Bug 1581859: Part 3a - Add do_GetProperty helper for nsIPropertyBag2. r=mccr8

There currently isn't a typesafe way to get a property with a given interface
from a property bag from C++. This patch adds one, following the pattern of
similar helpers, like `do_GetInterface`.

Differential Revision: https://phabricator.services.mozilla.com/D103210

xpcom/ds/nsHashPropertyBag.cpp

@@ -265,6 +265,23 @@ void nsHashPropertyBagBase::CopyFrom(nsIPropertyBag* aOther) {
   }
 }
 
+nsresult nsGetProperty::operator()(const nsIID& aIID,
+                                   void** aInstancePtr) const {
+  nsresult rv;
+
+  if (mPropBag) {
+    rv = mPropBag->GetPropertyAsInterface(mPropName, aIID, aInstancePtr);
+  } else {
+    rv = NS_ERROR_NULL_POINTER;
+    *aInstancePtr = 0;
+  }
+
+  if (mErrorPtr) {
+    *mErrorPtr = rv;
+  }
+  return rv;
+}
+
 /*
  * nsHashPropertyBag implementation.
  */

xpcom/ds/nsIPropertyBag2.idl

@@ -24,6 +24,9 @@ interface nsIPropertyBag2 : nsIPropertyBag
 
   /**
    * This method returns null if the value exists, but is null.
+   *
+   * Note: C++ callers should not use this method. They should use the
+   * typesafe `do_GetProperty` wrapper instead.
    */
   void        getPropertyAsInterface   (in AString prop,
                                         in nsIIDRef iid,
@@ -40,3 +43,38 @@ interface nsIPropertyBag2 : nsIPropertyBag
    */
   boolean     hasKey                   (in AString prop);
 };
+
+
+%{C++
+#include "nsCOMPtr.h"
+#include "nsAString.h"
+
+class MOZ_STACK_CLASS nsGetProperty final : public nsCOMPtr_helper {
+ public:
+  nsGetProperty(nsIPropertyBag2* aPropBag, const nsAString& aPropName, nsresult* aError)
+      : mPropBag(aPropBag), mPropName(aPropName), mErrorPtr(aError) {}
+
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const override;
+
+ private:
+  nsIPropertyBag2* MOZ_NON_OWNING_REF mPropBag;
+  const nsAString& mPropName;
+  nsresult* mErrorPtr;
+};
+
+/**
+ * A typesafe wrapper around nsIPropertyBag2::GetPropertyAsInterface. Similar
+ * to the `do_QueryInterface` family of functions, when assigned to a
+ * `nsCOMPtr` of a given type, attempts to query the given property to that
+ * type.
+ *
+ * If `aError` is passed, the return value of `GetPropertyAsInterface` is
+ * stored in it.
+ */
+inline const nsGetProperty do_GetProperty(nsIPropertyBag2* aPropBag,
+                                          const nsAString& aPropName,
+                                          nsresult* aError = 0) {
+  return nsGetProperty(aPropBag, aPropName, aError);
+}
+
+%}