[SPARK-20042][WEB UI] Fix log page buttons for reverse proxy mode

with spark.ui.reverseProxy=true, full path URLs like /log will point to
the master web endpoint which is serving the worker UI as reverse proxy.
To access a REST endpoint in the worker in reverse proxy mode , the
leading /proxy/"target"/ part of the base URI must be retained.

Added logic to log-view.js to handle this, similar to executorspage.js

Patch was tested manually

Author: Oliver Köth <okoeth@de.ibm.com>

Closes #17370 from okoethibm/master.

(cherry picked from commit 6f09dc70)
Signed-off-by: Sean Owen <sowen@cloudera.com>

core/src/main/resources/org/apache/spark/ui/static/log-view.js

@@ -51,13 +51,26 @@ function noNewAlert() {
   window.setTimeout(function () {alert.css("display", "none");}, 4000);
 }
 
+
+function getRESTEndPoint() {
+  // If the worker is served from the master through a proxy (see doc on spark.ui.reverseProxy), 
+  // we need to retain the leading ../proxy/<workerid>/ part of the URL when making REST requests.
+  // Similar logic is contained in executorspage.js function createRESTEndPoint.
+  var words = document.baseURI.split('/');
+  var ind = words.indexOf("proxy");
+  if (ind > 0) {
+      return words.slice(0, ind + 2).join('/') + "/log";
+  }
+  return "/log"
+}
+
 function loadMore() {
   var offset = Math.max(startByte - byteLength, 0);
   var moreByteLength = Math.min(byteLength, startByte);
 
   $.ajax({
     type: "GET",
-    url: "/log" + baseParams + "&offset=" + offset + "&byteLength=" + moreByteLength,
+    url: getRESTEndPoint() + baseParams + "&offset=" + offset + "&byteLength=" + moreByteLength,
     success: function (data) {
       var oldHeight = $(".log-content")[0].scrollHeight;
       var newlineIndex = data.indexOf('\n');
@@ -83,14 +96,14 @@ function loadMore() {
 function loadNew() {
   $.ajax({
     type: "GET",
-    url: "/log" + baseParams + "&byteLength=0",
+    url: getRESTEndPoint() + baseParams + "&byteLength=0",
     success: function (data) {
       var dataInfo = data.substring(0, data.indexOf('\n')).match(/\d+/g);
       var newDataLen = dataInfo[2] - totalLogLength;
       if (newDataLen != 0) {
         $.ajax({
           type: "GET",
-          url: "/log" + baseParams + "&byteLength=" + newDataLen,
+          url: getRESTEndPoint() + baseParams + "&byteLength=" + newDataLen,
           success: function (data) {
             var newlineIndex = data.indexOf('\n');
             var dataInfo = data.substring(0, newlineIndex).match(/\d+/g);