Hide "Parallelize position pickling" under a flag.

This disables #9619 as it lead to CI failures.

Author: smarter

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -146,6 +146,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val YshowPrintErrors: Setting[Boolean] = BooleanSetting("-Yshow-print-errors", "Don't suppress exceptions thrown during tree printing.")
   val YshowRawQuoteTrees: Setting[Boolean] = BooleanSetting("-Yshow-raw-tree", "Don't remove quote artifacts.")
   val YtestPickler: Setting[Boolean] = BooleanSetting("-Ytest-pickler", "Self-test for pickling functionality; should be used with -Ystop-after:pickler.")
+  val YparallelPickler: Setting[Boolean] = BooleanSetting("-Yparallel-pickler", "Run part of the pickling phase in parallel, disable because it breaks some tests.")
   val YcheckReentrant: Setting[Boolean] = BooleanSetting("-Ycheck-reentrant", "Check that compiled program does not contain vars that can be accessed from a global root.")
   val YdropComments: Setting[Boolean] = BooleanSetting("-Ydrop-comments", "Drop comments when scanning source files.")
   val YcookComments: Setting[Boolean] = BooleanSetting("-Ycook-comments", "Cook the comments (type check `@usecase`, etc.)")

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -86,7 +86,17 @@ class Pickler extends Phase {
         pickled
       }(using ExecutionContext.global)
       def force(): Array[Byte] = Await.result(pickledF, Duration.Inf)
-      if ctx.settings.YtestPickler.value then force()
+
+      // Turn off parallelism because it lead to non-deterministic CI failures:
+      // - https://github.com/lampepfl/dotty/runs/1029579877?check_suite_focus=true#step:10:967
+      // - https://github.com/lampepfl/dotty/runs/1027582846?check_suite_focus=true#step:10:564
+      //
+      // Turning off parallelism in -Ytest-pickler and Interactive mode seems to
+      // work around the problem but I would prefer to not enable this at all
+      // until the root cause of the issue is understood since it might manifest
+      // itself in other situations too.
+      if !ctx.settings.YparallelPickler.value then force()
+
       unit.pickled += (cls -> force)
     }
   }