Add field trial dump and retrieval methods from shared memory

base/metrics/field_trial.h

 // 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.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 55 matching lines @@
 #include "base/base_export.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
 #include "base/files/file.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/shared_memory.h"
 #include "base/metrics/persistent_memory_allocator.h"
 #include "base/observer_list_threadsafe.h"
+#include "base/pickle.h"
 #include "base/process/launch.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
 
 namespace base {
 
 class FieldTrialList;
 
 class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
@@ skipping 40 matching lines @@
   struct BASE_EXPORT State {
     StringPiece trial_name;
     StringPiece group_name;
     bool activated;
 
     State();
     State(const State& other);
     ~State();
   };
 
+  // We create one FieldTrialEntry per field trial in shared memory, via
+  // AddToAllocatorWhileLocked. The FieldTrialEntry is followed by a
+  // base::Pickle object that we unpickle and read from. Any changes to this
+  // structure requires a bump in kFieldTrialType id defined above.

[manzagop (departed), 2016/12/09 21:22:24]

nit: update comment as kFieldTrialType is defined in .cc, not above.

[lawrencewu, 2016/12/09 22:20:49]

Done.

+  struct FieldTrialEntry {
+    // Expected size for 32/64-bit check.
+    static constexpr size_t kExpectedInstanceSize = 8;
+
+    // Whether or not this field trial is activated. This is really just a
+    // boolean but marked as a uint32_t for portability reasons.
+    uint32_t activated;
+
+    // Size of the pickled structure, NOT the total size of this entry.
+    uint32_t pickle_size;
+
+    // Calling this is only valid when the entry is initialized. That is, it
+    // resides in shared memory and has a pickle containing the trial name and
+    // group name following it.
+    bool GetTrialAndGroupName(StringPiece* trial_name,
+                              StringPiece* group_name) const;
+
+    // Calling this is only valid when the entry is initialized as well. Reads
+    // the parameters following the trial and group name and stores them as
+    // key-value mappings in |params|.
+    bool GetParams(std::map<std::string, std::string>* params) const;
+
+   private:
+    // Returns an iterator over the data containing names and params.
+    PickleIterator GetPickleIterator() const;
+
+    // Takes the iterator and writes out the first two items into |trial_name|
+    // and |group_name|.
+    bool ReadStringPair(PickleIterator* iter,
+                        StringPiece* trial_name,
+                        StringPiece* group_name) const;
+  };

[Alexei Svitkine (slow), 2016/12/09 20:08:43]

Nit: Add DISALLOW_COPY_AND_ASSIGN(FieldTrialEntry)?

[bcwhite, 2016/12/09 20:13:29]

Allocator objects are always POD so can always be copied and assigned.  You may
get an error if you add ctor definitions because the compiler may no longer
consider it a POD.

[manzagop (departed), 2016/12/09 21:22:24]

Drive by: if relying on POD / trivial copy, use a is_pod / is_trivial or some
such static assert?

[bcwhite, 2016/12/09 21:38:42]

Already exists.  See PersistentMemoryAllocator::GetAsObject<>()

+
   typedef std::vector<ActiveGroup> ActiveGroups;
 
   // A return value to indicate that a given instance has not yet had a group
   // assignment (and hence is not yet participating in the trial).
   static const int kNotFinalized;
 
   // Disables this trial, meaning it always determines the default group
   // has been selected. May be called immediately after construction, or
   // at any time after initialization (should not be interleaved with
   // AppendGroup calls). Once disabled, there is no way to re-enable a
@@ skipping 424 matching lines @@
   // Gets the parameters for |field_trial| from shared memory and stores them in
   // |params|. This is only exposed for use by FieldTrialParamAssociator and
   // shouldn't be used by anything else.
   static bool GetParamsFromSharedMemory(
       FieldTrial* field_trial,
       std::map<std::string, std::string>* params);
 
   // Clears all the params in the allocator.
   static void ClearParamsFromSharedMemoryForTesting();
 
+  // We want to be able to dump field trial state to an allocator so that it

[Alexei Svitkine (slow), 2016/12/09 20:08:43]

Nit: Can you rephrase this comment and the one below without using "we want"
language. i.e. describe what it does.

[lawrencewu, 2016/12/09 20:31:11]

Done (as well as below).

+  // can be analyzed after a crash.
+  static void DumpAllFieldTrialsToPersistentAllocator(
+      PersistentMemoryAllocator* allocator);
+
+  // We want to be able to retrieve field trial state from an allocator so that
+  // it can be analyzed after a crash. The pointers in the returned vector are
+  // into the persistent memory segment and so are only valid as long as the
+  // allocator is valid.
+  static std::vector<const FieldTrial::FieldTrialEntry*>
+  GetAllFieldTrialsFromPersistentAllocator(
+      const PersistentMemoryAllocator* allocator);

[Alexei Svitkine (slow), 2016/12/09 20:08:43]

Can this be passed by const& instead of const*?

[lawrencewu, 2016/12/09 20:31:11]

Done.

+
  private:
   // Allow tests to access our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, InstantiateAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AddTrialsToAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest,
                            DoNotAddSimulatedFieldTrialsToAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AssociateFieldTrialParams);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, ClearParamsFromSharedMemory);
 
 #if defined(OS_WIN)
@@ skipping 23 matching lines @@
   static bool CreateTrialsFromSharedMemory(
       std::unique_ptr<base::SharedMemory> shm);
 
   // Instantiate the field trial allocator, add all existing field trials to it,
   // and duplicates its handle to a read-only handle, which gets stored in
   // |readonly_allocator_handle|.
   static void InstantiateFieldTrialAllocatorIfNeeded();
 
   // Adds the field trial to the allocator. Caller must hold a lock before
   // calling this.
-  static void AddToAllocatorWhileLocked(FieldTrial* field_trial);
+  static void AddToAllocatorWhileLocked(PersistentMemoryAllocator* allocator,
+                                        FieldTrial* field_trial);
 
   // Activate the corresponding field trial entry struct in shared memory.
   static void ActivateFieldTrialEntryWhileLocked(FieldTrial* field_trial);
 
   // A map from FieldTrial names to the actual instances.
   typedef std::map<std::string, FieldTrial*> RegistrationMap;
 
   // If one-time randomization is enabled, returns a weak pointer to the
   // corresponding EntropyProvider. Otherwise, returns NULL.
   static const FieldTrial::EntropyProvider*
@@ skipping 41 matching lines @@
 
   // Tracks whether CreateTrialsFromCommandLine() has been called.
   bool create_trials_from_command_line_called_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_