Fix sampling of V8.DetachedContextAgeInGC histogram.

src/isolate.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdlib.h>
 
 #include <fstream>  // NOLINT(readability/streams)
 #include <sstream>
 
 #include "src/v8.h"
@@ skipping 2580 matching lines @@
   int length = detached_contexts->length();
   if (length == 0) return;
   int new_length = 0;
   for (int i = 0; i < length; i += 2) {
     int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
     WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
     if (!cell->cleared()) {
       detached_contexts->set(new_length, Smi::FromInt(mark_sweeps + 1));
       detached_contexts->set(new_length + 1, cell);
       new_length += 2;
-    } else {
-      counters()->detached_context_age_in_gc()->AddSample(mark_sweeps);

[ulan, 2015/02/18 13:14:33]

We need to sample all detached contexts, not only the collected ones because if
there is a memory leak then the leaked context will never be collected.

     }
+    counters()->detached_context_age_in_gc()->AddSample(mark_sweeps + 1);
   }
   if (FLAG_trace_detached_contexts) {
     PrintF("%d detached contexts are collected out of %d\n",
            length - new_length, length);
     for (int i = 0; i < new_length; i += 2) {
       int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
       WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
       if (mark_sweeps > 3) {
         PrintF("detached context 0x%p\n survived %d GCs (leak?)\n",
                static_cast<void*>(cell->value()), mark_sweeps);
@@ skipping 26 matching lines @@
   if (prev_ && prev_->Intercept(flag)) return true;
   // Then check whether this scope intercepts.
   if ((flag & intercept_mask_)) {
     intercepted_flags_ |= flag;
     return true;
   }
   return false;
 }
 
 } }  // namespace v8::internal