Fix non-deterministic behavior in testMultipartFormData

[ZhanwangZhou]

Description

I encountered non-deterministic behavior while running the following tests using NonDex:

core.src.test.java.org.nanohttpd.junit.protocols.http.HttpServerTest.testMultipartFormData

Steps to reproduce

NonDex: https://github.com/TestingResearchIllinois/NonDex
Run the tests with NonDex:

mvn -pl core edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=org.nanohttpd.junit.protocols.http.HttpServerTest#testMultipartFormData

The error message shows:

Click to view

Running org.nanohttpd.junit.protocols.http.HttpServerTest
Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.549 sec <<< FAILURE! - in org.nanohttpd.junit.protocols.http.HttpServerTest
testMultipartFormData(org.nanohttpd.junit.protocols.http.HttpServerTest)  Time elapsed: 0.548 sec  <<< FAILURE!
org.junit.ComparisonFailure: Server failed multi-part data parsecommentbin expected:<[bincomment]> but was:<[commentbin]>
	at org.junit.Assert.assertEquals(Assert.java:115)
	at org.nanohttpd.junit.protocols.http.HttpServerTest.testMultipartFormData(HttpServerTest.java:265)


Results :

Failed tests:
  HttpServerTest.testMultipartFormData:265 Server failed multi-part data parsecommentbin expected:<[bincomment]> but was:<[commentbin]>

Tests run: 1, Failures: 1, Errors: 0, Skipped: 0

INFO: Surefire failed when running tests for ushu+f2EpyqhU1Vs8NANOwdxqiB0jaCdmLz34a24Yc=
INFO: Adding excluded groups to newly created one
INFO: Creating new argline for Surefire
CONFIG: nondexFilter=.*
nondexMode=FULL
nondexSeed=974622
nondexStart=0
nondexEnd=9223372036854775807
nondexPrintstack=false
nondexDir=/Users/zhouzhanwang/Desktop/nanohttpd/core/.nondex
nondexJarDir=/Users/zhouzhanwang/Desktop/nanohttpd/core/.nondex
nondexExecid=twViEYQ2+WftkXILPlPC9xKbHbzOzznkmS0MN2crxEU=
nondexLogging=CONFIG
test=
[INFO] Surefire report directory: /Users/zhouzhanwang/Desktop/nanohttpd/core/.nondex/twViEYQ2+WftkXILPlPC9xKbHbzOzznkmS0MN2crxEU=
[INFO] parallel='none', perCoreThreadCount=true, threadCount=0, useUnlimitedThreads=false, threadCountSuites=0, threadCountClasses=0, threadCountMethods=0, parallelOptimized=true

Potential Solution

The issue occurs due to non-deterministic behavior of function public Response serve(IHTTPSession session) in testMultipartFormData. The function iterates through the key set of Map<String, String> object files and then appends the keys to StringBuilder object responseMsg. However, the order of the key set of a Map object in Java is non-deterministic, which means the keys appended to responseMsg can be in any order. This causes the String variable line at line 266 to be either "bincomment" or "commentbin", and thus assertEquals at line 268 could fail.

Here I propose a solution that may fix the issue. Before we iterate the key set of the map object, we can store all the keys in a List<String> object and sort the list. Now, the order of keys in the list becomes deterministic, and we can iterate through the list and append the keys to responseMsg in a deterministic order. In this way, line variable will always be "bincomment", the same as the expected value.