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 50e9da10ea8da4c3747b3e33011a6949aee72dca..530561ac3c78605cb683c7d6c2f5308c3590f807 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"
@@ -756,8 +757,8 @@ int HttpCache::Transaction::HandleResult(int rv) {
 // 2. Cached entry, no validation:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
-//   SetupEntryForRead()
+//   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
+//   BeginCacheValidation() -> SetupEntryForRead()
 //
 //   Read():
 //   CacheReadData*
@@ -765,10 +766,10 @@ int HttpCache::Transaction::HandleResult(int rv) {
 // 3. Cached entry, validation (304):
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
-//   SendRequest* -> SuccessfulSendRequest -> UpdateCachedResponse ->
-//   CacheWriteResponse* -> UpdateCachedResponseComplete ->
-//   OverwriteCachedResponse -> PartialHeadersReceived
+//   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
+//   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
+//   UpdateCachedResponse -> CacheWriteResponse* -> UpdateCachedResponseComplete
+//   -> OverwriteCachedResponse -> PartialHeadersReceived
 //
 //   Read():
 //   CacheReadData*
@@ -776,10 +777,10 @@ int HttpCache::Transaction::HandleResult(int rv) {
 // 4. Cached entry, validation and replace (200):
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
-//   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse ->
-//   CacheWriteResponse* -> DoTruncateCachedData* -> TruncateCachedMetadata* ->
-//   PartialHeadersReceived
+//   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
+//   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
+//   OverwriteCachedResponse -> CacheWriteResponse* -> DoTruncateCachedData* ->
+//   TruncateCachedMetadata* -> PartialHeadersReceived
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
@@ -787,12 +788,12 @@ int HttpCache::Transaction::HandleResult(int rv) {
 // 5. Sparse entry, partially cached, byte range request:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> CacheQueryData* ->
-//   ValidateEntryHeadersAndContinue() -> StartPartialCacheValidation ->
-//   CompletePartialCacheValidation -> BeginCacheValidation() -> SendRequest* ->
-//   SuccessfulSendRequest -> UpdateCachedResponse -> CacheWriteResponse* ->
-//   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
-//   PartialHeadersReceived
+//   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
+//   CacheQueryData* -> ValidateEntryHeadersAndContinue() ->
+//   StartPartialCacheValidation -> CompletePartialCacheValidation ->
+//   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
+//   UpdateCachedResponse -> CacheWriteResponse* -> UpdateCachedResponseComplete
+//   -> OverwriteCachedResponse -> PartialHeadersReceived
 //
 //   Read() 1:
 //   NetworkRead* -> CacheWriteData*
@@ -831,8 +832,9 @@ int HttpCache::Transaction::HandleResult(int rv) {
 //   itself.
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
-//   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse
+//   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
+//   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
+//   OverwriteCachedResponse
 //
 // 10. HEAD. Sparse entry, partially cached:
 //   Serve the request from the cache, as long as it doesn't require
@@ -843,6 +845,19 @@ int HttpCache::Transaction::HandleResult(int rv) {
 //   Start(): Basically the same as example 7, as we never create a partial_
 //   object for this request.
 //
+// 11. Prefetch, not-cached entry:
+//   The same as example 1. The "unused_since_prefetch" bit is stored as true in
+//   UpdateCachedResponse.
+//
+// 12. Prefetch, cached entry:
+//   Like examples 2-4, only CacheToggleUnusedSincePrefetch* is inserted between
+//   CacheReadResponse* and CacheDispatchValidation if the unused_since_prefetch
+//   bit is unset.
+//
+// 13. Cached entry, unused_since_prefetch is true:
+//   Like examples 2-4, only CacheToggleUnusedSincePrefetch* is inserted between

[rvargas (doing something else), 2015/01/09 20:16:13]

mind making this explicit? Assuming the entry is used soon enough (hopefully the
common case), this morphs #s 3-4 to follow #2, so it's not really like them.

[jkarlin, 2015/01/12 13:01:14]

Done.

+//   CacheReadResponse* and CacheDispatchValidation and a validation request is
+//   not sent if the entry is less than 5 minutes old.
 int HttpCache::Transaction::DoLoop(int result) {
   DCHECK(next_state_ != STATE_NONE);
 
@@ -951,6 +966,17 @@ int HttpCache::Transaction::DoLoop(int result) {
       case STATE_CACHE_READ_RESPONSE_COMPLETE:
         rv = DoCacheReadResponseComplete(rv);
         break;
+      case STATE_CACHE_DISPATCH_VALIDATION:
+        DCHECK_EQ(OK, rv);
+        rv = DoCacheDispatchValidation();
+        break;
+      case STATE_TOGGLE_UNUSED_SINCE_PREFETCH:
+        DCHECK_EQ(OK, rv);
+        rv = DoCacheToggleUnusedSincePrefetch();
+        break;
+      case STATE_TOGGLE_UNUSED_SINCE_PREFETCH_COMPLETE:
+        rv = DoCacheToggleUnusedSincePrefetchComplete(rv);
+        break;
       case STATE_CACHE_WRITE_RESPONSE:
         DCHECK_EQ(OK, rv);
         rv = DoCacheWriteResponse();
@@ -1630,6 +1656,7 @@ 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) {
@@ -1848,6 +1875,42 @@ int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
   if (response_.headers->GetContentLength() == current_size)
     truncated_ = false;
 
+  if ((response_.unused_since_prefetch &&
+       !(request_->load_flags & LOAD_PREFETCH)) ||
+      (!response_.unused_since_prefetch &&
+       (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;
+    return OK;
+  }
+
+  next_state_ = STATE_CACHE_DISPATCH_VALIDATION;
+  return OK;
+}
+
+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;
+  next_state_ = STATE_CACHE_DISPATCH_VALIDATION;
+  return OK;
+}
+
+int HttpCache::Transaction::DoCacheDispatchValidation() {

[rvargas (doing something else), 2015/01/09 20:16:13]

Time to make sure that method definition follows the declaration order.

[jkarlin, 2015/01/12 13:01:14]

Done. Thanks!

   // We now have access to the cache entry.
   //
   //  o if we are a reader for the transaction, then we can start reading the
@@ -1861,6 +1924,7 @@ 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);
@@ -1875,7 +1939,6 @@ int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
     case WRITE:
     default:
       NOTREACHED();
-      result = ERR_FAILED;
   }
   return result;
 }
@@ -2545,6 +2608,16 @@ ValidationType HttpCache::Transaction::RequiresValidation() {
   if (effective_load_flags_ & LOAD_PREFERRING_CACHE)
     return VALIDATION_NONE;
 
+  if (response_.unused_since_prefetch &&
+      !(effective_load_flags_ & LOAD_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;
 
@@ -2552,8 +2625,9 @@ 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.
@@ -2614,7 +2688,8 @@ 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,