[Ask for sponsoring] Fix another NPE in jdk.compiler/com.sun.tools.javac.jvm.Code.emitop0(Code.java:571)

[JiaYanwei]

Hi all,

I had found & fixed a javac issue like
https://bugs.openjdk.java.net/browse/JDK-8229862 , and created a PR on
Github: https://github.com/openjdk/jdk/pull/1479 .

I have no account to create JIRA issue, and I'm not sure whether the code
style as well as the solution itself are appropriate. Could anyone sponsor
me? Thanks a lot.

Best Regards.

---

The Issue:
javac crashes with a NPE when compiling such code:

- a local class captured a reference type local variable of a enclosed
lambda (which contains multiple local variables with different types)
- instance the local class in a nested lambda
Affects all versions from Java 8 to 16-ea, here is a example output with
OpenJDK 15.0.1:

An exception has occurred in the compiler (15.0.1). Please file a bug
against the Java compiler via the Java bug reporting page (
http://bugreport.java.com) after checking the Bug Database (
http://bugs.java.com) for duplicates. Include your program, the following
diagnostic, and the parameters passed to the Java compiler in your report.
Thank you.
java.lang.NullPointerException: Cannot read field "sym" because
"this.lvar[1]" is null
    at jdk.compiler/com.sun.tools.javac.jvm.Code.emitop0(Code.java:571)
    at
jdk.compiler/com.sun.tools.javac.jvm.Items$LocalItem.load(Items.java:399)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genArgs(Gen.java:889)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitNewClass(Gen.java:1942)
    at
jdk.compiler/com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1800)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genExpr(Gen.java:864)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitExec(Gen.java:1723)
    at
jdk.compiler/com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1532)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:632)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:618)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStats(Gen.java:669)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitBlock(Gen.java:1084)
    at
jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1047)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:632)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genMethod(Gen.java:954)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitMethodDef(Gen.java:917)
    at
jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:893)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
    at jdk.compiler/com.sun.tools.javac.jvm.Gen.genClass(Gen.java:2395)
    at
jdk.compiler/com.sun.tools.javac.main.JavaCompiler.genCode(JavaCompiler.java:756)
    at
jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1644)
    at
jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1612)
    at
jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:973)
    at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:317)
    at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
    at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:59)
    at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:45)
printing javac parameters to: /tmp/javac.20201126_141249.args

The minimal reproducing code is:

class Main {
    static Runnable runnable = () -> {
        boolean b0 = false;
        String s0 = "hello";

        class Local {
            int i = s0.length();
        }

        Runnable dummy = () -> new Local();
    };
}

If the captured variable is a primitive type, the code will compile but
cause a runtime crash at startup. e.g.:

public class CaptureInt {
    static Runnable runnable = () -> {
        boolean b0 = false;
        int i0 = 5;

        class Local {
            int i = i0 + 2;
        }

        Runnable dummy = () -> new Local();
    };

    public static void main(String args[]) {
    }
}

Reason & Solution:

During compilation, the captured variable was correctly synthetized as a
synthetic argument in the synthetic methods for the nested lambda after
desugar().
But the synthetic argument was not used when loading free variables, and it
always use the original symbol for the original variable as if not in a
nested lambda.
My experimental solution is substituting the symbols in free variables that
were captured as synthetic arguments in nested lambda.
Welcome any advice and better solution.
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