Do not precache when the cache size is small

components/precache/content/precache_manager.h

Index: components/precache/content/precache_manager.h
diff --git a/components/precache/content/precache_manager.h b/components/precache/content/precache_manager.h
index e6bcc8094ab5e9a9bd5abed331d49c997cb20e85..04978dfae6797f5df1aa84733879a61b37e1dfad 100644
--- a/components/precache/content/precache_manager.h
+++ b/components/precache/content/precache_manager.h
@@ -21,6 +21,8 @@
 #include "components/history/core/browser/history_types.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "components/precache/core/precache_fetcher.h"
+#include "net/disk_cache/disk_cache.h"
+#include "net/http/http_cache.h"
 #include "url/gurl.h"
 
 namespace base {
@@ -53,6 +55,9 @@ class PrecacheUnfinishedWork;
 // Visible for test.
 size_t NumTopHosts();
 
+// Minimum required cache size (inclusive).
+constexpr int kMinCacheSizeBytes = 40e6;
+
 // Class that manages all precaching-related activities. Owned by the
 // BrowserContext that it is constructed for. Use
 // PrecacheManagerFactory::GetForBrowserContext to get an instance of this
@@ -129,6 +134,10 @@ class PrecacheManager : public KeyedService,
   FRIEND_TEST_ALL_PREFIXES(PrecacheManagerTest, RecordStatsForFetchWithNonHTTP);
   FRIEND_TEST_ALL_PREFIXES(PrecacheManagerTest,
                            RecordStatsForFetchWithSizeZero);
+  FRIEND_TEST_ALL_PREFIXES(PrecacheManagerTest,
+                           StartPrecachingStopsOnSmallCaches);
+  FRIEND_TEST_ALL_PREFIXES(PrecacheManagerTest,
+                           StartPrecachingWithGoodSizedCache);
 
   enum class AllowedType {
     ALLOWED,
@@ -156,6 +165,14 @@ class PrecacheManager : public KeyedService,
   // gets the list of TopHosts for metrics purposes, but otherwise does nothing.
   void OnHostsReceivedThenDone(const history::TopHostsList& host_counts);
 
+  // Chain of callbacks for StartPrecaching that make sure that we only precache
+  // if there is a cache big enough.
+  void PrecacheIfCacheIsBigEnough(
+      net::URLRequestContextGetter* url_request_context_getter);
+  void OnCacheBackendReceived(int net_error_code);
+  void OnCacheSizeReceived(int cache_size_bytes);
+  void OnCacheSizeReceivedInUIThread(int cache_size_bytes);
+
   // Returns true if precaching is allowed for the browser context.
   AllowedType PrecachingAllowed() const;
 
@@ -203,6 +220,15 @@ class PrecacheManager : public KeyedService,
   // Flag indicating whether or not precaching is currently in progress.
   bool is_precaching_;
 
+  // Pointer to the backend of the cache. Required to get the size of the cache.
+  // It is not owned and it is reset on demand via callbacks.
+  // It should only be accessed from the IO thread.
+  disk_cache::Backend* cache_backend_;
+
+  // The minimum cache size allowed for precaching. We allow this to be modified
+  // for testing purposes.
+  int min_cache_size_bytes_ = kMinCacheSizeBytes;
+
   // Work that hasn't yet finished.
   std::unique_ptr<PrecacheUnfinishedWork> unfinished_work_;