Do not precache when the cache size is small

components/precache/content/precache_manager.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/precache/content/precache_manager.h"
 
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "base/metrics/field_trial.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "components/history/core/browser/history_service.h"
 #include "components/precache/core/precache_database.h"
 #include "components/precache/core/precache_switches.h"
 #include "components/precache/core/proto/unfinished_work.pb.h"
 #include "components/prefs/pref_service.h"
 #include "components/sync/driver/sync_service.h"
 #include "components/variations/metrics_util.h"
 #include "components/variations/variations_associated_data.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/storage_partition.h"
 #include "net/base/network_change_notifier.h"
+#include "net/http/http_cache.h"
+#include "net/url_request/url_request_context.h"
+#include "net/url_request/url_request_context_getter.h"
 
 using content::BrowserThread;
 
 namespace {
 
 const char kPrecacheFieldTrialName[] = "Precache";
 const char kPrecacheFieldTrialEnabledGroup[] = "Enabled";
 const char kPrecacheFieldTrialControlGroup[] = "Control";
 const char kConfigURLParam[] = "config_url";
 const char kManifestURLPrefixParam[] = "manifest_url_prefix";
@@ skipping 65 matching lines @@
 
   // SyncService delegates to SyncPrefs, which must be called on the UI thread.
   if (history_service_ &&
       sync_service_->GetActiveDataTypes().Has(syncer::SESSIONS) &&
       !sync_service_->GetEncryptedDataTypes().Has(syncer::SESSIONS))
     return AllowedType::ALLOWED;
 
   return AllowedType::DISALLOWED;
 }
 
+void PrecacheManager::OnCacheBackendReceived(int net_error_code) {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
+  if (net_error_code == net::OK) {
+    DCHECK(cache_backend_);
+    int result = cache_backend_->CalculateSizeOfAllEntries(base::Bind(
+        &PrecacheManager::OnCacheSizeReceived, base::AsWeakPtr(this)));

[twifkak, 2016/11/18 20:27:37]

s/base::AsWeakPtr(this)/AsWeakPtr()/. Provided by the SupportsWeakPtr
superclass. Ditto below.

[jamartin, 2016/11/19 02:00:13]

Done.

+    if (result == net::ERR_IO_PENDING) {
+      // Wait for the callback.
+    } else if (result >= 0) {
+      // The result is the expected bytes already.
+      OnCacheSizeReceived(result);
+    } else {
+      // Error occurred. Couldn't get the size. Assume there is no cache.
+      OnCacheSizeReceived(0);
+    }
+  } else {  // net_error_code != net::OK
+    // Assume there is no cache.
+    OnCacheSizeReceived(0);
+  }
+  cache_backend_ = nullptr;
+}
+
+void PrecacheManager::OnCacheSizeReceived(int cache_size_bytes) {
+  // We can be either in IO or UI threads.
+
+  UMA_HISTOGRAM_MEMORY_KB("Precache.CacheSize.AllEntries",
+                          cache_size_bytes / 1024);
+  if (cache_size_bytes < min_cache_size_bytes_) {
+    BrowserThread::PostTask(
+        BrowserThread::UI, FROM_HERE,
+        base::Bind(&PrecacheManager::OnDone, base::AsWeakPtr(this)));
+  } else {
+    BrowserThread::PostTaskAndReplyWithResult(
+        BrowserThread::DB, FROM_HERE,
+        base::Bind(&PrecacheDatabase::GetUnfinishedWork,
+                   base::Unretained(precache_database_.get())),
+        base::Bind(&PrecacheManager::OnGetUnfinishedWorkDone, AsWeakPtr()));

[twifkak, 2016/11/18 20:27:37]

AFAICT, this means OnGetUnfinishedWorkDone may be called in the IO thread. This
is a problem, as it accesses some instance vars.

[jamartin, 2016/11/19 02:00:13]

Good catch.

+  }
+}
+
+void PrecacheManager::PrecacheIfCacheIsBigEnough() {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
+  CHECK(url_request_context_getter_);
+
+  // Continue with OnGetUnfinishedWorkDone only if the size of the cache is
+  // at least min_cache_size_bytes_.
+  // Class disk_cache::Backend does not expose its maximum size. However, caches
+  // are usually full, so we can use the size of all the entries stored in the
+  // cache (via CalculateSizeOfAllEntries) as a proxy of its maximum size.
+  net::URLRequestContext* context =
+      url_request_context_getter_->GetURLRequestContext();
+  url_request_context_getter_ = nullptr;
+  if (!context) {
+    OnCacheSizeReceived(0);
+    return;
+  }
+  net::HttpTransactionFactory* factory = context->http_transaction_factory();
+  if (!factory) {
+    OnCacheSizeReceived(0);
+    return;
+  }
+  net::HttpCache* cache = factory->GetCache();
+  if (cache) {
+    const int net_error_code = cache->GetBackend(
+        &cache_backend_, base::Bind(&PrecacheManager::OnCacheBackendReceived,
+                                    base::AsWeakPtr(this)));
+    if (net_error_code != net::ERR_IO_PENDING) {
+      // No need to wait for the callback. The callback hasn't been called with
+      // the appropriate code, so we call it directly.
+      OnCacheBackendReceived(net_error_code);
+    }
+  } else {  // !cache.
+    // There is no known cache. Assume that there is no cache.
+    OnCacheSizeReceived(0);
+  }
+}
+
 void PrecacheManager::StartPrecaching(
     const PrecacheCompletionCallback& precache_completion_callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   if (is_precaching_) {
     DLOG(WARNING) << "Cannot start precaching because precaching is already "
                      "in progress.";
     return;
   }
   precache_completion_callback_ = precache_completion_callback;
 
   is_precaching_ = true;
   BrowserThread::PostTask(
       BrowserThread::DB, FROM_HERE,
       base::Bind(&PrecacheDatabase::SetLastPrecacheTimestamp,
                  base::Unretained(precache_database_.get()),
                  base::Time::Now()));
-  BrowserThread::PostTaskAndReplyWithResult(
-      BrowserThread::DB,
-      FROM_HERE,
-      base::Bind(&PrecacheDatabase::GetUnfinishedWork,
-                 base::Unretained(precache_database_.get())),
-      base::Bind(&PrecacheManager::OnGetUnfinishedWorkDone, AsWeakPtr()));
+
+  url_request_context_getter_ =

[twifkak, 2016/11/18 20:27:37]

Why is this an instance var? Pass via base::Bind(..., base::Unretained(...)).

(BTW, when setting an instance var on one thread and reading it on another, you
need a memory barrier at least; usually a mutex is the safest bet.)

[jamartin, 2016/11/19 02:00:13]

Way better. Thanks.

+      content::BrowserContext::GetDefaultStoragePartition(browser_context_)
+          ->GetURLRequestContext();
+  if (url_request_context_getter_) {
+    BrowserThread::PostTask(
+        BrowserThread::IO, FROM_HERE,
+        base::Bind(&PrecacheManager::PrecacheIfCacheIsBigEnough,
+                   base::AsWeakPtr(this)));
+  } else {  // !url_request_context_getter_.
+    OnCacheSizeReceived(0);
+  }
 }
 
 void PrecacheManager::OnGetUnfinishedWorkDone(
     std::unique_ptr<PrecacheUnfinishedWork> unfinished_work) {
   // Reset progress on a prefetch that has taken too long to complete.
   if (unfinished_work->has_start_time() &&
       base::Time::Now() -
               base::Time::FromInternalValue(unfinished_work->start_time()) >
           base::TimeDelta::FromHours(6)) {
     PrecacheFetcher::RecordCompletionStatistics(
@@ skipping 208 matching lines @@
       this));
   precache_fetcher_->Start();
 }
 
 void PrecacheManager::OnHostsReceivedThenDone(
     const history::TopHostsList& host_counts) {
   OnDone();
 }
 
 }  // namespace precache