Introduce unique_fd

unique_fds are wrappers for integer file descriptors that automatically
close when they go out of scope. They are movable but not copyable and
generally mimic the semantics of std::unique_ptr

(cherry-pick from 918b9e8dc6)

Change-Id: I0657b848b6536d5ad9fc06686e240ea5c387b0ac
Test: Considerable exercise in new AIDL integration tests
Bug: None
Signed-off-by: Casey Dahlin <sadmac@google.com>

base/include/base/unique_fd.h

@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * 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 ANDROID_BASE_UNIQUE_FD_H
+#define ANDROID_BASE_UNIQUE_FD_H
+
+#include <unistd.h>
+
+#include <base/macros.h>
+
+/* Container for a file descriptor that automatically closes the descriptor as
+ * it goes out of scope.
+ *
+ *      unique_fd ufd(open("/some/path", "r"));
+ *
+ *      if (ufd.get() < 0) // invalid descriptor
+ *          return error;
+ *
+ *      // Do something useful
+ *
+ *      return 0; // descriptor is closed here
+ */
+namespace android {
+namespace base {
+
+class unique_fd final {
+ public:
+  unique_fd() : value_(-1) {}
+
+  explicit unique_fd(int value) : value_(value) {}
+  ~unique_fd() { clear(); }
+
+  unique_fd(unique_fd&& other) : value_(other.release()) {}
+  unique_fd& operator = (unique_fd&& s) {
+    reset(s.release());
+    return *this;
+  }
+
+  void reset(int new_value) {
+    if (value_ >= 0)
+      close(value_);
+    value_ = new_value;
+  }
+
+  void clear() {
+    reset(-1);
+  }
+
+  int get() const { return value_; }
+
+  int release() {
+    int ret = value_;
+    value_ = -1;
+    return ret;
+  }
+
+ private:
+  int value_;
+
+  DISALLOW_COPY_AND_ASSIGN(unique_fd);
+};
+
+}  // namespace base
+}  // namespace android
+
+#endif // ANDROID_BASE_UNIQUE_FD_H