Move the ClassPath cache to an explicit GlobalCache

[mbovel]

Attempt to move the ClassPath cache to an explicit GlobalCache to avoid global state, as suggested by @sjrd and @hamzaremmal.

We could add other kinds of caches there in the future, such as a file-contents cache as in #24650.

Given the wide variety of Driver implementations in the wild, this change is potentially risky: some may lose caching capabilities and become slower without noticing.

[mbovel]

Setting the global cache in Driver.setup doesn't catch all context usages. There are direct, driver-less usages of ContextBase.initialContext, such as in DottyTest:

scala3/compiler/test/dotty/tools/DottyTest.scala

Lines 25 to 27 in ba45875

	protected def initialCtx: FreshContext = {
	val base = new ContextBase {}
	val ctx = base.initialCtx.fresh

To be complete, we should probably set the cache in FreshContext.initial already, along with other store locations:

scala3/compiler/src/dotty/tools/dotc/core/Contexts.scala

Lines 773 to 777 in ba45875

	c._store = initialStore
	.updated(settingsStateLoc, settingsGroup.defaultState)
	.updated(notNullInfosLoc, Nil)
	.updated(compilationUnitLoc, NoCompilationUnit)
	.updated(profilerLoc, Profiler.NoOp)

But then, how to pass the cache instance down there? We don't even have access to settings there, I believe?

[mbovel]

Example failure:

Test dotty.tools.repl.ShadowingBatchTests.directory failed: java.lang.NullPointerException:
Cannot invoke "dotty.tools.dotc.GlobalCache.getOrCreateClassPath(dotty.tools.io.AbstractFile,
scala.Function0, dotty.tools.dotc.core.Contexts$Context)" because the return value of
"dotty.tools.dotc.core.Contexts$Context.globalCache()" is null, took 0.516 sec

Error:      at dotty.tools.dotc.classpath.ZipAndJarFileLookupFactory.create(ZipAndJarFileLookupFactory.scala:28)
Error:      at dotty.tools.dotc.classpath.ZipAndJarFileLookupFactory.create$(ZipAndJarFileLookupFactory.scala:24)
...
Error:      at dotty.tools.dotc.core.Definitions.syntheticCoreClasses(Definitions.scala:2132)
Error:      at dotty.tools.dotc.core.Definitions.init(Definitions.scala:2148)
Error:      at dotty.tools.dotc.core.Contexts$ContextBase.initialize(Contexts.scala:941)
Error:      at dotty.tools.DottyTest.initialCtx(DottyTest.scala:31)
Error:      at dotty.tools.DottyTest.initialCtx$(DottyTest.scala:19)
...

https://github.com/scala/scala3/actions/runs/20161879826/job/57879475677?pr=24737#step:5:1318

[bishabosha]

if its no longer based on weak references, then can we used a concurrent map rather than synchronized?

[bishabosha]

or i guess you could want a per-key synchronization

[mbovel]

Note that it is a different cache than the one we discussed in #24650. This one already exists and caches ClassPath instances, not file contents. I just moved that code and haven't changed it.

There might be room for improvement, but it should be addressed separately. The questions I am trying to answer in this PR are just: can and should we make that global cache less global by explicitly passing instances to drivers.

But I agree it might be smarter to use a specialized concurrent map.