MergeYaml recipe: Handle ParseError LSTs

[jevanlingen]

What's changed?

• The validate function can now handle wrong input yaml (it fails gracefully, as it is supposed to be)
• The incoming field is parsed once it is needed in the MergeYaml recipe.

What's your motivation?

The MergeYaml recipe throws a ClassCastException if invalid Yaml is provided (for instance a ParseError LST). Normally the YamlIsoVisitor does only run for yaml sources:

@Override
public boolean isAcceptable(SourceFile sourceFile, P p) {
    return sourceFile instanceof Yaml.Documents;
}

But the MergeYaml recipe does already do some parsing work for the incoming field, which is executed by the JVM before the isAcceptable has been called. This PR changes this behaviour to start parsing it once it is used.

In addition, this change improves performance for all use of the MergeYaml recipe with a $ key. In that case, the incoming field is not used at all, allowing an unnecessary parse step to be skipped.

[knutwannheden]

I am wondering if we should attempt to parse the YAML as part of validate() 🤔

[jevanlingen]

I am wondering if we should attempt to parse the YAML as part of validate() 🤔

Good one, I totally missed that one. We actually do this already, but the current implementation of the validate method does not handle invalid yaml input either:

@Test
void handleParseError() {
    rewriteRun(
      spec -> spec
        .recipe(new MergeYaml(
          "$.some.object",
          // language=yaml
          """
            script: |-ParseError
            """,
          false,
          "name",
          null,
          null,
          null
        ))
    );
}

Throws:

java.lang.ClassCastException: class org.openrewrite.tree.ParseError cannot be cast to class org.openrewrite.yaml.tree.Yaml$Documents (org.openrewrite.tree.ParseError and org.openrewrite.yaml.tree.Yaml$Documents are in unnamed module of loader 'app')

	at org.openrewrite.yaml.MergeYaml.lambda$validate$0(MergeYaml.java:98)
	at java.base/java.util.Optional.map(Optional.java:260)
	at org.openrewrite.yaml.MergeYaml.lambda$validate$1(MergeYaml.java:98)
        ..

I will make changes accordingly, so it does not break!

Edit: Done! I kept the other changes regarding the incoming field as it is still an improvement to the original code.