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..23742a5dbf602d052c7036bbdc06d831e3a76d7c 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"
@@ -741,7 +742,9 @@ int HttpCache::Transaction::HandleResult(int rv) {
   return rv;
 }
 
-// A few common patterns: (Foo* means Foo -> FooComplete)
+// A few common patterns: Foo* means Foo -> FooComplete
+//                        [Foo] means state Foo may be skipped

[rvargas (doing something else), 2015/01/09 02:25:34]

Instead of adding an optional state pairs to known patterns I think it is better
to leave the optional states out (the common case is no prefetch) and add
explicit cases for a prefetch load and, say, an expired load that is used
because it was prefetched.

[jkarlin, 2015/01/09 14:03:50]

Done.

+//
 //
 // 1. Not-cached entry:
 //   Start():
@@ -756,7 +759,8 @@ int HttpCache::Transaction::HandleResult(int rv) {
 // 2. Cached entry, no validation:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
+//   -> [CacheToggleUnusedSincePrefetch*] -> CacheReadResponseContinue ->
+//   BeginPartialCacheValidation() -> BeginCacheValidation() ->
 //   SetupEntryForRead()
 //
 //   Read():
@@ -765,10 +769,11 @@ 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
+//   -> [CacheToggleUnusedSincePrefetch*] -> CacheReadResponseContinue ->
+//   BeginPartialCacheValidation() -> BeginCacheValidation() -> SendRequest* ->
+//   SuccessfulSendRequest -> UpdateCachedResponse -> CacheWriteResponse* ->
+//   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
+//   PartialHeadersReceived
 //
 //   Read():
 //   CacheReadData*
@@ -776,10 +781,10 @@ 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
+//   -> [CacheToggleUnusedSincePrefetch*] -> CacheReadResponseContinue ->
+//   BeginPartialCacheValidation() -> BeginCacheValidation() -> SendRequest* ->
+//   SuccessfulSendRequest -> OverwriteCachedResponse -> CacheWriteResponse* ->
+//   DoTruncateCachedData* -> TruncateCachedMetadata* -> PartialHeadersReceived
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
@@ -787,7 +792,8 @@ int HttpCache::Transaction::HandleResult(int rv) {
 // 5. Sparse entry, partially cached, byte range request:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> CacheQueryData* ->
+//   -> [CacheToggleUnusedSincePrefetch*] -> CacheReadResponseContinue ->
+//   BeginPartialCacheValidation() -> CacheQueryData* ->
 //   ValidateEntryHeadersAndContinue() -> StartPartialCacheValidation ->
 //   CompletePartialCacheValidation -> BeginCacheValidation() -> SendRequest* ->
 //   SuccessfulSendRequest -> UpdateCachedResponse -> CacheWriteResponse* ->
@@ -831,8 +837,9 @@ int HttpCache::Transaction::HandleResult(int rv) {
 //   itself.
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
-//   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse
+//   -> [CacheToggleUnusedSincePrefetch*] -> CacheReadResponseContinue ->
+//   BeginPartialCacheValidation() -> BeginCacheValidation() -> SendRequest* ->
+//   SuccessfulSendRequest -> OverwriteCachedResponse
 //
 // 10. HEAD. Sparse entry, partially cached:
 //   Serve the request from the cache, as long as it doesn't require
@@ -842,7 +849,7 @@ int HttpCache::Transaction::HandleResult(int rv) {
 //
 //   Start(): Basically the same as example 7, as we never create a partial_
 //   object for this request.
-//
+
 int HttpCache::Transaction::DoLoop(int result) {
   DCHECK(next_state_ != STATE_NONE);
 
@@ -951,6 +958,16 @@ int HttpCache::Transaction::DoLoop(int result) {
       case STATE_CACHE_READ_RESPONSE_COMPLETE:
         rv = DoCacheReadResponseComplete(rv);
         break;
+      case STATE_CACHE_READ_RESPONSE_CONTINUE:
+        rv = DoCacheReadResponseContinue();

[rvargas (doing something else), 2015/01/09 02:25:33]

dcheck rv == ok

[jkarlin, 2015/01/09 14:03:50]

Done.

+        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 +1647,7 @@ 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 +1866,42 @@ 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_READ_RESPONSE_CONTINUE;
+  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_READ_RESPONSE_CONTINUE;
+  return OK;
+}
+
+int HttpCache::Transaction::DoCacheReadResponseContinue() {

[rvargas (doing something else), 2015/01/09 02:25:34]

Ideally states should be named by what is done in that state not how we got
there... and we are not concerned about reading the response anymore.

How about STATE_CACHE_DISPATCH_READ? STATE_CACHE_USE_RESPONSE? I'm not really
happy with either one but they stack on top of READ_RESPONSE_CONTINUE to me.

(-this is a dispatch but not of a read)
(-the response may not be really used, and it was used before this state)

STATE_CACHE_DISPATCH_VALIDATION ?

[jkarlin, 2015/01/09 14:03:50]

Yeah, I had a hell of a time coming up with an accurate name and just went with
Continue. At one point I had "CacheReadOrValidate" but that left out the UPDATE
case. READ_WRITE is the common case so I could see naming it for that.
STATE_CACHE_DISPATCH_VALIDATION it is.

   // 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 +1915,7 @@ 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 +1930,6 @@ int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
     case WRITE:
     default:
       NOTREACHED();
-      result = ERR_FAILED;
   }
   return result;
 }
@@ -2545,6 +2599,16 @@ 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 +2616,9 @@ 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 +2679,8 @@ 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,