offset, length);

According to the JavaDoc, 'length' specifies the number of bytes to write - however the bytes being returned seem to be length-offset which would suggest that undertow is treating 'length' as an end position. Changing our code so that 'length' includes the offset size fixes the issue but obviously this doesn't match the spec.