Do not precache when the cache size is small

components/precache/content/precache_manager.cc

Index: components/precache/content/precache_manager.cc
diff --git a/components/precache/content/precache_manager.cc b/components/precache/content/precache_manager.cc
index f4333c12d64e9b02779ca6a8de304a99f2e8bcf5..81a9514e2916e04f17ce98e77ab3487fdf78905c 100644
--- a/components/precache/content/precache_manager.cc
+++ b/components/precache/content/precache_manager.cc
@@ -12,6 +12,7 @@
 #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"
@@ -26,6 +27,9 @@
 #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;
 
@@ -111,6 +115,83 @@ PrecacheManager::AllowedType PrecacheManager::PrecachingAllowed() const {
   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);
@@ -128,12 +209,18 @@ void PrecacheManager::StartPrecaching(
       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(