Limit exposure to ConcurrentModificationException when sys props are replaced or mutated

port of scala/scala@f6859f2

[Cherry-picked 705c33c]

Author: jtjeferreira

Diff for: compiler/src/dotty/tools/dotc/config/PathResolver.scala

@@ -36,9 +36,16 @@ object PathResolver {
   /** Values found solely by inspecting environment or property variables.
    */
   object Environment {
-    private def searchForBootClasspath = (
-      systemProperties find (_._1 endsWith ".boot.class.path") map (_._2) getOrElse ""
-    )
+    private def searchForBootClasspath = {
+      import scala.jdk.CollectionConverters.*
+      val props = System.getProperties
+      // This formulation should be immune to ConcurrentModificationExceptions when system properties
+      // we're unlucky enough to witness a partially published result of System.setProperty or direct
+      // mutation of the System property map. stringPropertyNames internally uses the Enumeration interface,
+      // rather than Iterator, and this disables the fail-fast ConcurrentModificationException.
+      val propNames = props.stringPropertyNames()
+      propNames.asScala collectFirst { case k if k endsWith ".boot.class.path" => props.getProperty(k) } getOrElse ""
+    }
 
     /** Environment variables which java pays attention to so it
      *  seems we do as well.