Let prefetched resources skip cache revalidation once for a short duration.

net/http/http_cache_transaction.cc

Index: net/http/http_cache_transaction.cc
diff --git a/net/http/http_cache_transaction.cc b/net/http/http_cache_transaction.cc
index 7139abbdd79db0645bc37137d486365ad8fbbea8..955848c15b0954bddc28e724099790ac6c115611 100644
--- a/net/http/http_cache_transaction.cc
+++ b/net/http/http_cache_transaction.cc
@@ -27,6 +27,7 @@
 #include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
+#include "base/time/clock.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "net/base/completion_callback.h"
@@ -951,6 +952,12 @@ int HttpCache::Transaction::DoLoop(int result) {
       case STATE_CACHE_READ_RESPONSE_COMPLETE:
         rv = DoCacheReadResponseComplete(rv);
         break;
+      case STATE_TOGGLE_UNUSED_SINCE_PREFETCH:
+        rv = DoCacheToggleUnusedSincePrefetch();

[rvargas (doing something else), 2014/12/24 02:15:24]

DCHECK rv == ok?

[jkarlin, 2015/01/08 16:13:03]

Done.

+        break;
+      case STATE_TOGGLE_UNUSED_SINCE_PREFETCH_COMPLETE:

[rvargas (doing something else), 2014/12/24 02:15:24]

These states have to be incorporated on the comment preceding this method. 

[jkarlin, 2015/01/08 16:13:03]

Done.

+        rv = DoCacheToggleUnusedSincePrefetchComplete(rv);
+        break;
       case STATE_CACHE_WRITE_RESPONSE:
         DCHECK_EQ(OK, rv);
         rv = DoCacheWriteResponse();
@@ -1534,6 +1541,7 @@ int HttpCache::Transaction::DoUpdateCachedResponse() {
   response_.response_time = new_response_->response_time;
   response_.request_time = new_response_->request_time;
   response_.network_accessed = new_response_->network_accessed;
+  response_.unused_since_prefetch = new_response_->unused_since_prefetch;
 
   if (response_.headers->HasHeaderValue("cache-control", "no-store")) {
     if (!entry_->doomed) {
@@ -1567,6 +1575,7 @@ int HttpCache::Transaction::DoUpdateCachedResponseComplete(int result) {
       cache_->ConvertWriterToReader(entry_);
       mode_ = READ;
     }
+
     // We no longer need the network transaction, so destroy it.
     final_upload_progress_ = network_trans_->GetUploadProgress();
     ResetNetworkTransaction();
@@ -1707,6 +1716,40 @@ int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
   if (response_.headers->GetContentLength() == current_size)
     truncated_ = false;
 
+  if (response_.unused_since_prefetch ||
+      (!response_.unused_since_prefetch &&

[rvargas (doing something else), 2014/12/24 02:15:24]

!response_.unused_since_prefetch seems unnecessary.

[jkarlin, 2015/01/08 16:13:03]

Thanks!

+       request_->load_flags & LOAD_PREFETCH)) {
+    // Either this is the first use of an entry since it was prefetched or
+    // this is a prefetch. The value of response.unused_since_prefetch is valid
+    // for this transaction but the bit needs to be flipped in storage.
+    next_state_ = STATE_TOGGLE_UNUSED_SINCE_PREFETCH;

[rvargas (doing something else), 2014/12/24 02:15:24]

looks like this is not really toggle, as two consecutive prefetches do not reset
the flag. Actually, you reset the bit, which seems wrong.

[jkarlin, 2015/01/08 16:13:04]

Nice catch on two consecutive prefetches. Fixed and added tests.

+    return OK;
+  }
+
+  return DoCacheReadResponseCompleteContinuation();

[rvargas (doing something else), 2014/12/24 02:15:24]

This begs to be a dedicated state on the machine.

[jkarlin, 2015/01/08 16:13:03]

Done.

+}
+
+int HttpCache::Transaction::DoCacheToggleUnusedSincePrefetch() {
+  // Write back the toggled value for the next use of this entry.
+  response_.unused_since_prefetch = !response_.unused_since_prefetch;
+
+  // TODO(jkarlin): If DoUpdateCachedResponse is also called for this
+  // transaction then metadata will be written to cache twice. If prefetching
+  // becomes more common, consider combining the writes.
+  target_state_ = STATE_TOGGLE_UNUSED_SINCE_PREFETCH_COMPLETE;
+  next_state_ = STATE_CACHE_WRITE_RESPONSE;
+  return OK;
+}
+
+int HttpCache::Transaction::DoCacheToggleUnusedSincePrefetchComplete(
+    int result) {
+  // Restore the original value for this transaction.
+  response_.unused_since_prefetch = !response_.unused_since_prefetch;
+
+  return DoCacheReadResponseCompleteContinuation();
+}
+
+int HttpCache::Transaction::DoCacheReadResponseCompleteContinuation() {
   // We now have access to the cache entry.
   //
   //  o if we are a reader for the transaction, then we can start reading the
@@ -1720,6 +1763,7 @@ int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
   //    conditionalized request (if-modified-since / if-none-match). We check
   //    if the request headers define a validation request.
   //
+  int result = ERR_FAILED;
   switch (mode_) {
     case READ:
       UpdateTransactionPattern(PATTERN_ENTRY_USED);
@@ -2172,6 +2216,7 @@ int HttpCache::Transaction::BeginCacheValidation() {
     // TODO(ricea): Is this pattern okay for asynchronous revalidations?
     UpdateTransactionPattern(PATTERN_ENTRY_USED);
     RecordOfflineStatus(effective_load_flags_, OFFLINE_STATUS_FRESH_CACHE);
+

[rvargas (doing something else), 2014/12/24 02:15:24]

?

[jkarlin, 2015/01/08 16:13:03]

Done.

     return SetupEntryForRead();
   } else {
     // Make the network request conditional, to see if we may reuse our cached
@@ -2344,6 +2389,15 @@ ValidationType HttpCache::Transaction::RequiresValidation() {
   if (effective_load_flags_ & LOAD_PREFERRING_CACHE)
     return VALIDATION_NONE;
 
+  if (response_.unused_since_prefetch &&
+      response_.headers->GetCurrentAge(
+          response_.request_time, response_.response_time,
+          cache_->clock_->Now()) < TimeDelta::FromMinutes(kPrefetchReuseMins)) {
+    // The first use of a resource after prefetch within a short window skips
+    // validation.
+    return VALIDATION_NONE;
+  }
+
   if (effective_load_flags_ & (LOAD_VALIDATE_CACHE | LOAD_ASYNC_REVALIDATION))
     return VALIDATION_SYNCHRONOUS;
 
@@ -2351,8 +2405,9 @@ ValidationType HttpCache::Transaction::RequiresValidation() {
     return VALIDATION_SYNCHRONOUS;
 
   ValidationType validation_required_by_headers =
-      response_.headers->RequiresValidation(
-          response_.request_time, response_.response_time, Time::Now());
+      response_.headers->RequiresValidation(response_.request_time,
+                                            response_.response_time,
+                                            cache_->clock_->Now());
 
   if (validation_required_by_headers == VALIDATION_ASYNCHRONOUS) {
     // Asynchronous revalidation is only supported for GET and HEAD methods.
@@ -2413,7 +2468,8 @@ bool HttpCache::Transaction::ConditionalizeRequest() {
         response_.headers->GetFreshnessLifetimes(response_.response_time);
     if (lifetimes.staleness > TimeDelta()) {
       TimeDelta current_age = response_.headers->GetCurrentAge(
-          response_.request_time, response_.response_time, Time::Now());
+          response_.request_time, response_.response_time,
+          cache_->clock_->Now());
 
       custom_request_->extra_headers.SetHeader(
           kFreshnessHeader,