RFR: 8344911: Introduce test utility for asserting file open status

[Eirik Bjørsnøs]

On Mon, 25 Nov 2024 17:38:55 GMT, Eirik Bjørsnøs <eirbjo at openjdk.org> wrote:

> To the feature itself, it seems pretty complicated and invasive

As for complexity, the feature consists of:

* A Java agent `OpenFilesAgent`, containing 277 source code lines with:
  * A `main` method to package itself as a JAR for use with `-javaagent` on the test's command line
  * A `premain` method to add the `ClassFileTransformer`, make the runtime class `OpenFiles` available for the JDK's boot class loader and retransform any classes if needed.
  * The class file transformer itself, injecting hooks at around open and close operations in `FileInputStream`, `FileOutputStream`,  `RandomAccessFile` and `sun.nio.ch.FileChannelImpl`. This uses the `java.lang.classfile` API.
* An API support class `OpenFiles`, containing 79 source code lines which:
  * Allows instumented code to register a file as being opened  or closed. 
  * Provides an API for tests to assert files as open or closed
* A test for the implementation the the feature itself, verifying that the feature works as expected across a wide range of JDK file primitives. 

To make use of the feature, tests must:

* Add a `@run driver` tag to prepare the agent JAR files
* Run in `/othervm`
* Add the `-javaagent:OpenFilesAgent.jar` JVM option
* Import the `OpenFiles` API
* Add `assertOpen`, `assertClosed` calls as required by the test logic
* Remove / clean up any old assertion mechanisms (if so desired) 
 
Yes, the feature has a non-negligble maintenence cost. However, given the potential benefits of cathing bugs early, I think the implementation size or compexity is not too bad considering what is offered.

Byte code instrumentation is never pretty, but with the new `ClassFile` API I think the resulting code was relatively straightforward. (Although I have used ASM a lot in the past, this was my first experience using the ClassFile API. I'm sure my code can be improved). And with the `ClassFile` API we can hopefully expect more maintainers to be experienced with class file transformations in the future.

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PR Comment: https://git.openjdk.org/jdk/pull/22343#issuecomment-2498818742