Serve latest version of plugin resources when cache is stale

When serving HTTP plugin resources from cache,
we do need to check the cached lastModifiedDate
against the underlying plugin entry latest update.

If dates do not match, we need to fetch again the
entry and return it back to the client, updating
the in-memory cache.

Without this fix, the in-memory cached copy was
always returned even if the one on the file-system
was actually updated.

Change-Id: I36ee9989ade0640fb5ceda73749efb852d6c5801

gerrit-httpd/src/main/java/com/google/gerrit/httpd/plugins/HttpPluginServlet.java

@@ -24,6 +24,7 @@ import com.google.common.base.Strings;
 import com.google.common.cache.Cache;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
+import com.google.common.net.HttpHeaders;
 import com.google.gerrit.extensions.registration.RegistrationHandle;
 import com.google.gerrit.httpd.restapi.RestApiServlet;
 import com.google.gerrit.server.MimeUtilFileTypeRegistry;
@@ -260,7 +261,7 @@ class HttpPluginServlet extends HttpServlet
     String file = pathInfo.substring(1);
     ResourceKey key = new ResourceKey(holder.plugin, file);
     Resource rsc = resourceCache.getIfPresent(key);
-    if (rsc != null) {
+    if (rsc != null && req.getHeader(HttpHeaders.IF_MODIFIED_SINCE) == null) {
       rsc.send(req, res);
       return;
     }
@@ -278,7 +279,11 @@ class HttpPluginServlet extends HttpServlet
         PluginContentScanner scanner = holder.plugin.getContentScanner();
         Optional<PluginEntry> entry = scanner.getEntry(file);
         if (entry.isPresent()) {
-          sendResource(scanner, entry.get(), key, res);
+          if (hasUpToDateCachedResource(rsc, entry.get().getTime())) {
+            rsc.send(req, res);
+          } else {
+            sendResource(scanner, entry.get(), key, res);
+          }
         } else {
           resourceCache.put(key, Resource.NOT_FOUND);
           Resource.NOT_FOUND.send(req, res);
@@ -297,12 +302,26 @@ class HttpPluginServlet extends HttpServlet
       }
       if (!entry.isPresent() && file.endsWith("/index.html")) {
         String pfx = file.substring(0, file.length() - "index.html".length());
-        sendAutoIndex(scanner, pfx, holder.plugin.getName(), key, res,
-            holder.plugin.getSrcFile().lastModified());
+        long pluginLastModified = holder.plugin.getSrcFile().lastModified();
+        if (hasUpToDateCachedResource(rsc, pluginLastModified)) {
+          rsc.send(req, res);
+        } else {
+          sendAutoIndex(scanner, pfx, holder.plugin.getName(), key, res,
+              pluginLastModified);
+        }
       } else if (entry.isPresent() && entry.get().getName().endsWith(".md")) {
-        sendMarkdownAsHtml(scanner, entry.get(), holder.plugin.getName(), key, res);
+        if (hasUpToDateCachedResource(rsc, entry.get().getTime())) {
+          rsc.send(req, res);
+        } else {
+          sendMarkdownAsHtml(scanner, entry.get(), holder.plugin.getName(),
+              key, res);
+        }
       } else if (entry.isPresent()) {
-        sendResource(scanner, entry.get(), key, res);
+        if (hasUpToDateCachedResource(rsc, entry.get().getTime())) {
+          rsc.send(req, res);
+        } else {
+          sendResource(scanner, entry.get(), key, res);
+        }
       } else {
         resourceCache.put(key, Resource.NOT_FOUND);
         Resource.NOT_FOUND.send(req, res);
@@ -313,6 +332,11 @@ class HttpPluginServlet extends HttpServlet
     }
   }
 
+  private boolean hasUpToDateCachedResource(Resource cachedResource, long lastUpdateTime) {
+    return cachedResource != null
+        && cachedResource.isUnchanged(lastUpdateTime);
+  }
+
   private void appendEntriesSection(PluginContentScanner scanner, List<PluginEntry> entries,
       String sectionTitle, StringBuilder md, String prefix,
       int nameOffset) throws IOException {

gerrit-httpd/src/main/java/com/google/gerrit/httpd/plugins/Resource.java

@@ -34,9 +34,17 @@ abstract class Resource {
       CacheHeaders.setNotCacheable(res);
       res.sendError(HttpServletResponse.SC_NOT_FOUND);
     }
+
+    @Override
+    boolean isUnchanged(long latestModifiedDate) {
+      return false;
+    }
   };
 
+  abstract boolean isUnchanged(long latestModifiedDate);
+
   abstract int weigh();
+
   abstract void send(HttpServletRequest req, HttpServletResponse res)
       throws IOException;
-}
+}

gerrit-httpd/src/main/java/com/google/gerrit/httpd/plugins/SmallResource.java

@@ -71,4 +71,9 @@ final class SmallResource extends Resource {
     res.setContentLength(data.length);
     res.getOutputStream().write(data);
   }
+
+  @Override
+  boolean isUnchanged(long lastModified) {
+    return this.lastModified == lastModified;
+  }
 }