JRI: use attribute-name length in rniGetAttrNames

[r-suzuki]

Thank you for maintaining rJava/JRI.

This PR proposes a small fix for what appears to be an issue in rniGetAttrNames on R 4.6.0.

In the R 4.6.0 code path, the attribute names are obtained with R_getAttribNames(o) and stored in ans, but the Java array size is currently taken from XLENGTH(o).

Since the length of the target object is independent of the number of attributes, zero-length objects with attributes can return null.

This change uses XLENGTH(ans) for the Java array size, matching the later loop over ans.

- ac = (unsigned int) XLENGTH(o);
+ ac = (unsigned int) XLENGTH(ans);

The minimal failing case is:

structure(numeric(0), foo = 1)

This object has length 0, but it has one attribute name, foo.

Test program

Save as rtest_attrnames.java and run with:

./run rtest_attrnames --vanilla

import org.rosuda.JRI.Rengine;
import java.util.Arrays;

public class rtest_attrnames {
    private static void check(Rengine re, String expr, String[] expected) {
        long xp = re.rniEval(re.rniParse(expr, 1), 0);
        String[] got = re.rniGetAttrNames(xp);

        System.out.println(expr);
        System.out.println("got      = " + Arrays.toString(got));
        System.out.println("expected = " + Arrays.toString(expected));

        if (!Arrays.equals(got, expected)) {
            throw new RuntimeException("unexpected attribute names for: " + expr);
        }
    }
    
    public static void main(String[] args) {
        if (!Rengine.versionCheck()) {
            throw new RuntimeException("JRI version mismatch");
        }

        Rengine re = new Rengine(new String[] {"--vanilla"}, false, null);

        if (!re.waitForR()) {
            throw new RuntimeException("cannot load R");
        }

        try {
            check(re,
                "structure(numeric(0), foo = 1)",
                new String[] {"foo"});

            check(re,
                "structure(1, foo = 1, bar = 2)",
                new String[] {"foo", "bar"});

            check(re,
                "structure(1:10, foo = 1)",
                new String[] {"foo"});

            check(re,
                "1:10",
                null);
        } finally {
            re.end();
        }
    }
}

Result with R 4.5.0

The test succeeds.

structure(numeric(0), foo = 1)
got      = [foo]
expected = [foo]
structure(1, foo = 1, bar = 2)
got      = [foo, bar]
expected = [foo, bar]
structure(1:10, foo = 1)
got      = [foo]
expected = [foo]
1:10
got      = null
expected = null

Result with R 4.6.0 before this change

The first case fails, and the program terminates with a RuntimeException.

structure(numeric(0), foo = 1)
got      = null
expected = [foo]
Exception in thread "main" java.lang.RuntimeException: unexpected attribute names for: structure(numeric(0), foo = 1)
        at rtest_attrnames.check(rtest_attrnames.java:14)
        at rtest_attrnames.main(rtest_attrnames.java:30)

Result with R 4.6.0 after this change

The test succeeds.

structure(numeric(0), foo = 1)
got      = [foo]
expected = [foo]
structure(1, foo = 1, bar = 2)
got      = [foo, bar]
expected = [foo, bar]
structure(1:10, foo = 1)
got      = [foo]
expected = [foo]
1:10
got      = null
expected = null

Thank you for taking a look. Please let me know if I missed anything.