Support saving browser metrics to disk and reading them during next run.

base/metrics/persistent_histogram_allocator.cc

Index: base/metrics/persistent_histogram_allocator.cc
diff --git a/base/metrics/persistent_histogram_allocator.cc b/base/metrics/persistent_histogram_allocator.cc
index 62e8cd6d68baa4ca567e97f3c095ce093969eb1c..268a7a0a1122f66b2a10af58ebb347db36df882b 100644
--- a/base/metrics/persistent_histogram_allocator.cc
+++ b/base/metrics/persistent_histogram_allocator.cc
@@ -6,6 +6,7 @@
 
 #include <memory>
 
+#include "base/files/important_file_writer.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ -46,6 +47,7 @@ enum : uint32_t {
 // anything essential at exit anyway due to the fact that they depend on data
 // managed elsewhere and which could be destructed first.
 GlobalHistogramAllocator* g_allocator;
+GlobalHistogramAllocator* g_allocator_disabled;

[Ilya Sherman, 2016/04/26 20:01:24]

Please use a boolean for enabled/disabled state, rather than having two separate
pointers.

[bcwhite, 2016/04/26 21:04:26]

Done.

 
 // Take an array of range boundaries and create a proper BucketRanges object
 // which is returned to the caller. A return of nullptr indicates that the
@@ -651,12 +653,36 @@ void GlobalHistogramAllocator::CreateWithSharedMemoryHandle(
 }
 
 // static
+void GlobalHistogramAllocator::Enable() {
+  if (g_allocator) {
+    // Already enabled.
+    DCHECK(!g_allocator_disabled);
+    return;
+  }
+
+  DCHECK(g_allocator_disabled);
+  std::swap(g_allocator, g_allocator_disabled);
+}
+
+// static
+void GlobalHistogramAllocator::Disable() {
+  if (g_allocator_disabled) {
+    // Already disabled.
+    DCHECK(!g_allocator);
+    return;
+  }
+
+  DCHECK(g_allocator);
+  std::swap(g_allocator, g_allocator_disabled);
+}
+
+// static
 void GlobalHistogramAllocator::Set(
     std::unique_ptr<GlobalHistogramAllocator> allocator) {
   // Releasing or changing an allocator is extremely dangerous because it
   // likely has histograms stored within it. If the backing memory is also
   // also released, future accesses to those histograms will seg-fault.
-  CHECK(!g_allocator);
+  CHECK(!g_allocator && !g_allocator_disabled);
   g_allocator = allocator.release();
   size_t existing = StatisticsRecorder::GetHistogramCount();
 
@@ -670,9 +696,14 @@ GlobalHistogramAllocator* GlobalHistogramAllocator::Get() {
 }
 
 // static
+GlobalHistogramAllocator* GlobalHistogramAllocator::GetEvenIfDisabled() {
+  return g_allocator ? g_allocator : g_allocator_disabled;
+}
+
+// static
 std::unique_ptr<GlobalHistogramAllocator>
 GlobalHistogramAllocator::ReleaseForTesting() {
-  GlobalHistogramAllocator* histogram_allocator = g_allocator;
+  GlobalHistogramAllocator* histogram_allocator = GetEvenIfDisabled();
   if (!histogram_allocator)
     return nullptr;
   PersistentMemoryAllocator* memory_allocator =
@@ -700,10 +731,41 @@ GlobalHistogramAllocator::ReleaseForTesting() {
     DCHECK_NE(kResultHistogram, histogram_data->name);
   }
 
+  DCHECK_NE(g_allocator != nullptr, g_allocator_disabled != nullptr);
   g_allocator = nullptr;
+  g_allocator_disabled = nullptr;
   return WrapUnique(histogram_allocator);
 };
 
+void GlobalHistogramAllocator::SetPersistentLocation(const FilePath& location) {
+  persistent_location_ = location;
+}
+
+bool GlobalHistogramAllocator::WriteToPersistentLocation() {
+#if defined(OS_NACL)
+  // NACL doesn't support file operations, including ImportantFileWriter.
+  NOTREACHED();
+  return false;
+#else
+  // Stop if no destination is set, perhaps because it has not been enabled.
+  if (persistent_location_.empty()) {
+    NOTREACHED() << "Could not write \"" << Name() << "\" persistent histograms"
+                 << " to file because no location was set.";
+    return false;
+  }
+
+  StringPiece contents(static_cast<const char*>(data()), used());
+  if (!ImportantFileWriter::WriteFileAtomically(persistent_location_,
+                                                contents)) {
+    LOG(ERROR) << "Could not write \"" << Name() << "\" persistent histograms"
+               << " to file: " << persistent_location_.value();
+    return false;
+  }
+
+  return true;
+#endif
+}
+
 GlobalHistogramAllocator::GlobalHistogramAllocator(
     std::unique_ptr<PersistentMemoryAllocator> memory)
     : PersistentHistogramAllocator(std::move(memory)),