#726 interface methods, closures and applymapping don't work together (original method names kept invokedynamic)

[Dawid Weiss]

The scenario is as follows. There are two JAR files: A.jar and B.jar, obfuscated one after another with the obfuscation mapping of A applied to B (via applymapping).

A declares an functional interface Foo with a single method bar.
B declares a class where variable of type Foo is assigned a lambda.

At runtime we received an exception about an undeclared abstract method error ('bar'). This was a bit weird because 'bar' was renamed to something different in A and we had mapping file to prove it.

The problem was difficult to figure out because for certain interfaces in A everything worked perfectly well in B. Turns out the outcome actually depends on whether A has ANY invokedynamic (lambda) of Foo inside any of its classes -- if so, Proguard will keep method name intact and not obfuscate it. This is confirmed by looking at Obfuscator.java:

    // We also keep the names of the abstract methods of functional
    // interfaces referenced from bootstrap method arguments (additional
    // interfaces with LambdaMetafactory.altMetafactory).
    // The functional method names have to match the names in the
    // dynamic method invocations with LambdaMetafactory.

I spent a good few hours trying to figure it out. Can't offer a solution or even an idea on how to solve it (or work around it), but it's definitely a trappy thing and caught us by surprise.

[Dawid Weiss]

This issue is describing the same problem:
https://sourceforge.net/p/proguard/bugs/620/