[JENKINS-62006] Move getChannelToMaster to a separate class

This should avoid classloading issues I've seen, such as

Caused by: java.lang.NoClassDefFoundError:
javax/servlet/ServletException
  at hudson.util.ProcessTree.get(ProcessTree.java:432)
...

core/src/main/java/hudson/slaves/SlaveComputer.java

@@ -1067,17 +1067,11 @@ public synchronized void publish(LogRecord record) {
      *
      * @return null if the calling thread doesn't have any trace of where its master is.
      * @since 1.362
+     * @deprecated Use {@link SlaveComputerUtil#getChannelToMaster()} instead.
      */
+    @Deprecated
     public static VirtualChannel getChannelToMaster() {
-        if (Jenkins.getInstanceOrNull()!=null) // check if calling thread is on master or on slave
-            return FilePath.localChannel;
-
-        // if this method is called from within the agent computation thread, this should work
-        Channel c = Channel.current();
-        if (c!=null && Boolean.TRUE.equals(c.getProperty("slave")))
-            return c;
-
-        return null;
+        return SlaveComputerUtil.getChannelToMaster();
     }
 
     /**

core/src/main/java/hudson/slaves/SlaveComputerUtil.java

@@ -0,0 +1,30 @@
+package hudson.slaves;
+
+import hudson.FilePath;
+import hudson.remoting.Channel;
+import hudson.remoting.VirtualChannel;
+import jenkins.model.Jenkins;
+
+public final class SlaveComputerUtil {
+    private SlaveComputerUtil() {}
+    /**
+     * Obtains a {@link VirtualChannel} that allows some computation to be performed on the master.
+     * This method can be called from any thread on the master, or from agent (more precisely,
+     * it only works from the remoting request-handling thread in agents, which means if you've started
+     * separate thread on agents, that'll fail.)
+     *
+     * @return null if the calling thread doesn't have any trace of where its master is.
+     * @since XXX
+     */
+    public static VirtualChannel getChannelToMaster() {
+        if (Jenkins.getInstanceOrNull()!=null) // check if calling thread is on master or on slave
+            return FilePath.localChannel;
+
+        // if this method is called from within the agent computation thread, this should work
+        Channel c = Channel.current();
+        if (c!=null && Boolean.TRUE.equals(c.getProperty("slave")))
+            return c;
+
+        return null;
+    }
+}

core/src/main/java/hudson/util/ProcessTree.java

@@ -33,7 +33,7 @@
 import hudson.Util;
 import hudson.remoting.Channel;
 import hudson.remoting.VirtualChannel;
-import hudson.slaves.SlaveComputer;
+import hudson.slaves.SlaveComputerUtil;
 import hudson.util.ProcessKillingVeto.VetoCause;
 import hudson.util.ProcessTree.OSProcess;
 import hudson.util.ProcessTreeRemoting.IOSProcess;
@@ -193,7 +193,7 @@ public void killAll(@CheckForNull Process proc, @CheckForNull Map<String, String
     /*package*/ final List<ProcessKiller> getKillers() throws InterruptedException {
         if (killers==null)
             try {
-                VirtualChannel channelToMaster = SlaveComputer.getChannelToMaster();
+                VirtualChannel channelToMaster = SlaveComputerUtil.getChannelToMaster();
                 if (channelToMaster!=null) {
                     killers = channelToMaster.call(new ListAll());
                 } else {
@@ -288,7 +288,7 @@ void killByKiller() throws InterruptedException {
             // Quick check, does anything exist to check against
             if (!skipVetoes) {
                 try {
-                    VirtualChannel channelToMaster = SlaveComputer.getChannelToMaster();
+                    VirtualChannel channelToMaster = SlaveComputerUtil.getChannelToMaster();
                     if (channelToMaster!=null) {
                         CheckVetoes vetoCheck = new CheckVetoes(this);
                         causeMessage = channelToMaster.call(vetoCheck);
@@ -429,7 +429,7 @@ public static ProcessTree get() {
         // Check for the existance of vetoers if I don't know already
         if (vetoersExist == null) {
             try {
-                VirtualChannel channelToMaster = SlaveComputer.getChannelToMaster();
+                VirtualChannel channelToMaster = SlaveComputerUtil.getChannelToMaster();
                 if (channelToMaster != null) {
                     vetoersExist = channelToMaster.call(new DoVetoersExist());
                 }