Backport "Limit exposure to ConcurrentModificationException when sys …

…props are replaced or mutated" to 3.6 (#22275)

Backports #22180 to the 3.6.3.

PR submitted by the release tooling.
[skip ci]

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

@@ -36,17 +36,25 @@ 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.
      */
     def classPathEnv: String        =  envOrElse("CLASSPATH", "")
     def sourcePathEnv: String       =  envOrElse("SOURCEPATH", "")
 
-    def javaBootClassPath: String   = propOrElse("sun.boot.class.path", searchForBootClasspath)
+    //using propOrNone/getOrElse instead of propOrElse so that searchForBootClasspath is lazy evaluated
+    def javaBootClassPath: String   = propOrNone("sun.boot.class.path") getOrElse searchForBootClasspath
 
     def javaExtDirs: String         = propOrEmpty("java.ext.dirs")
     def scalaHome: String           = propOrEmpty("scala.home")