RFR(S): 8240226: DeflateIn_InflateOut.java test incorrectly assumes size of compressed file

[Volker Simonis]

Hi,

can I please get a review for this trivial test fix:

http://cr.openjdk.java.net/~simonis/webrevs/2020/8240226/
https://bugs.openjdk.java.net/browse/JDK-8240226

The SkipBytes() subtest in DeflateIn_InflateOut.java executes several
steps and for each step it deflates an array of 1024*1024 bytes. In
every step, before deflation is performed, the array is refilled with
random bytes. However, only after the first step, the size of the
generated, deflated stream is checked and recorded. The following
subtests all assume that deflating the array filled with the same
number of, but different, random bytes will produce a deflated stream
of the same size.

This assumption is wrong. Depending on the exact contents of the
source array, the size of the generated, deflated result might
slightly differ. For some reason, the deflated result produced by the
default zlib implementation seems to always have the same size
(although there's no guarantee for this), but other zlib
implementations sometimes produce differently sized, deflated results.

The problem can easily be reproduced by placing a version of
Cloudflare's zlib clone (https://github.com/cloudflare/zlib) on the
LD_LIBRARY_PATH (for jdk versions configured with
"--with-zlib=system", which is the default since jdk11):

$ LD_LIBRARY_PATH=/ziptests/build/zlib-cloudflare
./images/jdk/bin/java DeflateIn_InflateOut
java.lang.Exception: Stack trace
at java.base/java.lang.Thread.dumpStack(Thread.java:1379)
at DeflateIn_InflateOut.fail(DeflateIn_InflateOut.java:235)
at DeflateIn_InflateOut.check(DeflateIn_InflateOut.java:238)
at DeflateIn_InflateOut.SkipBytes(DeflateIn_InflateOut.java:204)
at DeflateIn_InflateOut.realMain(DeflateIn_InflateOut.java:229)
at DeflateIn_InflateOut.main(DeflateIn_InflateOut.java:243)

Passed = 16 failed = 1
Exception in thread "main" java.lang.AssertionError: Some tests failed
at DeflateIn_InflateOut.main(DeflateIn_InflateOut.java:245)

The test may (and actually does) randomly fail at any of the subtests
executed in SkipBytes() which reuse "numReadable", the size of the
first deflated stream generated in that method.

The fix is trivial. It is unnecessary to refill the source array with
random bytes before each subtest. Filling the array one time at test
startup time should be sufficient.

Thank you and best regards,
Volker