Bug 1708643 - Add generic predicate and fallback functions for QM_OR_…

…ELSE_(WARN|NOTE|LOG)_IF; r=dom-storage-reviewers,asuth

Differential Revision: https://phabricator.services.mozilla.com/D114935

dom/quota/QuotaCommon.h

@@ -1022,6 +1022,34 @@ auto ErrToDefaultOkOrErr(nsresult aValue) -> Result<V, nsresult> {
   return Err(aValue);
 }
 
+// Helper template function so that QM_TRY predicates checking for a specific
+// error can be concisely written as IsSpecificError<NS_SOME_ERROR> instead of
+// as a more verbose lambda.
+template <nsresult ErrorValue>
+bool IsSpecificError(const nsresult aValue) {
+  return aValue == ErrorValue;
+}
+
+// Helper template function so that QM_TRY fallback functions that are
+// converting errors into specific in-band success values can be concisely
+// written as ErrToOk<SuccessValueToReturn> (with the return type inferred).
+// For example, many file-related APIs that access information about a file may
+// return an nsresult error code if the file does not exist. From an
+// application perspective, the file not existing is not actually exceptional
+// and can instead be handled by the success case.
+template <auto SuccessValue, typename V = decltype(SuccessValue)>
+auto ErrToOk(const nsresult aValue) -> Result<V, nsresult> {
+  return V{SuccessValue};
+}
+
+// Helper template function so that QM_TRY fallback functions that are
+// suppressing errors by converting them into (generic) success can be
+// concisely written as ErrToDefaultOk<>.
+template <typename V = mozilla::Ok>
+auto ErrToDefaultOk(const nsresult aValue) -> Result<V, nsresult> {
+  return V{};
+}
+
 // TODO: Maybe move this to mfbt/ResultExtensions.h
 template <typename R, typename Func, typename... Args>
 Result<R, nsresult> ToResultGet(const Func& aFunc, Args&&... aArgs) {

dom/quota/test/gtest/TestQuotaCommon.cpp

@@ -1478,6 +1478,53 @@ TEST(QuotaCommon_ErrToDefaultOkOrErr, NsCOMPtr_Err)
   EXPECT_EQ(res.unwrapErr(), NS_ERROR_UNEXPECTED);
 }
 
+TEST(QuotaCommon_IsSpecificError, Match)
+{ EXPECT_TRUE(IsSpecificError<NS_ERROR_FAILURE>(NS_ERROR_FAILURE)); }
+
+TEST(QuotaCommon_IsSpecificError, Mismatch)
+{ EXPECT_FALSE(IsSpecificError<NS_ERROR_FAILURE>(NS_ERROR_UNEXPECTED)); }
+
+TEST(QuotaCommon_ErrToOk, Bool_True)
+{
+  auto res = ErrToOk<true>(NS_ERROR_FAILURE);
+  EXPECT_TRUE(res.isOk());
+  EXPECT_EQ(res.unwrap(), true);
+}
+
+TEST(QuotaCommon_ErrToOk, Bool_False)
+{
+  auto res = ErrToOk<false>(NS_ERROR_FAILURE);
+  EXPECT_TRUE(res.isOk());
+  EXPECT_EQ(res.unwrap(), false);
+}
+
+TEST(QuotaCommon_ErrToOk, Int_42)
+{
+  auto res = ErrToOk<42>(NS_ERROR_FAILURE);
+  EXPECT_TRUE(res.isOk());
+  EXPECT_EQ(res.unwrap(), 42);
+}
+
+TEST(QuotaCommon_ErrToOk, NsCOMPtr_nullptr)
+{
+  auto res = ErrToOk<nullptr, nsCOMPtr<nsISupports>>(NS_ERROR_FAILURE);
+  EXPECT_TRUE(res.isOk());
+  EXPECT_EQ(res.unwrap(), nullptr);
+}
+
+TEST(QuotaCommon_ErrToDefaultOk, Ok)
+{
+  auto res = ErrToDefaultOk<Ok>(NS_ERROR_FAILURE);
+  EXPECT_TRUE(res.isOk());
+}
+
+TEST(QuotaCommon_ErrToDefaultOk, NsCOMPtr)
+{
+  auto res = ErrToDefaultOk<nsCOMPtr<nsISupports>>(NS_ERROR_FAILURE);
+  EXPECT_TRUE(res.isOk());
+  EXPECT_EQ(res.unwrap(), nullptr);
+}
+
 class StringPairParameterized
     : public ::testing::TestWithParam<std::pair<const char*, const char*>> {};