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

net/http/http_cache_transaction.cc

 // Copyright (c) 2012 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 "net/http/http_cache_transaction.h"
 
 #include "build/build_config.h"
 
 #if defined(OS_POSIX)
 #include <unistd.h>
 #endif
 
 #include <algorithm>
 #include <string>
 
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/format_macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/rand_util.h"
 #include "base/strings/string_number_conversions.h"
 #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"
 #include "net/base/io_buffer.h"
 #include "net/base/load_flags.h"
 #include "net/base/load_timing_info.h"
 #include "net/base/net_errors.h"
 #include "net/base/net_log.h"
 #include "net/base/upload_data_stream.h"
 #include "net/cert/cert_status_flags.h"
@@ skipping 694 matching lines @@
 }
 
 int HttpCache::Transaction::HandleResult(int rv) {
   DCHECK(rv != ERR_IO_PENDING);
   if (!callback_.is_null())
     DoCallback(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():
 //   GetBackend* -> InitEntry -> OpenEntry* -> CreateEntry* -> AddToEntry* ->
 //   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse ->
 //   CacheWriteResponse* -> TruncateCachedData* -> TruncateCachedMetadata* ->
 //   PartialHeadersReceived
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
 //
 // 2. Cached entry, no validation:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
-//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
+//   -> [CacheToggleUnusedSincePrefetch*] -> CacheReadResponseContinue ->
+//   BeginPartialCacheValidation() -> BeginCacheValidation() ->
 //   SetupEntryForRead()
 //
 //   Read():
 //   CacheReadData*
 //
 // 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*
 //
 // 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*
 //
 // 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* ->
 //   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
 //   PartialHeadersReceived
 //
 //   Read() 1:
 //   NetworkRead* -> CacheWriteData*
 //
 //   Read() 2:
@@ skipping 23 matching lines @@
 //   Start(): Same as for a GET request (example #3)
 //
 //   Read():
 //   CacheReadData (returns 0)
 //
 // 9. HEAD. Cached entry, validation and replace (200):
 //   Pass through. The request dooms the old entry, as a HEAD won't be stored by
 //   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
 //   revalidation. Ignore missing ranges when deciding to revalidate. If the
 //   entry requires revalidation, ignore the whole request and go to full pass
 //   through (the result of the HEAD request will NOT update the entry).
 //
 //   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);
 
   int rv = result;
   do {
     State state = next_state_;
     next_state_ = STATE_NONE;
     switch (state) {
       case STATE_GET_BACKEND:
         DCHECK_EQ(OK, rv);
@@ skipping 88 matching lines @@
         DCHECK_EQ(OK, rv);
         rv = DoPartialHeadersReceived();
         break;
       case STATE_CACHE_READ_RESPONSE:
         DCHECK_EQ(OK, rv);
         rv = DoCacheReadResponse();
         break;
       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();
         break;
       case STATE_CACHE_WRITE_TRUNCATED_RESPONSE:
         DCHECK_EQ(OK, rv);
         rv = DoCacheWriteTruncatedResponse();
         break;
       case STATE_CACHE_WRITE_RESPONSE_COMPLETE:
         rv = DoCacheWriteResponseComplete(rv);
@@ skipping 659 matching lines @@
           "422516 HttpCache::Transaction::DoUpdateCachedResponse"));
 
   next_state_ = STATE_UPDATE_CACHED_RESPONSE_COMPLETE;
   int rv = OK;
   // Update cached response based on headers in new_response.
   // TODO(wtc): should we update cached certificate (response_.ssl_info), too?
   response_.headers->Update(*new_response_->headers.get());
   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) {
       int ret = cache_->DoomEntry(cache_key_, NULL);
       DCHECK_EQ(OK, ret);
     }
   } else {
     // If we are already reading, we already updated the headers for this
     // request; doing it again will change Content-Length.
     if (!reading_) {
@@ skipping 198 matching lines @@
 
   // cert_cache() will be null if the CertCacheTrial field trial is disabled.
   if (cache_->cert_cache() && response_.ssl_info.is_valid())
     ReadCertChain();
 
   // Some resources may have slipped in as truncated when they're not.
   int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
   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
   //    cache entry.
   //
   //  o if we can read or write, then we should check if the cache entry needs
   //    to be validated and then issue a network request if needed or just read
   //    from the cache if the cache entry is already valid.
   //
   //  o if we are set to UPDATE, then we are handling an externally
   //    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);
       result = BeginCacheRead();
       break;
     case READ_WRITE:
       result = BeginPartialCacheValidation();
       break;
     case UPDATE:
       result = BeginExternallyConditionalizedRequest();
       break;
     case WRITE:
     default:
       NOTREACHED();
-      result = ERR_FAILED;
   }
   return result;
 }
 
 int HttpCache::Transaction::DoCacheWriteResponse() {
   // TODO(vadimt): Remove ScopedTracker below once crbug.com/422516 is fixed.
   tracked_objects::ScopedTracker tracking_profile(
       FROM_HERE_WITH_EXPLICIT_FUNCTION(
           "422516 HttpCache::Transaction::DoCacheWriteResponse"));
 
@@ skipping 649 matching lines @@
   if (response_.vary_data.is_valid() &&
       !response_.vary_data.MatchesRequest(*request_,
                                           *response_.headers.get())) {
     vary_mismatch_ = true;
     return VALIDATION_SYNCHRONOUS;
   }
 
   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;
 
   if (request_->method == "PUT" || request_->method == "DELETE")
     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.
     if (request_->method != "GET" && request_->method != "HEAD")
       return VALIDATION_SYNCHRONOUS;
   }
 
   return validation_required_by_headers;
 }
 
@@ skipping 40 matching lines @@
   bool use_if_range = partial_.get() && !partial_->IsCurrentRangeCached() &&
                       !invalid_range_;
 
   if (!use_if_range) {
     // stale-while-revalidate is not useful when we only have a partial response
     // cached, so don't set the header in that case.
     HttpResponseHeaders::FreshnessLifetimes lifetimes =
         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,
           base::StringPrintf("max-age=%" PRId64
                              ",stale-while-revalidate=%" PRId64 ",age=%" PRId64,
                              lifetimes.freshness.InSeconds(),
                              lifetimes.staleness.InSeconds(),
                              current_age.InSeconds()));
     }
   }
@@ skipping 537 matching lines @@
 
 void HttpCache::Transaction::OnIOComplete(int result) {
   // TODO(vadimt): Remove ScopedTracker below once crbug.com/422516 is fixed.
   tracked_objects::ScopedTracker tracking_profile(
       FROM_HERE_WITH_EXPLICIT_FUNCTION("422516 Transaction::OnIOComplete"));
 
   DoLoop(result);
 }
 
 }  // namespace net