Improve GWT SDM debug session recompilation time by a factor of 10

One of the major features of upcoming GWT release 2.7 is incremental
compilation in SDM session.  Because Buck recompilation is
integrated as HTTP filter on every request, this optimization is
jeopardized: Buck is unaware that a SDM debug session is active.
We cannot entirely skip the Buck integration in SDM debug session
as the site must be initialized at least once.

Pass a Java property from Eclipse launch configuration and maintain
initialization map with initialized flag per user agent for the site.
This improves the time by a factor of 10, ca. 2 sec. for incremental
recompilation on my laptop:

   Compilation succeeded -- 1,822s
   Linking into
   <site>/gerrit-gwtui/com.google.gerrit.GerritGwtUI/compile-5/war/gerrit_ui;
   Writing extras to
   <site>/gerrit-gwtui/com.google.gerrit.GerritGwtUI/compile-5/extras/gerrit_ui
   Link succeeded
   Linking succeeded -- 0,190s
   2,140s total -- Compile completed

Change-Id: Id2cb19a675d500c04e6748216a77dbb4f26fa1ab

Documentation/dev-eclipse.txt

@@ -58,7 +58,7 @@ link:http://www.gwtproject.org/articles/superdevmode.html[Super Dev Mode].
 * Make sure to activate source maps feature in your browser
 * Load Gerrit page `http://localhost:8080`
 * Open developer tools, source tab
-* Click on `Dev Mode On` bookmark
+* Click on `Dev Mode On` bookmark to incrementally recompile changed files
 * Select `gerrit_ui` module to compile (the `Compile` button can also be used
 as a bookmarklet).
 * Navigate on the left to: sourcemaps/gerrit_ui folder (`Ctrl+O` key shortcut

gerrit-pgm/src/main/java/com/google/gerrit/pgm/http/jetty/JettyServer.java

@@ -578,7 +578,10 @@ public class JettyServer {
     p.deleteOnExit();
 
     app.addFilter(new FilterHolder(new Filter() {
+      private final boolean gwtuiRecompile =
+          System.getProperty("gerrit.disable-gwtui-recompile") == null;
       private final UserAgentRule rule = new UserAgentRule();
+      private final Set<String> uaInitialized = new HashSet<>();
       private String lastTarget;
       private long lastTime;
 
@@ -587,30 +590,32 @@ public class JettyServer {
           FilterChain chain) throws IOException, ServletException {
         String pkg = "gerrit-gwtui";
         String target = "ui_" + rule.select((HttpServletRequest) request);
-        String rule = "//" + pkg + ":" + target;
-        // TODO(davido): instead of assuming specific Buck's internal
-        // target directory for gwt_binary() artifacts, ask Buck for
-        // the location of user agent permutation GWT zip, e. g.:
-        // $ buck targets --show_output //gerrit-gwtui:ui_safari \
-        //    | awk '{print $2}'
-        String child = String.format("%s/__gwt_binary_%s__", pkg, target);
-        File zip = new File(new File(gen, child), target + ".zip");
+        if (gwtuiRecompile || !uaInitialized.contains(target)) {
+          String rule = "//" + pkg + ":" + target;
+          // TODO(davido): instead of assuming specific Buck's internal
+          // target directory for gwt_binary() artifacts, ask Buck for
+          // the location of user agent permutation GWT zip, e. g.:
+          // $ buck targets --show_output //gerrit-gwtui:ui_safari \
+          //    | awk '{print $2}'
+          String child = String.format("%s/__gwt_binary_%s__", pkg, target);
+          File zip = new File(new File(gen, child), target + ".zip");
 
-        synchronized (this) {
-          try {
-            build(root, gen, rule);
-          } catch (BuildFailureException e) {
-            displayFailure(rule, e.why, (HttpServletResponse) res);
-            return;
-          }
+          synchronized (this) {
+            try {
+              build(root, gen, rule);
+            } catch (BuildFailureException e) {
+              displayFailure(rule, e.why, (HttpServletResponse) res);
+              return;
+            }
 
-          if (!target.equals(lastTarget) || lastTime != zip.lastModified()) {
-            lastTarget = target;
-            lastTime = zip.lastModified();
-            unpack(zip, dstwar);
+            if (!target.equals(lastTarget) || lastTime != zip.lastModified()) {
+              lastTarget = target;
+              lastTime = zip.lastModified();
+              unpack(zip, dstwar);
+            }
           }
+          uaInitialized.add(target);
         }
-
         chain.doFilter(request, res);
       }
 

tools/eclipse/gerrit_gwt_debug.launch

@@ -18,5 +18,5 @@
 <stringAttribute key="org.eclipse.jdt.launching.MAIN_TYPE" value="com.google.gerrit.gwtdebug.GerritGwtDebugLauncher"/>
 <stringAttribute key="org.eclipse.jdt.launching.PROGRAM_ARGUMENTS" value="-noprecompile -src ${resource_loc:/gerrit} -workDir ${resource_loc:/gerrit}/buck-out/gen/gerrit-gwtui com.google.gerrit.GerritGwtUI -- --console-log --show-stack-trace -d ${resource_loc:/gerrit}/../gerrit_testsite"/>
 <stringAttribute key="org.eclipse.jdt.launching.PROJECT_ATTR" value="gerrit"/>
-<stringAttribute key="org.eclipse.jdt.launching.VM_ARGUMENTS" value="-Xmx1024M&#10;-XX:MaxPermSize=256M"/>
+<stringAttribute key="org.eclipse.jdt.launching.VM_ARGUMENTS" value="-Xmx1024M&#10;-XX:MaxPermSize=256M&#10;-Dgerrit.disable-gwtui-recompile=true"/>
 </launchConfiguration>