Store and retrieve features from shared memory

base/feature_list.cc

Index: base/feature_list.cc
diff --git a/base/feature_list.cc b/base/feature_list.cc
index 89b105defc6df0119688cb3b0e708f1b905f36df..c2f268b138b479ad1bdca59a855ab61f9b9425b2 100644
--- a/base/feature_list.cc
+++ b/base/feature_list.cc
@@ -12,6 +12,7 @@
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial.h"
+#include "base/pickle.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 
@@ -27,6 +28,42 @@ FeatureList* g_instance = nullptr;
 // Tracks whether the FeatureList instance was initialized via an accessor.
 bool g_initialized_from_accessor = false;
 
+const uint32_t kFeatureType = 0x06567CA6 + 1;  // SHA1(FeatureEntry) v1
+
+// An allocator entry for a feature in shared memory. The FeatureEntry is
+// followed by a base::Pickle object that contains the feature and trial name.
+// Any changes to this structure requires a bump in kFeatureType defined above.
+struct FeatureEntry {
+  // Expected size for 32/64-bit check.
+  static constexpr size_t kExpectedInstanceSize = 8;
+
+  // Specifies whether a feature override enables or disables the feature. Same
+  // values as the OverrideState enum in feature_list.h
+  uint32_t override_state;
+
+  // Size of the pickled structure, NOT the total size of this entry.
+  uint32_t pickle_size;
+
+  // Reads the feature and trial name from the pickle. Calling this is only
+  // valid on an initialized entry that's in shared memory.
+  bool GetFeatureAndTrialName(StringPiece* feature_name,
+                              StringPiece* trial_name) const {
+    const char* src =
+        reinterpret_cast<const char*>(this) + sizeof(FeatureEntry);
+
+    Pickle pickle(src, pickle_size);
+    PickleIterator pickle_iter(pickle);
+
+    if (!pickle_iter.ReadStringPiece(feature_name))
+      return false;
+
+    // Return true because we are not guaranteed to have a trial name anyways.
+    auto sink = pickle_iter.ReadStringPiece(trial_name);
+    ALLOW_UNUSED_LOCAL(sink);
+    return true;
+  }
+};
+
 // Some characters are not allowed to appear in feature names or the associated
 // field trial names, as they are used as special characters for command-line
 // serialization. This function checks that the strings are ASCII (since they
@@ -56,6 +93,31 @@ void FeatureList::InitializeFromCommandLine(
   initialized_from_command_line_ = true;
 }
 
+void FeatureList::InitializeFromSharedMemory(
+    PersistentMemoryAllocator* allocator) {
+  DCHECK(!initialized_);
+
+  PersistentMemoryAllocator::Iterator iter(allocator);
+
+  PersistentMemoryAllocator::Reference ref;
+  while ((ref = iter.GetNextOfType(kFeatureType)) !=
+         PersistentMemoryAllocator::kReferenceNull) {
+    const FeatureEntry* entry =
+        allocator->GetAsObject<const FeatureEntry>(ref, kFeatureType);
+
+    OverrideState override_state =
+        static_cast<OverrideState>(entry->override_state);
+
+    StringPiece feature_name;
+    StringPiece trial_name;
+    if (!entry->GetFeatureAndTrialName(&feature_name, &trial_name))
+      continue;
+
+    FieldTrial* trial = FieldTrialList::Find(trial_name.as_string());
+    RegisterOverride(feature_name, override_state, trial);
+  }
+}
+
 bool FeatureList::IsFeatureOverriddenFromCommandLine(
     const std::string& feature_name,
     OverrideState state) const {
@@ -98,6 +160,33 @@ void FeatureList::RegisterFieldTrialOverride(const std::string& feature_name,
   RegisterOverride(feature_name, override_state, field_trial);
 }
 
+void FeatureList::AddFeaturesToAllocator(PersistentMemoryAllocator* allocator) {
+  DCHECK(initialized_);
+
+  for (const auto& override : overrides_) {
+    Pickle pickle;
+    pickle.WriteString(override.first);
+    if (override.second.field_trial)
+      pickle.WriteString(override.second.field_trial->trial_name());
+
+    size_t total_size = sizeof(FeatureEntry) + pickle.size();
+    PersistentMemoryAllocator::Reference ref =
+        allocator->Allocate(total_size, kFeatureType);
+    if (!ref)
+      return;
+
+    FeatureEntry* entry =
+        allocator->GetAsObject<FeatureEntry>(ref, kFeatureType);
+    entry->override_state = override.second.overridden_state;
+    entry->pickle_size = pickle.size();
+
+    char* dst = reinterpret_cast<char*>(entry) + sizeof(FeatureEntry);
+    memcpy(dst, pickle.data(), pickle.size());
+
+    allocator->MakeIterable(ref);
+  }
+}
+
 void FeatureList::GetFeatureOverrides(std::string* enable_overrides,
                                       std::string* disable_overrides) {
   DCHECK(initialized_);