Add retry to the rt.jar copying stuff

main/api/src/mill/api/ClassLoader.scala

@@ -2,10 +2,10 @@ package mill.api
 
 import java.net.{URL, URLClassLoader}
 
-import java.nio.file.FileAlreadyExistsException
+import java.nio.file.{FileAlreadyExistsException, FileSystemException}
 
 import mill.java9rtexport.Export
-import scala.util.Try
+import scala.util.{Properties, Try}
 
 /**
  * Utilities for creating classloaders for running compiled Java/Scala code in
@@ -78,19 +78,37 @@ object ClassLoader {
     if (java9OrAbove) {
       val java90rtJar = ctx.home / Export.rtJarName
       if (!os.exists(java90rtJar)) {
-        Try {
-          os.copy(os.Path(Export.rt()), java90rtJar, createFolders = true)
-        }.recoverWith { case e: FileAlreadyExistsException =>
-          // some race?
-          if (os.exists(java90rtJar) && PathRef(java90rtJar) == PathRef(os.Path(Export.rt()))) Try {
-            // all good
-            ()
+        // Time between retries should go from 100 ms to around 10s, which should
+        // leave plenty of time for another process to write fully this 50+ MB file
+        val retry = Retry(
+          count = 7,
+          backoffMillis = 100,
+          filter = {
+            case (_, _: FileSystemException) if Properties.isWin => true
+            case _ => false
           }
-          else Try {
-            // retry
-            os.copy(os.Path(Export.rt()), java90rtJar, replaceExisting = true, createFolders = true)
-          }
-        }.get
+        )
+        retry {
+          Try {
+            os.copy(os.Path(Export.rt()), java90rtJar, createFolders = true)
+          }.recoverWith { case e: FileAlreadyExistsException =>
+            // some race?
+            if (os.exists(java90rtJar) && PathRef(java90rtJar) == PathRef(os.Path(Export.rt())))
+              Try {
+                // all good
+                ()
+              }
+            else Try {
+              // retry
+              os.copy(
+                os.Path(Export.rt()),
+                java90rtJar,
+                replaceExisting = true,
+                createFolders = true
+              )
+            }
+          }.get
+        }
       }
       urls :+ java90rtJar.toIO.toURI().toURL()
     } else {