test_windows_full in the CI is broken: pickling difference in stdlibPatches

[smarter]

It appears to have started with 96ced02, the CI run is https://github.com/lampepfl/dotty/runs/2057136722:

 class dotty.tools.dotc.reporting.Diagnostic$Error at ?: pickling difference for module class language$ in library\src\scala\runtime\stdLibPatches\language.scala, for details:
Fatal compiler crash when compiling: library\src\scala\runtime\stdLibPatches:

  diff before-pickling.txt after-pickling.txt while compiling library\src\scala\runtime\stdLibPatches\language.scala, library\src\scala\runtime\stdLibPatches\Predef.scala
pickling difference for module class language$ in library\src\scala\runtime\stdLibPatches\language.scala, for details:

  diff before-pickling.txt after-pickling.txt
dotty.tools.dotc.report$.error(report.scala:71)
dotty.tools.dotc.transform.Pickler.testSame(Pickler.scala:145)
dotty.tools.dotc.transform.Pickler.testUnpickler$$anonfun$2(Pickler.scala:135)
scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
scala.collection.IterableOnceOps.foreach(IterableOnce.scala:563)
scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:561)
scala.collection.AbstractIterable.foreach(Iterable.scala:919)
scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:889)
dotty.tools.dotc.transform.Pickler.testUnpickler(Pickler.scala:136)
dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:119)
dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:205)
scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
dotty.tools.dotc.Run.runPhases$5(Run.scala:215)
dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:223)
scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
dotty.tools.dotc.Run.compileUnits(Run.scala:230)
dotty.tools.dotc.Run.compileSources(Run.scala:166)
dotty.tools.dotc.Run.compile(Run.scala:150)
dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
dotty.tools.dotc.Driver.process(Driver.scala:198)
dotty.tools.dotc.Driver.process(Driver.scala:166)
dotty.tools.vulpix.ParallelTesting$Test.compile(ParallelTesting.scala:490)
dotty.tools.vulpix.ParallelTesting$CompilationLogic.compileTestSource$$anonfun$2$$anonfun$1(ParallelTesting.scala:219)
scala.collection.immutable.List.map(List.scala:246)
dotty.tools.vulpix.ParallelTesting$CompilationLogic.compileTestSource$$anonfun$1(ParallelTesting.scala:219)
scala.util.Try$.apply(Try.scala:210)
dotty.tools.vulpix.ParallelTesting$CompilationLogic.dotty$tools$vulpix$ParallelTesting$CompilationLogic$$compileTestSource(ParallelTesting.scala:220)
dotty.tools.vulpix.ParallelTesting$$anon$2.checkTestSource$$anonfun$1(ParallelTesting.scala:265)
dotty.tools.vulpix.ParallelTesting$Test.tryCompile(ParallelTesting.scala:433)
dotty.tools.vulpix.ParallelTesting$$anon$2.checkTestSource(ParallelTesting.scala:268)
dotty.tools.vulpix.ParallelTesting$Test$LoggedRunnable.run(ParallelTesting.scala:336)
dotty.tools.vulpix.ParallelTesting$Test$LoggedRunnable.run$(ParallelTesting.scala:318)
dotty.tools.vulpix.ParallelTesting$$anon$2.run(ParallelTesting.scala:263)
java.util.concurrent.ForkJoinTask$AdaptedRunnableAction.exec(ForkJoinTask.java:1386)
java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:175)
Error:  Test dotty.tools.dotc.BootstrappedOnlyCompilationTests.picklingWithCompiler failed: java.lang.AssertionError: Expected no errors when compiling, failed for the following reason(s):
Error:  
Error:    - encountered 1 test failures(s), took 25.844 sec
Error:      at dotty.tools.vulpix.ParallelTesting$CompilationTest.checkCompile(ParallelTesting.scala:897)
Error:      at dotty.tools.dotc.BootstrappedOnlyCompilationTests.picklingWithCompiler(BootstrappedOnlyCompilationTests.scala:177)
Error:      ...
[info] Test dotty.tools.dotc.BootstrappedOnlyCompilationTests.negWithCompiler started
[info] Test dotty.tools.dotc.BootstrappedOnlyCompilationTests.posMacros started
Foo[T]
[info] Test dotty.tools.dotc.BootstrappedOnlyCompilationTests.runWithCompiler started
foo
[info] Test dotty.tools.dotc.BootstrappedOnlyCompilationTests.runBootstrappedOnly started

================================================================================
Test Report
================================================================================

10 suites passed, 1 failed, 11 total
    library\src\scala\runtime\stdLibPatches failed

[liufengyun]

We have the following difference:

[smarter]

When we compile scala.runtime.stdLibPatches.language, we apply stdlibPatches on that file itself, which changes the owner of its members to scala.language, this doesn't really make sense and apparently doesn't survive unpickling (but why only on Windows?

I think we need to:

• Change the logic in stdlibPatches: instead of modifying the denotation of existing symbols, create new symbols to insert in scala.language and leave the original definitions alone
• Make sure all these definitions are @compileTimeOnly so that we never try to lookup non-existing classfiles for them.

[liufengyun]

but why only on Windows?

That's the most mysterious thing. I conjectured it's related to SymDenocation.fullNameCache, and caching behavior is different on Windows.

However, when I disable the fullNameCache, we still have the same pickling test failure. So the conjecture is incorect.

[smarter]

Reopened since the underlying issue isn't fixed.