Add defer, a general facility for ad-hoc RAII. (#5554)

Author: wilhuff

Firestore/Example/Firestore.xcodeproj/project.pbxproj

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+		A6A9946A006AA87240B37E31 /* defer_test.cc in Sources */ = {isa = PBXBuildFile; fileRef = 8ABAC2E0402213D837F73DC3 /* defer_test.cc */; };
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+		8ABAC2E0402213D837F73DC3 /* defer_test.cc */ = {isa = PBXFileReference; includeInIndex = 1; path = defer_test.cc; sourceTree = "<group>"; };
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@@ -1743,6 +1750,7 @@
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 				548DB928200D59F600E00ABC /* comparison_test.cc */,
+				8ABAC2E0402213D837F73DC3 /* defer_test.cc */,
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+				17DC97DE15D200932174EC1F /* defer_test.cc in Sources */,
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 				B99452AB7E16B72D1C01FBBC /* datastore_test.cc in Sources */,
+				6325D0E43A402BC5866C9C0E /* defer_test.cc in Sources */,
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Firestore/core/src/util/defer.h

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_UTIL_DEFER_H_
+#define FIRESTORE_CORE_SRC_UTIL_DEFER_H_
+
+#include <functional>
+#include <utility>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+/**
+ * Creates a deferred action object that will execute the given `action` when
+ * the `Defer` object is destroyed at the close of the lexical scope containing
+ * it. You must declare a local variable of type `Defer` for it to have any
+ * useful effect; otherwise the `Defer` is destroyed at the end of the
+ * statement, which is equivalent to just directly running the `action`.
+ *
+ * `Defer` is useful for performing ad-hoc RAII-style actions, without having
+ * to create the wrapper class. For example:
+ *
+ *     FILE* file = fopen(filename, "rb");
+ *     Defer cleanup([&] {
+ *       if (file) {
+ *         fclose(file);
+ *       }
+ *     });
+ */
+class Defer {
+  // TODO(C++17): Make Action a template argument and use CTAD in the callers.
+ public:
+  using Action = std::function<void()>;
+
+  /**
+   * Constructs a `Defer` object.
+   *
+   * @param action a callable object; usually a lambda. Even if exceptions are
+   *     enabled, when `action` is invoked it must not throw. This is similar
+   *     to the restriction that exists on destructors generally.
+   */
+  explicit Defer(Action&& action) : action_(std::move(action)) {
+  }
+
+  ~Defer() {
+    action_();
+  }
+
+  // Defer can be neither copied nor moved.
+  Defer(const Defer&) = delete;
+  Defer& operator=(const Defer&) = delete;
+
+ private:
+  Action action_;
+};
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_UTIL_DEFER_H_

Firestore/core/src/util/filesystem_posix.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Google
+ * Copyright 2018 Google LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -168,7 +168,7 @@ Status Filesystem::IsDirectory(const Path& path) {
 
 StatusOr<int64_t> Filesystem::FileSize(const Path& path) {
   struct stat st {};
-  if (stat(path.c_str(), &st) == 0) {
+  if (::stat(path.c_str(), &st) == 0) {
     return st.st_size;
   } else {
     return Status::FromErrno(

Firestore/core/src/util/filesystem_win.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Google
+ * Copyright 2018 Google LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,6 +24,7 @@
 #include <cerrno>
 #include <string>
 
+#include "Firestore/core/src/util/defer.h"
 #include "Firestore/core/src/util/hard_assert.h"
 #include "Firestore/core/src/util/path.h"
 #include "Firestore/core/src/util/statusor.h"
@@ -36,16 +37,16 @@ namespace util {
 
 StatusOr<Path> Filesystem::AppDataDir(absl::string_view app_name) {
   wchar_t* path = nullptr;
+  auto cleanup = defer([&] { CoTaskMemFree(path); });
+
   HRESULT hr = SHGetKnownFolderPath(FOLDERID_LocalAppData, 0, nullptr, &path);
   if (FAILED(hr)) {
-    CoTaskMemFree(path);
     return Status::FromLastError(
         HRESULT_CODE(hr),
         "Failed to find the local application data directory");
   }
 
   Path result = Path::FromUtf16(path, wcslen(path)).AppendUtf8(app_name);
-  CoTaskMemFree(path);
   return std::move(result);
 }
 

Firestore/core/src/util/string_apple.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Google
+ * Copyright 2018 Google LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -46,6 +46,15 @@ inline CFStringRef MakeCFString(absl::string_view contents) {
                                  kCFStringEncodingUTF8, false);
 }
 
+/**
+ * CFRelease that safely ignores null values.
+ */
+inline void SafeCFRelease(CFTypeRef cf) {
+  if (cf) {
+    CFRelease(cf);
+  }
+}
+
 #if defined(__OBJC__)
 
 // Translates a string_view string to the equivalent NSString by making a copy.

Firestore/core/test/unit/model/field_value_test.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Google
+ * Copyright 2018 Google LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,6 +28,8 @@
 #include "Firestore/core/src/model/field_mask.h"
 #include "Firestore/core/src/model/field_path.h"
 #include "Firestore/core/src/nanopb/byte_string.h"
+#include "Firestore/core/src/util/defer.h"
+#include "Firestore/core/src/util/string_apple.h"
 #include "Firestore/core/test/unit/testutil/equals_tester.h"
 #include "Firestore/core/test/unit/testutil/testutil.h"
 #include "Firestore/core/test/unit/testutil/time_testing.h"
@@ -312,10 +314,10 @@ TEST_F(FieldValueTest, Equality) {
 TEST_F(FieldValueTest, CanonicalBitsAreCanonical) {
   double input = ToDouble(kAlternateNanBits);
   CFNumberRef number = CFNumberCreate(nullptr, kCFNumberDoubleType, &input);
+  util::Defer cleanup([&] { util::SafeCFRelease(number); });
 
   double actual = 0.0;
   CFNumberGetValue(number, kCFNumberDoubleType, &actual);
-  CFRelease(number);
 
   ASSERT_EQ(kCanonicalNanBits, ToBits(actual));
 }

Firestore/core/test/unit/util/defer_test.cc

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "Firestore/core/src/util/defer.h"
+
+#include "gtest/gtest.h"
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+TEST(DeferTest, Defers) {
+  int value = 0;
+  {
+    Defer cleanup([&] { ++value; });
+    ASSERT_EQ(value, 0);
+  }
+
+  ASSERT_EQ(value, 1);
+}
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase

Firestore/core/test/unit/util/filesystem_test.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Google
+ * Copyright 2018 Google LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,6 +22,7 @@
 #include <fstream>
 
 #include "Firestore/core/src/util/autoid.h"
+#include "Firestore/core/src/util/defer.h"
 #include "Firestore/core/src/util/log.h"
 #include "Firestore/core/src/util/path.h"
 #include "Firestore/core/src/util/statusor.h"
@@ -43,9 +44,12 @@ using testutil::Touch;
 static void WriteStringToFile(const Path& path, const std::string& text) {
   std::ofstream out{path.native_value()};
   ASSERT_TRUE(out.good());
+  Defer cleanup([&] {
+    out.close();
+    ASSERT_TRUE(out.good());
+  });
+
   out << text;
-  out.close();
-  ASSERT_TRUE(out.good());
 }
 
 static void WriteBytesToFile(const Path& path, int byte_count) {

Firestore/core/test/unit/util/string_apple_test.mm

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Google
+ * Copyright 2018 Google LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@
 #include <string>
 #include <vector>
 
+#include "Firestore/core/src/util/defer.h"
 #include "gtest/gtest.h"
 
 namespace firebase {
@@ -50,9 +51,10 @@
 TEST_F(StringAppleTest, MakeStringFromCFStringRef) {
   for (const std::string& string_value : StringTestCases()) {
     CFStringRef cf_string = MakeCFString(string_value);
+    Defer cleanup([&] { SafeCFRelease(cf_string); });
+
     std::string actual = MakeString(cf_string);
     EXPECT_EQ(string_value, actual);
-    CFRelease(cf_string);
   }
 }