Revert "Revert "Add a way to turn off unique_fd's operator int.""

The original commit broke aosp-master-with-phones, because of
vendor libraries the depended on the int versions of libbase functions.

This patch reverts the revert, and also adds ABI-compatibility shims for
the replaced functions.

This reverts commit 2c58e1924a7bf1b44049764fddb40c1704dc288f.

Bug: http://b/131312539
Test: treehugger
Test: forrest run of aosp-master-with-phones
Change-Id: I75cc84ec8d963e20862f7662e8e2f409471f41cc
diff --git a/base/abi_compatibility.cpp b/base/abi_compatibility.cpp
new file mode 100644
index 0000000..06a7801
--- /dev/null
+++ b/base/abi_compatibility.cpp
@@ -0,0 +1,77 @@
+/*
+ * Copyright (C) 2019 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.
+ */
+
+#include <memory>
+
+#include "android-base/cmsg.h"
+#include "android-base/file.h"
+#include "android-base/mapped_file.h"
+#include "android-base/unique_fd.h"
+
+namespace android {
+namespace base {
+
+// These ABI-compatibility shims are in a separate file for two reasons:
+//   1. If they were in the file with the actual functions, it prevents calls to
+//      those functions by other functions in the file, due to ambiguity.
+//   2. We will hopefully be able to delete these quickly.
+
+#if !defined(_WIN32)
+ssize_t SendFileDescriptorVector(int sockfd, const void* data, size_t len,
+                                 const std::vector<int>& fds) {
+  return SendFileDescriptorVector(borrowed_fd(sockfd), data, len, fds);
+}
+
+ssize_t ReceiveFileDescriptorVector(int sockfd, void* data, size_t len, size_t max_fds,
+                                    std::vector<unique_fd>* fds) {
+  return ReceiveFileDescriptorVector(borrowed_fd(sockfd), data, len, max_fds, fds);
+}
+#endif
+
+bool ReadFdToString(int fd, std::string* content) {
+  return ReadFdToString(borrowed_fd(fd), content);
+}
+
+bool WriteStringToFd(const std::string& content, int fd) {
+  return WriteStringToFd(content, borrowed_fd(fd));
+}
+
+bool ReadFully(int fd, void* data, size_t byte_count) {
+  return ReadFully(borrowed_fd(fd), data, byte_count);
+}
+
+bool ReadFullyAtOffset(int fd, void* data, size_t byte_count, off64_t offset) {
+  return ReadFullyAtOffset(borrowed_fd(fd), data, byte_count, offset);
+}
+
+bool WriteFully(int fd, const void* data, size_t byte_count) {
+  return WriteFully(borrowed_fd(fd), data, byte_count);
+}
+
+#if defined(__LP64__)
+#define MAPPEDFILE_FROMFD _ZN10MappedFile6FromFdEilmi
+#else
+#define MAPPEDFILE_FROMFD _ZN10MappedFile6FromFdEixmi
+#endif
+
+#pragma clang diagnostic ignored "-Wreturn-type-c-linkage"
+extern "C" std::unique_ptr<MappedFile> MAPPEDFILE_FROMFD(int fd, off64_t offset, size_t length,
+                                                         int prot) {
+  return MappedFile::FromFd(fd, offset, length, prot);
+}
+
+}  // namespace base
+}  // namespace android