file seek in wlst / Jython 2.2.1 fails for lines longer than 8091 characters - jython

Question

For a CSV file generated in WLST / Jython 2.2.1 i want to update the header, the first line of the output file, when new metrics have been detected. This works fine by using seek to go to the first line and overwriting the line. But it fails when the number of characters of the first line exceeds 8091 characters.
I made simplified script which does reproduce the issue i am facing here.
#!/usr/bin/python
#
import sys
global maxheaderlength
global initheader
maxheaderlength=8092
logFilename = "test.csv"
# Create (overwrite existing) file
logfileAppender = open(logFilename,"w",0)
logfileAppender.write("." * maxheaderlength)
logfileAppender.write("\n")
logfileAppender.close()
# Append some lines
logfileAppender = open(logFilename,"a",0)
logfileAppender.write("2nd line\n")
logfileAppender.write("3rd line\n")
logfileAppender.write("4th line\n")
logfileAppender.write("5th line\n")
logfileAppender.close()
# Seek back to beginning of file and add data
logfileAppender = open(logFilename,"r+",0)
logfileAppender.seek(0) ;
header = "New Header Line" + "." * maxheaderlength
header = header[:maxheaderlength]
logfileAppender.write(header)
logfileAppender.close()
When maxheaderlength is 8091 or lower i do get the results as expected. The file test.csv starts with “New Header Line" followed by 8076 dots and
followed by the lines
2nd line
3rd line
4th line
5th line
When maxheaderlength is 8092> the test.csv results as a file starting with 8092 dots followed by "New Header Line" and then followed by 8077 dots. The 2nd ... 5th line are now show, probably overwritten by the dots.
Any idea how to work around or fix this ?

Answer 1

I too was able to reproduce this extremely odd behaviour and indeed it works correctly in Jython 2.5.3 so I think we can safely say this is a bug in 2.2.1 (which unfortunately you're stuck with for WLST).
My usual recourse in these circumstances is to fall back to using native Java methods. Changing the last block of code as follows seems to work as expected :-
# Seek back to beginning of file and add data
from java.io import RandomAccessFile
logfileAppender = RandomAccessFile(logFilename, "rw")
logfileAppender.seek(0) ;
header = "New Header Line" + "." * maxheaderlength
header = header[:maxheaderlength]
logfileAppender.writeBytes(header)
logfileAppender.close()