Bug 1752331: Expose a HasDefaultValue preference r=KrisWright

Differential Revision: https://phabricator.services.mozilla.com/D137149

modules/libpref/Preferences.cpp

@@ -2493,6 +2493,16 @@ nsPrefBranch::PrefHasUserValue(const char* aPrefName, bool* aRetVal) {
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsPrefBranch::PrefHasDefaultValue(const char* aPrefName, bool* aRetVal) {
+  NS_ENSURE_ARG_POINTER(aRetVal);
+  NS_ENSURE_ARG(aPrefName);
+
+  const PrefName& pref = GetPrefName(aPrefName);
+  *aRetVal = Preferences::HasDefaultValue(pref.get());
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsPrefBranch::LockPref(const char* aPrefName) {
   NS_ENSURE_ARG(aPrefName);
@@ -5011,6 +5021,14 @@ bool Preferences::HasUserValue(const char* aPrefName) {
   return pref.isSome() && pref->HasUserValue();
 }
 
+/* static */
+bool Preferences::HasDefaultValue(const char* aPrefName) {
+  NS_ENSURE_TRUE(InitStaticMembers(), false);
+
+  Maybe<PrefWrapper> pref = pref_Lookup(aPrefName);
+  return pref.isSome() && pref->HasDefaultValue();
+}
+
 /* static */
 int32_t Preferences::GetType(const char* aPrefName) {
   NS_ENSURE_TRUE(InitStaticMembers(), nsIPrefBranch::PREF_INVALID);

modules/libpref/Preferences.h

@@ -223,6 +223,9 @@ class Preferences final : public nsIPrefService,
   // Whether the pref has a user value or not.
   static bool HasUserValue(const char* aPref);
 
+  // Whether the pref has a user value or not.
+  static bool HasDefaultValue(const char* aPref);
+
   // Adds/Removes the observer for the root pref branch. See nsIPrefBranch.idl
   // for details.
   static nsresult AddStrongObserver(nsIObserver* aObserver,

modules/libpref/nsIPrefBranch.idl

@@ -278,6 +278,25 @@ interface nsIPrefBranch : nsISupports
    */
   boolean prefHasUserValue(in string aPrefName);
 
+  /**
+   * Called to check if a specific preference has a default value associated to
+   * it.
+   *
+   * @param aPrefName The preference to be tested.
+   *
+   * @note
+   * This method can be called on either a default or user branch but, in
+   * effect, always operates on the user branch.
+   *
+   * @note
+   * This method can be used to distinguish between a built-in preference and a
+   * user-added preference.
+   *
+   * @return boolean  true  The preference has a default value.
+   *                  false The preference only has a user value.
+   */
+  boolean prefHasDefaultValue(in string aPrefName);
+
   /**
    * Called to check if a specific preference is locked. If a preference is
    * locked calling its Get method will always return the default value.