gsub's implementation causes stack overflows

[Jiehong]

Hello,

Using jackson-jq v0.0.13 (happens in v1.0 too), the implementation of gsub is prone to stack overflows.

Here is a reproducer:

  @Test
  public void gsubStackoverflow() throws IOException {
    final Scope rootScope = Scope.newEmptyScope();
    rootScope.loadFunctions(Thread.currentThread().getContextClassLoader());
    final Scope childScope = Scope.newChildScope(rootScope);

    final String s = IntStream.range(0, 3500).mapToObj(i -> Integer.valueOf(i).toString()).collect(Collectors.joining(","));
    final String json = String.format("\"%s\"", s);
    final JsonQuery query = JsonQuery.compile(". | gsub(\",\";\", \")");
    final JsonNode input = new ObjectMapper().readTree(json);
    query.apply(childScope, input);
  }

(3500 is just the value that triggers it on my machine, but the value can be a lot lower depending on the available memory. This only simulates a long string containing "1,2,3..." and where gsub is used to add a space after each comma)

And the result is:

java.lang.StackOverflowError
	at net.thisptr.jackson.jq.internal.functions._SubImplFunction.concat(_SubImplFunction.java:50)
	at net.thisptr.jackson.jq.internal.functions._SubImplFunction.concat(_SubImplFunction.java:63)
	at net.thisptr.jackson.jq.internal.functions._SubImplFunction.concat(_SubImplFunction.java:63)
	at net.thisptr.jackson.jq.internal.functions._SubImplFunction.concat(_SubImplFunction.java:63)
        (...cut for brievety...)

The code seems to indeed show a stack is used: https://github.com/eiiches/jackson-jq/blob/master/0.x/jackson-jq/src/main/java/net/thisptr/jackson/jq/internal/functions/_SubImplFunction.java#L62

In v1, the code hasn't changed much, and should still suffer the same fate: https://github.com/eiiches/jackson-jq/blob/develop/1.x/jackson-jq/src/main/java/net/thisptr/jackson/jq/internal/functions/_SubImplFunction.java#L74

In this specific case, there is a workaround: replace gsub(","; ", ") by split(",") | join(", "), but gsub shouldn't blow up like that.