[FIXED JENKINS-11251]

The actual meat of the change is in remoting.

changelog.html

@@ -55,6 +55,9 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=bug>
+    Fixed a synchronization problem between master/slave data communication.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-11251">issue 11251</a>)
   <li class=rfe>
     Added a mechanism to filter extension points as they are discovered.
   <li class=rfe>

core/src/main/java/hudson/FilePath.java

@@ -1456,6 +1456,16 @@ public final class FilePath implements Serializable {
      * Writes to this file.
      * If this file already exists, it will be overwritten.
      * If the directory doesn't exist, it will be created.
+     *
+     * <P>
+     * I/O operation to remote {@link FilePath} happens asynchronously, meaning write operations to the returned
+     * {@link OutputStream} will return without receiving a confirmation from the remote that the write happened.
+     * I/O operations also happens asynchronously from the {@link Channel#call(Callable)} operations, so if
+     * you write to a remote file and then execute {@link Channel#call(Callable)} and try to access the newly copied
+     * file, it might not be fully written yet.
+     *
+     * <p>
+     *
      */
     public OutputStream write() throws IOException, InterruptedException {
         if(channel==null) {
@@ -1596,10 +1606,16 @@ public final class FilePath implements Serializable {
             }
         });
 
-        // make sure the write fully happens before we return.
+        // make sure the writes fully got delivered to 'os' before we return.
+        // this is needed because I/O operation is asynchronous
         syncIO();
     }
 
+    /**
+     * With fix to JENKINS-11251 (remoting 2.15), this is no longer necessary.
+     * But I'm keeping it for a while so that users who manually deploy slave.jar has time to deploy new version
+     * before this goes away.
+     */
     private void syncIO() throws InterruptedException {
         try {
             if (channel!=null)

pom.xml

@@ -189,7 +189,7 @@ THE SOFTWARE.
       <dependency>
         <groupId>org.jenkins-ci.main</groupId>
         <artifactId>remoting</artifactId>
-        <version>2.13</version>
+        <version>2.16</version>
       </dependency>
 
       <dependency>