Support "shared" histograms between processes.

base/metrics/histogram_base.h

Index: base/metrics/histogram_base.h
diff --git a/base/metrics/histogram_base.h b/base/metrics/histogram_base.h
index f45cc9c106e323e83791265a57cbefaa3352011b..309e31af68c8d320fc0f72dbd154694e7b0a65e5 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -13,12 +13,14 @@
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/memory/persistent_memory_allocator.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string_piece.h"
 #include "base/time/time.h"
 
 namespace base {
 
+class BucketRanges;
 class DictionaryValue;
 class HistogramBase;
 class HistogramSamples;
@@ -79,6 +81,13 @@ class BASE_EXPORT HistogramBase {
     // to shortcut looking up the callback if it doesn't exist.
     kCallbackExists = 0x20,
 
+    // Indicates that the histogram is held in "persistent" memory and may
+    // be accessible between processes. This is only possible if such a
+    // memory segment has been created/attached, used to create a Persistent-
+    // MemoryAllocator, and that loaded into the Histogram module before this
+    // histogram is created.
+    kIsPersistent = 0x40,
+
     // Only for Histogram and its sub classes: fancy bucket-naming support.
     kHexRangePrintingFlag = 0x8000,
   };
@@ -158,7 +167,53 @@ class BASE_EXPORT HistogramBase {
   // customize the output.
   void WriteJSON(std::string* output) const;
 
+  // Access a PersistentMemoryAllocator for storing histograms in space that
+  // can be persisted or shared between processes. There is only ever one
+  // allocator for all such histograms created by a single process though one
+  // process may access the histograms created by other processes if it has a
+  // handle on its memory segment. This takes ownership of the object and
+  // should not be changed without great care as it is likely that there will
+  // be pointers to data held in that space.
+  static void SetDefaultPersistentMemoryAllocator(PersistentMemoryAllocator* a);
+  static PersistentMemoryAllocator* GetDefaultPersistentMemoryAllocator() {
+    return allocator_.get();
+  }
+
+  // This access to the persistent allocator is only for testing; it extracts
+  // the current allocator completely. This allows easy creation of histograms
+  // within persistent memory segments which can then be extracted and used
+  // in other ways.
+  static PersistentMemoryAllocator* ReleaseDefaultPersistentMemoryAllocator() {
+    return allocator_.release();
+  };
+
+  // Recreate a Histogram from data held in persistent memory. Though this
+  // object will be local to the current process, the sample data will be
+  // shared with all other threads referencing it. This method takes a |ref|
+  // to the top- level histogram data and the |allocator| on which it is found.
+  // This method will return nullptr if any problem is detected with the data.
+  // The |allocator| may or may not be the same as the PersistentMemoryAllocator
+  // set for general use so that this method can be used to extract Histograms
+  // from persistent memory segments other than the default place that this
+  // process is creating its own histograms. The caller must take ownership of
+  // the returned object and destroy it when no longer needed.
+  static HistogramBase* GetPersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      int32_t ref);
+
+  // Get the next histogram in persistent data based on iterator. The caller
+  // must take ownership of the returned object and destroy it when no longer
+  // needed.
+  static HistogramBase* GetNextPersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      PersistentMemoryAllocator::Iterator* iter);

[Alexei Svitkine (slow), 2015/12/04 18:27:36]

Can you put these in a separate file? HistogramBase is an abstract base class
and shouldn't be concerned with how histograms are allocated.

[bcwhite, 2015/12/07 23:27:19]

Done.

+
  protected:
+  struct PersistentHistogramData;
+
+  // Global allocator used when histograms should be persistent.
+  static scoped_ptr<PersistentMemoryAllocator> allocator_;
+
   // Subclasses should implement this function to make SerializeInfo work.
   virtual bool SerializeInfoImpl(base::Pickle* pickle) const = 0;
 
@@ -190,6 +245,32 @@ class BASE_EXPORT HistogramBase {
   // passing |sample| as the parameter.
   void FindAndRunCallback(Sample sample) const;
 
+  // Create a histogram based on persistent data.
+  static HistogramBase* CreatePersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      PersistentHistogramData* histogram_data);
+
+  // Allocate a new persistent histogram. This does *not* make the object
+  // iterable in the allocator; call MakeIterable(ref) directly if that is
+  // desired.
+  static HistogramBase* AllocatePersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      HistogramType histogram_type,
+      const std::string& name,
+      int minimum,
+      int maximum,
+      const BucketRanges* bucket_ranges,
+      int32 flags,
+      PersistentMemoryAllocator::Reference* ref_ptr);
+
+  // Import new histograms from attached PersistentMemoryAllocator. It's
+  // possible for other processes to create histograms in the attached memory
+  // segment; this adds those to the internal list of known histograms to
+  // avoid creating duplicates that would have to merged during reporting.
+  // Every call to this method resumes from the last entry it saw so it costs
+  // nothing if nothing new has been added.
+  static void ImportPersistentHistograms();
+
  private:
   const std::string histogram_name_;
   AtomicCount flags_;