Fix spawning of terminfo thread in server mode (#3833)

Previously we only spawned the terminfo updater when spawning the Mill
server process, but that only handles the no-server case and
first-time-spawning-server case, without handling the
subsequently-re-use-server case.

This moves the logic into a helper method so we can call it from both
server and no-server launchers at the first point at which we have a
dedicated `serverDir` available. Since the `serverDIr` is only available
deep inside the `ServerLauncher` object, as it needs to do the work of
testing different locks to pick a server, we add a `preRun` hook that we
can override to start the terminfo updater thread

main/client/src/mill/main/client/ServerLauncher.java

@@ -52,6 +52,7 @@ public static class Result{
     final int serverProcessesLimit = 5;
     final int serverInitWaitMillis = 10000;
     public abstract void initServer(Path serverDir, boolean b, Locks locks) throws Exception;
+    public abstract void preRun(Path serverDir) throws Exception;
     InputStream stdin;
     PrintStream stdout;
     PrintStream stderr;
@@ -101,6 +102,7 @@ public Result acquireLocksAndRun(String outDir) throws Exception {
             ) {
                 if (clientLocked.isLocked()) {
                     Result result = new Result();
+                    preRun(serverDir);
                     result.exitCode = run(serverDir, setJnaNoSys, locks);
                     result.serverDir = serverDir;
                     return result;

main/server/test/src/mill/main/server/ClientServerTests.scala

@@ -96,6 +96,7 @@ object ClientServerTests extends TestSuite {
         memoryLocks,
         forceFailureForTestingMillisDelay
       ) {
+        def preRun(serverDir: Path) = { /*do nothing*/ }
         def initServer(serverDir: Path, b: Boolean, locks: Locks) = {
           val serverId = "server-" + nextServerId
           nextServerId += 1

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

@@ -40,6 +40,9 @@ public static void main(String[] args) throws Exception {
                 public void initServer(Path serverDir, boolean setJnaNoSys, Locks locks) throws Exception{
                     MillProcessLauncher.launchMillServer(serverDir, setJnaNoSys);
                 }
+                public void preRun(Path serverDir) throws Exception {
+                    MillProcessLauncher.runTermInfoThread(serverDir);
+                }
             };
             int exitCode = launcher.acquireLocksAndRun(OutFiles.out).exitCode;
             if (exitCode == Util.ExitServerCodeWhenVersionMismatch()) {

runner/client/src/mill/runner/client/MillProcessLauncher.java

@@ -10,6 +10,7 @@
 import java.nio.file.Paths;
 import java.util.*;
 
+import mill.main.client.DebugLog;
 import org.jline.terminal.TerminalBuilder;
 import org.jline.terminal.Terminal;
 
@@ -37,7 +38,9 @@ static int launchMillNoServer(String[] args) throws Exception {
         boolean interrupted = false;
 
         try {
-            return configureRunMillProcess(builder, processDir).waitFor();
+            Process p = configureRunMillProcess(builder, processDir);
+            MillProcessLauncher.runTermInfoThread(processDir);
+            return p.waitFor();
 
         } catch (InterruptedException e) {
             interrupted = true;
@@ -71,30 +74,9 @@ static Process configureRunMillProcess(
         ProcessBuilder builder,
         Path serverDir
     ) throws Exception {
-        Terminal term = TerminalBuilder.builder().dumb(true).build();
+
         Path sandbox = serverDir.resolve(ServerFiles.sandbox);
         Files.createDirectories(sandbox);
-        Files.write(
-            serverDir.resolve(ServerFiles.terminfo),
-            (term.getWidth() + " " + term.getHeight()).getBytes()
-        );
-        Thread termInfoPropagatorThread = new Thread(
-            () -> {
-                try {
-
-                    while(true){
-                        Files.write(
-                            serverDir.resolve(ServerFiles.terminfo),
-                            (term.getWidth() + " " + term.getHeight()).getBytes()
-                        );
-
-                        Thread.sleep(100);
-                    }
-                }catch (Exception e){}
-            },
-            "TermInfoPropagatorThread"
-        );
-        termInfoPropagatorThread.start();
         builder.environment().put(EnvVars.MILL_WORKSPACE_ROOT, new File("").getCanonicalPath());
 
         builder.directory(sandbox.toFile());
@@ -214,4 +196,24 @@ static List<String> millLaunchJvmCommand(boolean setJnaNoSys) throws Exception {
     static List<String> readMillJvmOpts() {
         return Util.readOptsFileLines(millJvmOptsFile());
     }
+
+    public static void runTermInfoThread(Path serverDir) throws Exception{
+        Terminal term = TerminalBuilder.builder().dumb(true).build();
+        Thread termInfoPropagatorThread = new Thread(
+            () -> {
+                try {
+                    while(true){
+                        Files.write(
+                            serverDir.resolve(ServerFiles.terminfo),
+                            (term.getWidth() + " " + term.getHeight()).getBytes()
+                        );
+
+                        Thread.sleep(100);
+                    }
+                }catch (Exception e){}
+            },
+            "TermInfoPropagatorThread"
+        );
+        termInfoPropagatorThread.start();
+    }
 }