Adds kind-of-live thumbnail generation for a potential tab switcher.

chrome/browser/renderer_host/backing_store_manager.cc

Index: chrome/browser/renderer_host/backing_store_manager.cc
diff --git a/chrome/browser/renderer_host/backing_store_manager.cc b/chrome/browser/renderer_host/backing_store_manager.cc
index 3ff2e76b5ddebe1bdfeb171a0bdb9b3eacb4b59c..7b16a43464cc9264dceb817c2ebe787a807e6a17 100644
--- a/chrome/browser/renderer_host/backing_store_manager.cc
+++ b/chrome/browser/renderer_host/backing_store_manager.cc
@@ -7,6 +7,7 @@
 #include "base/sys_info.h"
 #include "chrome/browser/renderer_host/backing_store.h"
 #include "chrome/browser/renderer_host/render_widget_host.h"
+#include "chrome/browser/renderer_host/render_widget_host_painting_observer.h"
 #include "chrome/common/chrome_constants.h"
 
 namespace {
@@ -15,35 +16,59 @@ typedef OwningMRUCache<RenderWidgetHost*, BackingStore*> BackingStoreCache;
 static BackingStoreCache* cache = NULL;
 
 // Returns the size of the backing store cache.
-static int GetBackingStoreCacheSize() {
+static size_t GetBackingStoreCacheSize() {
   // This uses a similar approach to GetMaxRendererProcessCount.  The goal
   // is to reduce memory pressure and swapping on low-resource machines.
-  static const int kMaxDibCountByRamTier[] = {
+  static const size_t kMaxDibCountByRamTier[] = {
     2,  // less than 256MB
     3,  // 256MB
     4,  // 512MB
     5   // 768MB and above
   };
 
-  static int max_size = kMaxDibCountByRamTier[
+  static size_t max_size = kMaxDibCountByRamTier[
           std::min(base::SysInfo::AmountOfPhysicalMemoryMB() / 256,
                    static_cast<int>(arraysize(kMaxDibCountByRamTier)) - 1)];
   return max_size;
 }
 
+// Expires the given backing store from the cache.
+void ExpireBackingStoreAt(BackingStoreCache::iterator backing_store) {
+  RenderWidgetHost* rwh = backing_store->second->render_widget_host();
+  if (rwh->painting_observer()) {
+    rwh->painting_observer()->WidgetWillDestroyBackingStore(
+        backing_store->first,
+        backing_store->second);
+  }
+  cache->Erase(backing_store);
+}
+
 // Creates the backing store for the host based on the dimensions passed in.
 // Removes the existing backing store if there is one.
 BackingStore* CreateBackingStore(RenderWidgetHost* host,
                                  const gfx::Size& backing_store_size) {
+  // Remove any existing backing store in case we're replacing it.
   BackingStoreManager::RemoveBackingStore(host);
 
+  size_t max_cache_size = GetBackingStoreCacheSize();
+  if (max_cache_size > 0 && !cache)
+    cache = new BackingStoreCache(BackingStoreCache::NO_AUTO_EVICT);
+
+  if (cache->size() >= max_cache_size) {
+    // Need to remove an old backing store to make room for the new one. We
+    // don't want to do this when the backing store is being replace by a new
+    // one for the same tab, but this case won't get called then: we'll have
+    // removed the onld one in the RemoveBackingStore above, and the cache
+    // won't be over-sized.
+    //
+    // Crazy C++ alert: rbegin.base() is a forward iterator pointing to end(),
+    // so we need to do -- to move one back to the actual last item.
+    ExpireBackingStoreAt(--cache->rbegin().base());
+  }
+
   BackingStore* backing_store = host->AllocBackingStore(backing_store_size);
-  int backing_store_cache_size = GetBackingStoreCacheSize();
-  if (backing_store_cache_size > 0) {
-    if (!cache)
-      cache = new BackingStoreCache(backing_store_cache_size);
+  if (max_cache_size > 0)
     cache->Put(host, backing_store);
-  }
   return backing_store;
 }
 
@@ -110,7 +135,6 @@ void BackingStoreManager::RemoveBackingStore(RenderWidgetHost* host) {
   BackingStoreCache::iterator it = cache->Peek(host);
   if (it == cache->end())
     return;
-
   cache->Erase(it);
 
   if (cache->empty()) {
@@ -118,3 +142,12 @@ void BackingStoreManager::RemoveBackingStore(RenderWidgetHost* host) {
     cache = NULL;
   }
 }
+
+// static
+bool BackingStoreManager::ExpireBackingStoreForTest(RenderWidgetHost* host) {
+  BackingStoreCache::iterator it = cache->Peek(host);
+  if (it == cache->end())
+    return false;
+  ExpireBackingStoreAt(it);
+  return true;
+}