StackOverflowError when OS has a lot of environment variables

[extbe]

Hi!

Versions
config v1.4.0
Java 11.0.2

Description
I try to run my application on OS with about 700 environment variables. I have a config file with substitution test = ${ENV_VARIABLE} and next code to create Config:

Config conf = ConfigFactory.systemProperties()
                .withFallback(ConfigFactory.parseResources("test.conf"))
                .withFallback(ConfigFactory.systemEnvironment())
                .resolve();

Also I limit thread stack size to 210k using -Xss210k, so the application fails with an error (I skipped the rest of stack trace because it is about 900 lines long)

Exception in thread "main" java.lang.StackOverflowError
	at java.base/java.util.Collections$UnmodifiableCollection$1.<init>(Collections.java:1041)
	at java.base/java.util.Collections$UnmodifiableCollection.iterator(Collections.java:1041)
	at java.base/java.util.Collections$UnmodifiableCollection$1.<init>(Collections.java:1042)
	at java.base/java.util.Collections$UnmodifiableCollection.iterator(Collections.java:1041)
	at java.base/java.util.Collections$UnmodifiableCollection$1.<init>(Collections.java:1042)
	at java.base/java.util.Collections$UnmodifiableCollection.iterator(Collections.java:1041)
	at java.base/java.util.Collections$UnmodifiableCollection$1.<init>(Collections.java:1042)
	at java.base/java.util.Collections$UnmodifiableCollection.iterator(Collections.java:1041)

I noticed that if I remove the substitution or .withFallback(ConfigFactory.systemEnvironment()) then the application starts.

I debugged and found that in the ResolveContext each call to pushTrace or popTrace wraps cycleMarkers into another UnmodifiableSet (because the constructor does it), even though it's already an UnmodifiableSet.

[extbe]

@havocp, is there anything else that has to be done before accepting the pull request? I've checked a modified version from the pull request and it seems that the issue has been solved.

[havocp]

Nothing else afaik, thanks for testing. Just not a lot of active maintenance happening here unfortunately so things are slow.