False-positive globally reachable private Java vars under -Ycheck-reentrant

[WojciechMazur]

Compiler version

3.5.0-RC1-bin-20240501-e6bc130-NIGHTLY

Minimized example

Found in Scala 2 compiler forward-port #19897

private val Close = java.nio.CharBuffer.allocate(0)

Output

> scala-cli compile --server=false sandbox.scala -S 3.nightly -Ycheck-reentrant -Ylog:checkReentrant+ 
possible data race involving globally reachable variable isReadOnly in class CharBuffer: Boolean
  use -Ylog:checkReentrant+ to find out more about why the variable is reachable.
possible data race involving globally reachable variable address in class Buffer: Long
  use -Ylog:checkReentrant+ to find out more about why the variable is reachable.
2 errors found
Compilation failed

Expectation

isReadOnly and address are private/package private? variable, first of them can be only accessed via the accessor method. These should not be reported.

[bishabosha]

my thought was that it doesn't care about private because all that matters is "is there a mutable field" and so someone could call something public that changes the field

[WojciechMazur]

Maybe, that would also make sense. I don't know the scope of -Ycheck-reentrant flag and neither understand it yet, so I'm leaving it to decide how it should be handled.

[Gedochao]

@odersky thoughts on this behaviour?

[odersky]

I think that's a reasonable warning. The fix is to annotate Close with @sharable.