Make Computer#getLogDir thread-safe (#4730)

core/src/main/java/hudson/model/Computer.java

@@ -195,6 +195,8 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
 
     protected final Object statusChangeLock = new Object();
 
+    private final Object logDirLock = new String("logDirLock");
+
     /**
      * Keeps track of stack traces to track the termination requests for this computer.
      *
@@ -303,10 +305,12 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
      */
     protected @NonNull File getLogDir() {
         File dir = new File(Jenkins.get().getRootDir(),"logs/slaves/"+nodeName);
-        try {
-            IOUtils.mkdirs(dir);
-        } catch (IOException x) {
-            LOGGER.log(Level.SEVERE, "Failed to create agent log directory " + dir, x);
+        synchronized (logDirLock) {
+            try {
+                IOUtils.mkdirs(dir);
+            } catch (IOException x) {
+                LOGGER.log(Level.SEVERE, "Failed to create agent log directory " + dir, x);
+            }
         }
         return dir;
     }