Adjust logback outfile file upon startup

runner/client/src/mill/runner/client/MillClientMain.java

@@ -17,6 +17,10 @@
  */
 public class MillClientMain {
   public static void main(String[] args) throws Exception {
+
+    if (System.getProperty("mill.log.file") == null)
+      System.setProperty("mill.log.file", OutFiles.out + "/mill.log");
+
     boolean runNoServer = false;
     if (args.length > 0) {
       String firstArg = args[0];

runner/resources/logback.xml

@@ -1,6 +1,6 @@
 <configuration debug="false" scan="false">
   <appender name="FILE" class="ch.qos.logback.core.FileAppender">
-    <file>out/mill.log</file>
+    <file>${mill.log.file:-out/mill.log}</file>
       <encoder>
         <pattern>%date %level [%thread] %logger{36} %msg%n</pattern>
       </encoder>

runner/src/mill/runner/MillMain.scala

@@ -41,6 +41,7 @@ object MillMain {
   }
 
   def main(args: Array[String]): Unit = SystemStreams.withTopLevelSystemStreamProxy {
+    adjustLogbackOutputFile()
     val initialSystemStreams = SystemStreams.original
     // setup streams
     val (runnerStreams, cleanupStreams, bspLog) =
@@ -432,6 +433,17 @@ object MillMain {
     for ((k, v) <- desiredProps) System.setProperty(k, v)
   }
 
+  // Logback is set up via a logback.xml file in resources, so that if an app
+  // tries to log via slf4j, its logs will go somewhere. We need to tell logback
+  // where to write its output file. The out directory might not be "out/", so we
+  // set a Java property, used in logback.xml and then read by logback, so that
+  // the logs go to a file under the actual out directory.
+  // This method should be called early on in Mill processes, not to miss any
+  // slf4j use.
+  def adjustLogbackOutputFile(): Unit =
+    if (!sys.props.contains("mill.log.file"))
+      sys.props("mill.log.file") = s"${OutFiles.out}/mill.log"
+
   def withOutLock[T](
       noBuildLock: Boolean,
       noWaitForBuildLock: Boolean,

runner/src/mill/runner/MillServerMain.scala

@@ -23,6 +23,8 @@ object MillServerMain {
       }
     )
 
+    MillMain.adjustLogbackOutputFile()
+
     val acceptTimeoutMillis =
       Try(System.getProperty("mill.server_timeout").toInt).getOrElse(30 * 60 * 1000) // 30 minutes