Add File::Duplicate to duplicate a file handle.

BUG=462584
R=rvargas@chromium.org

Review URL: https://codereview.chromium.org/1017243002

Cr-Commit-Position: refs/heads/master@{#321389}

base/files/file.h

@@ -288,6 +288,13 @@ class BASE_EXPORT File {
   // Unlock a file previously locked.
   Error Unlock();
 
+  // Returns a new object referencing this file for use within the current
+  // process. Handling of FLAG_DELETE_ON_CLOSE varies by OS. On POSIX, the File
+  // object that was created or initialized with this flag will have unlinked
+  // the underlying file when it was created or opened. On Windows, the
+  // underlying file is deleted when the last handle to it is closed.
+  File Duplicate();
+
   bool async() const { return async_; }
 
 #if defined(OS_WIN)

base/files/file_posix.cc

@@ -463,6 +463,20 @@ File::Error File::Unlock() {
   return CallFctnlFlock(file_.get(), false);
 }
 
+File File::Duplicate() {
+  if (!IsValid())
+    return File();
+
+  PlatformFile other_fd = dup(GetPlatformFile());
+  if (other_fd == -1)
+    return File(OSErrorToFileError(errno));
+
+  File other(other_fd);
+  if (async())
+    other.async_ = true;
+  return other.Pass();
+}
+
 // Static.
 File::Error File::OSErrorToFileError(int saved_errno) {
   switch (saved_errno) {

base/files/file_unittest.cc

@@ -443,6 +443,49 @@ TEST(FileTest, Seek) {
   EXPECT_EQ(kOffset, file.Seek(base::File::FROM_END, -kOffset));
 }
 
+TEST(FileTest, Duplicate) {
+  base::ScopedTempDir temp_dir;
+  ASSERT_TRUE(temp_dir.CreateUniqueTempDir());
+  FilePath file_path = temp_dir.path().AppendASCII("file");
+  File file(file_path,(base::File::FLAG_CREATE |
+                       base::File::FLAG_READ |
+                       base::File::FLAG_WRITE));
+  ASSERT_TRUE(file.IsValid());
+
+  File file2(file.Duplicate());
+  ASSERT_TRUE(file2.IsValid());
+
+  // Write through one handle, close it, read through the other.
+  static const char kData[] = "now is a good time.";
+  static const int kDataLen = sizeof(kData) - 1;
+
+  ASSERT_EQ(0, file.Seek(base::File::FROM_CURRENT, 0));
+  ASSERT_EQ(0, file2.Seek(base::File::FROM_CURRENT, 0));
+  ASSERT_EQ(kDataLen, file.WriteAtCurrentPos(kData, kDataLen));
+  ASSERT_EQ(kDataLen, file.Seek(base::File::FROM_CURRENT, 0));
+  ASSERT_EQ(kDataLen, file2.Seek(base::File::FROM_CURRENT, 0));
+  file.Close();
+  char buf[kDataLen];
+  ASSERT_EQ(kDataLen, file2.Read(0, &buf[0], kDataLen));
+  ASSERT_EQ(std::string(kData, kDataLen), std::string(&buf[0], kDataLen));
+}
+
+TEST(FileTest, DuplicateDeleteOnClose) {
+  base::ScopedTempDir temp_dir;
+  ASSERT_TRUE(temp_dir.CreateUniqueTempDir());
+  FilePath file_path = temp_dir.path().AppendASCII("file");
+  File file(file_path,(base::File::FLAG_CREATE |
+                       base::File::FLAG_READ |
+                       base::File::FLAG_WRITE |
+                       base::File::FLAG_DELETE_ON_CLOSE));
+  ASSERT_TRUE(file.IsValid());
+  File file2(file.Duplicate());
+  ASSERT_TRUE(file2.IsValid());
+  file.Close();
+  file2.Close();
+  ASSERT_FALSE(base::PathExists(file_path));
+}
+
 #if defined(OS_WIN)
 TEST(FileTest, GetInfoForDirectory) {
   base::ScopedTempDir temp_dir;

base/files/file_win.cc

@@ -309,6 +309,28 @@ File::Error File::Unlock() {
   return FILE_OK;
 }
 
+File File::Duplicate() {
+  if (!IsValid())
+    return File();
+
+  HANDLE other_handle = nullptr;
+
+  if (!::DuplicateHandle(GetCurrentProcess(),  // hSourceProcessHandle
+                         GetPlatformFile(),
+                         GetCurrentProcess(),  // hTargetProcessHandle
+                         &other_handle,
+                         0,  // dwDesiredAccess ignored due to SAME_ACCESS
+                         FALSE,  // !bInheritHandle
+                         DUPLICATE_SAME_ACCESS)) {
+    return File(OSErrorToFileError(GetLastError()));
+  }
+
+  File other(other_handle);
+  if (async())
+    other.async_ = true;
+  return other.Pass();
+}
+
 // Static.
 File::Error File::OSErrorToFileError(DWORD last_error) {
   switch (last_error) {