Rename the non-const version of output_partition(i) to

mutable_output_partition(i) and change call-sites as needed.

Add an AtomicBool used for checking that we should not be modifying
the CachedResult in a RewriteContext after it is serialized to the
cache, and a new private method RewriteContext::CheckNotFrozen() to
check it.

net/instaweb/rewriter/cache_extender.cc

@@ -47,6 +47,7 @@
 #include "pagespeed/kernel/html/html_name.h"
 #include "pagespeed/kernel/http/content_type.h"
 #include "pagespeed/kernel/http/google_url.h"
+#include "pagespeed/kernel/http/request_headers.h"
 #include "pagespeed/kernel/http/response_headers.h"
 #include "pagespeed/kernel/http/semantic_type.h"
 #include "pagespeed/opt/logging/enums.pb.h"
@@ -252,7 +253,7 @@ void CacheExtender::Context::RewriteSingle(
   }
   RewriteDone(
       extender_->RewriteLoadedResource(
-          input_resource, output_resource, output_partition(0)), 0);
+          input_resource, output_resource, mutable_output_partition(0)), 0);
 }
 
 void CacheExtender::Context::Render() {

net/instaweb/rewriter/collect_dependencies_filter.cc

@@ -22,6 +22,7 @@
 #include "base/logging.h"
 #include "net/instaweb/rewriter/cached_result.pb.h"
 #include "net/instaweb/rewriter/dependencies.pb.h"
+#include "net/instaweb/rewriter/input_info.pb.h"
 #include "net/instaweb/rewriter/public/css_util.h"
 #include "net/instaweb/rewriter/public/dependency_tracker.h"
 #include "net/instaweb/rewriter/public/output_resource_kind.h"
@@ -38,6 +39,7 @@
 #include "pagespeed/kernel/base/thread_annotations.h"
 #include "pagespeed/kernel/base/thread_system.h"
 #include "pagespeed/kernel/html/html_element.h"
+#include "pagespeed/kernel/html/html_name.h"
 #include "pagespeed/kernel/http/data_url.h"
 #include "pagespeed/kernel/http/google_url.h"
 #include "pagespeed/kernel/http/semantic_type.h"
@@ -232,13 +234,15 @@ class CollectDependenciesFilter::Context : public RewriteContext {
     // the first dependency we collected, or nullptr.
     if (num_output_partitions() == 1 &&
         output_partition(0)->collected_dependency_size() > 0) {
-      CachedResult* result = output_partition(0);
+      // Deep copy here because output_partition is already written, and it
+      // makes no sense to mutate it.
+      CachedResult result = *output_partition(0);
 
       // Top-level stuff just gets its dep_id_ as the sorting key.
-      result->mutable_collected_dependency(0)->add_order_key(dep_id_);
+      result.mutable_collected_dependency(0)->add_order_key(dep_id_);
 
       dep_tracker->ReportDependencyCandidate(dep_id_,
-                                             &result->collected_dependency(0));
+                                             &result.collected_dependency(0));
 
       // Any other dependencies stored in result->collected_dependency >= 1
       // are things we discovered *inside* whatever is described by
@@ -252,9 +256,9 @@ class CollectDependenciesFilter::Context : public RewriteContext {
       // (dep_id_ + 1) or some larger number. Note that we produce order keys
       // at most 2 deep because we (for now?) only collect dependencies that
       // deep.
-      for (int c = 1; c < result->collected_dependency_size(); ++c) {
+      for (int c = 1; c < result.collected_dependency_size(); ++c) {
         int additional_dep_id = dep_tracker->RegisterDependencyCandidate();
-        Dependency* child_dep = result->mutable_collected_dependency(c);
+        Dependency* child_dep = result.mutable_collected_dependency(c);
         child_dep->add_order_key(dep_id_);
         child_dep->add_order_key(c);
         dep_tracker->ReportDependencyCandidate(additional_dep_id, child_dep);

net/instaweb/rewriter/css_combine_filter.cc

@@ -27,6 +27,7 @@
 #include "base/logging.h"
 #include "net/instaweb/http/public/log_record.h"
 #include "net/instaweb/rewriter/cached_result.pb.h"
+#include "net/instaweb/rewriter/input_info.pb.h"
 #include "net/instaweb/rewriter/public/css_tag_scanner.h"
 #include "net/instaweb/rewriter/public/output_resource.h"
 #include "net/instaweb/rewriter/public/output_resource_kind.h"
@@ -40,7 +41,6 @@
 #include "net/instaweb/rewriter/public/rewrite_result.h"
 #include "net/instaweb/rewriter/public/server_context.h"
 #include "pagespeed/kernel/base/charset_util.h"
-#include "pagespeed/kernel/base/proto_util.h"
 #include "pagespeed/kernel/base/scoped_ptr.h"
 #include "pagespeed/kernel/base/statistics.h"
 #include "pagespeed/kernel/base/string.h"
@@ -50,6 +50,7 @@
 #include "pagespeed/kernel/html/html_name.h"
 #include "pagespeed/kernel/http/content_type.h"
 #include "pagespeed/kernel/http/google_url.h"
+#include "pagespeed/kernel/http/semantic_type.h"
 #include "pagespeed/opt/logging/enums.pb.h"
 #include "webutil/css/parser.h"
 
@@ -271,7 +272,7 @@ class CssCombineFilter::Context : public RewriteContext {
 
   virtual void Render() {
     for (int p = 0, np = num_output_partitions(); p < np; ++p) {
-      CachedResult* partition = output_partition(p);
+      const CachedResult* partition = output_partition(p);
       if (partition->input_size() == 0) {
         continue;
       }
@@ -321,7 +322,7 @@ class CssCombineFilter::Context : public RewriteContext {
     }
   }
 
-  void DisableRemovedSlots(CachedResult* partition) {
+  void DisableRemovedSlots(const CachedResult* partition) {
     // Slot 0 will be replaced by the combined resource as part of
     // rewrite_context.cc.  But we still need to delete links for slots 1-N,
     // and to prevent further acting on them.

net/instaweb/rewriter/css_filter.cc

@@ -437,7 +437,7 @@ bool CssFilter::Context::RewriteCssText(const GoogleUrl& css_base_gurl,
     // comment? There are often a lot and they can be quite long, so I'm not
     // sure it's the best idea. Perhaps better to ask users to use the command
     // line utility? Or is it better to give them all the info in one place?
-    output_partition(0)->add_debug_message(StrCat(
+    mutable_output_partition(0)->add_debug_message(StrCat(
         "CSS rewrite failed: Parse error in ", css_base_gurl.Spec()));
   } else {
     // Edit stylesheet.
@@ -550,7 +550,7 @@ bool CssFilter::Context::FallbackRewriteUrls(
             resource, fallback_transformer_->map(), url.Spec()));
         css_image_rewriter_->RewriteSlot(slot, ImageInlineMaxBytes(), this);
       } else if (!is_authorized) {
-        output_partition(0)->add_debug_message(
+        mutable_output_partition(0)->add_debug_message(
             StrCat("A resource was not rewritten because ", url.Host(),
                    " is not an authorized domain"));
       }
@@ -580,7 +580,7 @@ void CssFilter::Context::Harvest() {
       filter_->num_fallback_failures_->Add(1);
       GoogleUrl css_base_gurl;
       GetCssBaseUrlToUse(input_resource_, &css_base_gurl);
-      output_partition(0)->add_debug_message(StrCat(
+      mutable_output_partition(0)->add_debug_message(StrCat(
           "CSS rewrite failed: Fallback transformer error in ",
           css_base_gurl.Spec()));
     }
@@ -656,8 +656,8 @@ void CssFilter::Context::Harvest() {
       server_context->MergeNonCachingResponseHeaders(input_resource_,
                                                      output_resource_);
     } else {
-      output_partition(0)->set_inlined_data(out_text);
-      output_partition(0)->set_is_inline_output_resource(true);
+      mutable_output_partition(0)->set_inlined_data(out_text);
+      mutable_output_partition(0)->set_is_inline_output_resource(true);
     }
     ok = Driver()->Write(ResourceVector(1, input_resource_),
                          out_text,
@@ -667,7 +667,7 @@ void CssFilter::Context::Harvest() {
   }
 
   if (!hierarchy_.flattening_failure_reason().empty()) {
-    output_partition(0)->add_debug_message(
+    mutable_output_partition(0)->add_debug_message(
         hierarchy_.flattening_failure_reason());
   }
 
@@ -716,7 +716,7 @@ bool CssFilter::Context::SerializeCss(int64 in_text_size,
                          Integer64ToString(-bytes_saved).c_str());
       }
       filter_->num_rewrites_dropped_->Add(1);
-      output_partition(0)->add_debug_message(StrCat(
+      mutable_output_partition(0)->add_debug_message(StrCat(
           "CSS rewrite failed: Cannot improve ", css_base_gurl.Spec()));
     }
   }

net/instaweb/rewriter/css_image_rewriter.cc

@@ -196,7 +196,8 @@ void CssImageRewriter::InheritChildImageInfo(RewriteContext* context) {
   for (AssociatedImageInfoMap::iterator i = child_image_info.begin(),
                                         e = child_image_info.end();
        i != e; ++i) {
-    *context->output_partition(0)->add_associated_image_info() = i->second;
+    *context->mutable_output_partition(0)->add_associated_image_info() =
+        i->second;
   }
 }
 

net/instaweb/rewriter/css_image_rewriter_test.cc

@@ -1175,10 +1175,10 @@ TEST_F(CssImageRewriterTest, DebugMessage) {
       ")}");
 
   debug_message_ =
+      "<!--The image was not inlined because it has too many bytes.-->"
       "<!--Image http://test.com/foo.png does not appear to need resizing.-->"
       "<!--Image http://test.com/foo.png has no transparent pixels, "
-      "is not sensitive to compression noise, and has no animation.-->"
-      "<!--The image was not inlined because it has too many bytes.-->";
+      "is not sensitive to compression noise, and has no animation.-->";
   ValidateRewriteInlineCss("recompress_css_images", kCss, kCssAfter,
                            kNoStatCheck | kExpectCached);
 }

net/instaweb/rewriter/css_summarizer_base.cc

@@ -19,7 +19,6 @@
 #include "net/instaweb/rewriter/public/css_summarizer_base.h"
 
 #include <cstddef>
-#include <memory>
 
 #include "base/logging.h"
 #include "net/instaweb/rewriter/cached_result.pb.h"
@@ -49,12 +48,11 @@
 #include "pagespeed/kernel/html/html_node.h"
 #include "pagespeed/kernel/http/content_type.h"
 #include "pagespeed/kernel/http/data_url.h"
+#include "pagespeed/kernel/util/url_segment_encoder.h"
 #include "webutil/css/parser.h"
 
 namespace net_instaweb {
 
-class UrlSegmentEncoder;
-
 // Rewrite context for CssSummarizerBase --- it invokes the filter's
 // summarization functions on parsed CSS ASTs when available, and synchronizes
 // them with the summaries_ table in the CssSummarizerBase.
@@ -206,7 +204,7 @@ void CssSummarizerBase::Context::RewriteSingle(
   parser.set_quirks_mode(false);
 
   scoped_ptr<Css::Stylesheet> stylesheet(parser.ParseRawStylesheet());
-  CachedResult* result = output_partition(0);
+  CachedResult* result = mutable_output_partition(0);
   if (stylesheet.get() == NULL ||
       parser.errors_seen_mask() != Css::Parser::kNoError) {
     // TODO(morlovich): do we want a stat here?

net/instaweb/rewriter/image_combine_filter.cc

@@ -21,7 +21,6 @@
 #include <cstddef>                     // for size_t
 #include <iterator>
 #include <map>
-#include <memory>
 #include <set>
 #include <utility>
 #include <vector>
@@ -844,7 +843,7 @@ class ImageCombineFilter::Context : public RewriteContext {
   // except for one, don't use it.
   virtual void Render() {
     for (int p = 0, np = num_output_partitions(); p < np; ++p) {
-      CachedResult* partition = output_partition(p);
+      const CachedResult* partition = output_partition(p);
       int num_inputs = partition->input_size();
       if (num_inputs > 1) {
         if (!partition->has_spriter_result()) {

net/instaweb/rewriter/image_rewrite_filter.cc

@@ -571,7 +571,7 @@ void ImageRewriteFilter::Context::Render() {
 
   CHECK_EQ(1, num_slots());
 
-  CachedResult* result = output_partition(0);
+  const CachedResult* result = output_partition(0);
   bool rewrote_url = false;
   ResourceSlot* resource_slot = slot(0).get();
   if (place_ == Place::kCss || !has_parent()) {
@@ -579,6 +579,19 @@ void ImageRewriteFilter::Context::Render() {
     if (place_ == Place::kCss) {
       rewrote_url = filter_->FinishRewriteCssImageUrl(
           css_image_inline_max_bytes_, result, resource_slot, &inline_result);
+      if (Driver()->options()->Enabled(RewriteOptions::kInlineImages)) {
+        const char* message = MessageForInlineResult(inline_result);
+        if (message != nullptr) {
+          RewriteContext* context = parent();
+          if (context != nullptr) {
+            CachedResult* css_result = context->mutable_output_partition(0);
+            // We want to have the CSS context retain this, so we can
+            // annotate the HTML that references the CSS with the image.
+            // So we write the debug message into the CSS context.
+            filter_->SaveDebugMessageToCache(message, css_result);
+          }
+        }
+      }
     } else if (place_ == Place::kHtmlAttr) {
       // We use manual rendering for HTML, as we have to consider whether to
       // inline, and may also pass in width and height attributes.
@@ -598,20 +611,12 @@ void ImageRewriteFilter::Context::Render() {
           filter_->RegisterImageInfo(aii);
         }
       }
-    }
-
-    if (Driver()->options()->Enabled(RewriteOptions::kInlineImages) &&
-        (place_ == Place::kCss || place_ == Place::kHtmlAttr)) {
-      // Show the debug message for inlining only when this option has been
-      // enabled. We also don't inline srcset images.
-      // Note: Although debug message is saved to the cached_result, it is
-      // *not* cached because the cached_result has already been stored in
-      // the cache previously. This is good because the exact debug message
-      // here depends upon factors that may not be in the cache key (such
-      // as whether this is a responsive image). So we should not be storing
-      // the result in the cache.
-      filter_->SaveDebugMessageToCache(
-          MessageForInlineResult(inline_result), this, result);
+      if (Driver()->options()->Enabled(RewriteOptions::kInlineImages)) {
+        const char* message = MessageForInlineResult(inline_result);
+        if (message != nullptr) {
+          Driver()->InsertDebugComment(message, html_slot->element());
+        }
+      }
     }
     // Use standard rendering in case the rewrite is nested and not inside CSS.
   }
@@ -1227,7 +1232,7 @@ RewriteResult ImageRewriteFilter::RewriteLoadedResourceImpl(
   cached->set_optimized_image_type(optimized_image_type);
   cached->set_size(rewrite_result == kRewriteOk ? image->output_size() :
                    image->input_size());
-  SaveDebugMessageToCache(image->debug_message(), rewrite_context, cached);
+  SaveDebugMessageToCache(image->debug_message(), cached);
 
   // Try inlining input image if output hasn't been inlined already.
   if (!cached->has_inlined_data()) {
@@ -1801,7 +1806,6 @@ bool ImageRewriteFilter::FinishRewriteImageUrl(
 }
 
 void ImageRewriteFilter::SaveDebugMessageToCache(const GoogleString& message,
-                                                 Context* rewrite_context,
                                                  CachedResult* cached_result) {
   if (!message.empty()) {
     // We always save our result to our cache entry, since it will be propagated

net/instaweb/rewriter/in_place_rewrite_context.cc

@@ -310,8 +310,9 @@ void InPlaceRewriteContext::Harvest() {
     if (nested_context->num_slots() == 1 && num_output_partitions() == 1 &&
         nested_context->slot(0)->was_optimized()) {
       ResourcePtr nested_resource = nested_context->slot(0)->resource();
-      CachedResult* partition = output_partition(0);
-      CachedResult* nested_partition = nested_context->output_partition(0);
+      CachedResult* partition = mutable_output_partition(0);
+      const CachedResult* nested_partition =
+          nested_context->output_partition(0);
       VLOG(1) << "In-place rewrite succeeded for " << url_
               << " and the rewritten resource is "
               << nested_resource->url();

net/instaweb/rewriter/javascript_filter.cc

@@ -245,7 +245,7 @@ class JavascriptFilter::Context : public SingleRewriteContext {
     if (num_output_partitions() != 1) {
       return;
     }
-    CachedResult* result = output_partition(0);
+    const CachedResult* result = output_partition(0);
     ResourceSlot* output_slot = slot(0).get();
     if (!result->url_relocatable()) {
       Driver()->InsertDebugComment(

net/instaweb/rewriter/js_combine_filter.cc

@@ -31,6 +31,7 @@
 
 #include "base/logging.h"
 #include "net/instaweb/rewriter/cached_result.pb.h"
+#include "net/instaweb/rewriter/input_info.pb.h"
 #include "net/instaweb/rewriter/public/javascript_code_block.h"
 #include "net/instaweb/rewriter/public/javascript_filter.h"
 #include "net/instaweb/rewriter/public/output_resource.h"
@@ -338,7 +339,7 @@ class JsCombineFilter::Context : public RewriteContext {
   // original script for that tag.
   virtual void Render() {
     for (int p = 0, np = num_output_partitions(); p < np; ++p) {
-      CachedResult* partition = output_partition(p);
+      const CachedResult* partition = output_partition(p);
       int partition_size = partition->input_size();
       if (partition_size > 1) {
         // Make sure we can edit every element here.
@@ -404,7 +405,7 @@ class JsCombineFilter::Context : public RewriteContext {
 
   // Create an element for the combination of all the elements in the
   // partition. Insert it before first one.
-  void MakeCombinedElement(CachedResult* partition) {
+  void MakeCombinedElement(const CachedResult* partition) {
     int first_index = partition->input(0).index();
     HtmlResourceSlot* first_slot =
         static_cast<HtmlResourceSlot*>(slot(first_index).get());

net/instaweb/rewriter/public/css_flatten_imports_context.h

@@ -155,7 +155,8 @@ class CssFlattenImportsContext : public SingleRewriteContext {
     hierarchy_->RollUpContents();
 
     // Our result is the combination of all our imports and our own rules.
-    output_partition(0)->set_inlined_data(hierarchy_->minified_contents());
+    mutable_output_partition(0)->set_inlined_data(
+        hierarchy_->minified_contents());
 
     ServerContext* server_context = FindServerContext();
     server_context->MergeNonCachingResponseHeaders(input_resource_,

net/instaweb/rewriter/public/image_rewrite_filter.h

@@ -284,7 +284,6 @@ class ImageRewriteFilter : public RewriteFilter {
   bool StoreUrlInPropertyCache(const StringPiece& url);
 
   void SaveDebugMessageToCache(const GoogleString& message,
-                               Context* rewrite_context,
                                CachedResult* cached_result);
 
   GoogleString UrlForDebugMessages(const Context* context) const;

net/instaweb/rewriter/public/rewrite_context.h

@@ -24,12 +24,14 @@
 
 #include "net/instaweb/http/public/http_cache.h"
 #include "net/instaweb/rewriter/cached_result.pb.h"
+#include "net/instaweb/rewriter/input_info.pb.h"
 #include "net/instaweb/rewriter/public/output_resource_kind.h"
 #include "net/instaweb/rewriter/public/resource.h"
 #include "net/instaweb/rewriter/public/resource_slot.h"
 #include "net/instaweb/rewriter/public/rewrite_result.h"
 #include "net/instaweb/rewriter/public/server_context.h"
 #include "pagespeed/controller/schedule_rewrite_callback.h"
+#include "pagespeed/kernel/base/atomic_bool.h"
 #include "pagespeed/kernel/base/basictypes.h"
 #include "pagespeed/kernel/base/function.h"
 #include "pagespeed/kernel/base/scoped_ptr.h"
@@ -202,7 +204,7 @@ class RewriteContext {
   // but may also be accessed in ::Render.
   int num_output_partitions() const;
   const CachedResult* output_partition(int i) const;
-  CachedResult* output_partition(int i);
+  CachedResult* mutable_output_partition(int i);
 
   // Returns true if this context is chained to some predecessors, and
   // must therefore be started by a predecessor and not RewriteDriver.
@@ -263,6 +265,10 @@ class RewriteContext {
   // id.
   bool IsNestedIn(StringPiece id) const;
 
+  // Checks to make sure that partitions_ is not frozen when it is
+  // about to be modified, calling LOG(DFATAL) if there is a problem.
+  void CheckNotFrozen();
+
   // Allows a nested rewriter to walk up its parent hierarchy.
   RewriteContext* parent() { return parent_; }
   const RewriteContext* parent() const { return parent_; }
@@ -890,6 +896,12 @@ class RewriteContext {
   // UrlAsyncFetcher.
 
   bool started_;
+
+  // This is only used in debug, but it's better not to have conditionally
+  // compiled member variables in case someone wants to compile only some
+  // PSOL modules for debug.
+  AtomicBool frozen_;
+
   scoped_ptr<OutputPartitions> partitions_;
   OutputResourceVector outputs_;
   int outstanding_fetches_;

net/instaweb/rewriter/public/rewrite_context_test_base.h

@@ -343,7 +343,7 @@ class CombiningFilter : public RewriteFilter {
     virtual void Render();
     virtual void WillNotRender();
     virtual void Cancel();
-    void DisableRemovedSlots(CachedResult* partition);
+    void DisableRemovedSlots(const CachedResult* partition);
     virtual const UrlSegmentEncoder* encoder() const { return &encoder_; }
     virtual const char* id() const { return kFilterId; }
     virtual OutputResourceKind kind() const {

net/instaweb/rewriter/rewrite_context.cc

@@ -1056,7 +1056,8 @@ const CachedResult* RewriteContext::output_partition(int i) const {
   return &partitions_->partition(i);
 }
 
-CachedResult* RewriteContext::output_partition(int i) {
+CachedResult* RewriteContext::mutable_output_partition(int i) {
+  CheckNotFrozen();
   return partitions_->mutable_partition(i);
 }
 
@@ -1315,6 +1316,7 @@ void RewriteContext::AddRecheckDependency() {
                    resource->fetch_response_status()) * Timer::kSecondMs;
     }
   }
+  CheckNotFrozen();
   InputInfo* force_recheck = partitions_->add_other_dependency();
   force_recheck->set_type(InputInfo::CACHED);
   force_recheck->set_expiration_time_ms(now_ms + ttl_ms);
@@ -1394,6 +1396,7 @@ void RewriteContext::OutputCacheHit(bool write_partitions) {
 void RewriteContext::OutputCacheMiss() {
   is_metadata_cache_miss_ = true;
   outputs_.clear();
+  CheckNotFrozen();
   partitions_->Clear();
   ServerContext* server_context = FindServerContext();
   if (server_context->shutting_down()) {
@@ -1510,6 +1513,7 @@ void RewriteContext::RepeatedSuccess(const RewriteContext* primary) {
   if (primary->was_too_busy_) {
     MarkTooBusy();
   }
+  CheckNotFrozen();
   partitions_->CopyFrom(*primary->partitions_);
   for (int i = 0, n = primary->num_outputs(); i < n; ++i) {
     outputs_.push_back(primary->outputs_[i]);
@@ -1716,6 +1720,7 @@ void RewriteContext::PartitionDone(RewriteResult result_or_busy) {
   }
 
   if (!result) {
+    CheckNotFrozen();
     partitions_->clear_partition();
     outputs_.clear();
   }
@@ -1777,8 +1782,8 @@ void RewriteContext::WritePartition() {
             DCHECK(gurl.IsWebValid()) << partition.url();
           }
         }
+        frozen_.set_value(true);
 #endif
-
         StringOutputStream sstream(&buf);  // finalizes buf in destructor
         partitions_->SerializeToZeroCopyStream(&sstream);
       }
@@ -1888,6 +1893,7 @@ void RewriteContext::CheckAndAddOtherDependency(const InputInfo& input_info) {
     return;
   }
 
+  CheckNotFrozen();
   InputInfo* dep = partitions_->add_other_dependency();
   *dep = input_info;
   // The input index here is with respect to the nested context's inputs,
@@ -1949,6 +1955,7 @@ void RewriteContext::RewriteDoneImpl(RewriteResult result,
   if (result == kTooBusy) {
     MarkTooBusy();
   } else {
+    CheckNotFrozen();
     CachedResult* partition =
         partitions_->mutable_partition(partition_index);
     bool optimizable = (result == kRewriteOk);
@@ -2014,7 +2021,7 @@ void RewriteContext::Propagate(bool render_slots) {
                                     slot(0)->element());
     }
     for (int p = 0, np = num_output_partitions(); p < np; ++p) {
-      CachedResult* partition = output_partition(p);
+      const CachedResult* partition = output_partition(p);
       int n = partition->input_size();
       if (partition->debug_message_size() > 0) {
         if (has_parent()) {
@@ -2090,7 +2097,7 @@ void RewriteContext::Finalize() {
 }
 
 void RewriteContext::RenderPartitionOnDetach(int rewrite_index) {
-  CachedResult* partition = output_partition(rewrite_index);
+  const CachedResult* partition = output_partition(rewrite_index);
   for (int i = 0; i < partition->input_size(); ++i) {
     int slot_index = partition->input(i).index();
     slot(slot_index)->set_was_optimized(true);
@@ -2152,6 +2159,7 @@ void RewriteContext::RunSuccessors() {
 void RewriteContext::StartRewriteForFetch() {
   // Make a fake partition that has all the inputs, since we are
   // performing the rewrite for only one output resource.
+  CheckNotFrozen();
   CachedResult* partition = partitions_->add_partition();
   bool ok_to_rewrite = true;
   for (int i = 0, n = slots_.size(); i < n; ++i) {
@@ -2584,12 +2592,13 @@ void RewriteContext::FetchCacheDone(CacheLookupResult* cache_result) {
   // locking things, fetching inputs, rewriting, and so on.
 
   scoped_ptr<CacheLookupResult> owned_cache_result(cache_result);
+  CheckNotFrozen();
   partitions_.reset(owned_cache_result->partitions.release());
   LogMetadataCacheInfo(owned_cache_result->cache_ok,
                        owned_cache_result->can_revalidate);
 
   if (owned_cache_result->cache_ok && (num_output_partitions() == 1)) {
-    CachedResult* result = output_partition(0);
+    const CachedResult* result = output_partition(0);
     OutputResourcePtr output_resource;
     if (result->optimizable() &&
         CreateOutputResourceForCachedOutput(result, &output_resource)) {
@@ -2715,6 +2724,7 @@ void RewriteContext::StartFetchImpl() {
 void RewriteContext::StartFetchReconstruction() {
   // Note that in case of fetches we continue even if we didn't manage to
   // take the lock.
+  CheckNotFrozen();
   partitions_->Clear();
   FetchInputs();
 }
@@ -2853,6 +2863,14 @@ bool RewriteContext::IsNestedIn(StringPiece id) const {
   return parent_ != NULL && id == parent_->id();
 }
 
+void RewriteContext::CheckNotFrozen() {
+#ifndef NDEBUG
+  if (frozen_.value()) {
+    LOG(DFATAL) << "output_partitions_ mutated after being written";
+  }
+#endif
+}
+
 GoogleString RewriteContext::ToString() const {
   return ToStringWithPrefix("");
 }

net/instaweb/rewriter/rewrite_context_test_base.cc

@@ -25,6 +25,7 @@
 #include "net/instaweb/http/public/http_cache.h"
 #include "net/instaweb/http/public/logging_proto_impl.h"
 #include "net/instaweb/rewriter/cached_result.pb.h"
+#include "net/instaweb/rewriter/input_info.pb.h"
 #include "net/instaweb/rewriter/public/output_resource.h"
 #include "net/instaweb/rewriter/public/rewrite_options.h"
 #include "net/instaweb/rewriter/public/rewrite_result.h"
@@ -301,7 +302,8 @@ void CombiningFilter::Context::Cancel() {
   ++filter_->num_cancel_;
 }
 
-void CombiningFilter::Context::DisableRemovedSlots(CachedResult* partition) {
+void CombiningFilter::Context::DisableRemovedSlots(
+    const CachedResult* partition) {
   if (filter_->disable_successors_) {
     slot(0)->set_disable_further_processing(true);
   }