Bug 1425613 - Part 1 - Record and expose pref access statistics in nsIPrefService in debug mode. r=njn

These statistics will be used by browser tests to analyze frequently accessed
preferences so that we can recommend using preference observers instead.

MozReview-Commit-ID: 9uZnwmjZL4U

modules/libpref/Preferences.cpp

@@ -725,6 +725,10 @@ private:
 class PrefEntry : public PLDHashEntryHdr
 {
 public:
+#ifdef DEBUG
+  // This field is before mPref to minimize sizeof(PrefEntry) on 64-bit.
+  uint32_t mAccessCount;
+#endif
   Pref* mPref; // Note: this is never null in a live entry.
 
   static bool MatchEntry(const PLDHashEntryHdr* aEntry, const void* aKey)
@@ -740,6 +744,9 @@ public:
     auto entry = static_cast<PrefEntry*>(aEntry);
     auto prefName = static_cast<const char*>(aKey);
 
+#ifdef DEBUG
+    entry->mAccessCount = 0;
+#endif
     entry->mPref = new Pref(prefName);
   }
 
@@ -941,7 +948,15 @@ static Pref*
 pref_HashTableLookup(const char* aPrefName)
 {
   PrefEntry* entry = pref_HashTableLookupInner(aPrefName);
-  return entry ? entry->mPref : nullptr;
+  if (!entry) {
+    return nullptr;
+  }
+
+#ifdef DEBUG
+  entry->mAccessCount += 1;
+#endif
+
+  return entry->mPref;
 }
 
 static nsresult
@@ -3615,6 +3630,34 @@ Preferences::GetDefaultBranch(const char* aPrefRoot, nsIPrefBranch** aRetVal)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+Preferences::ReadStats(nsIPrefStatsCallback* aCallback)
+{
+#ifdef DEBUG
+  for (auto iter = gHashTable->Iter(); !iter.Done(); iter.Next()) {
+    PrefEntry* entry = static_cast<PrefEntry*>(iter.Get());
+    aCallback->Visit(entry->mPref->Name(), entry->mAccessCount);
+  }
+
+  return NS_OK;
+#else
+  return NS_ERROR_NOT_IMPLEMENTED;
+#endif
+}
+
+NS_IMETHODIMP
+Preferences::ResetStats()
+{
+#ifdef DEBUG
+  for (auto iter = gHashTable->Iter(); !iter.Done(); iter.Next()) {
+    static_cast<PrefEntry*>(iter.Get())->mAccessCount = 0;
+  }
+  return NS_OK;
+#else
+  return NS_ERROR_NOT_IMPLEMENTED;
+#endif
+}
+
 NS_IMETHODIMP
 Preferences::GetDirty(bool* aRetVal)
 {