file_util: Export CopyFileContents function

This is a variant of CopyFile to use when the file paths are not
accessible, e.g. passing a file descriptor across containers.

Change-Id: I87a674b389139ced34cfc3ece11df5c2765df640
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536652
Commit-Queue: Alexander Bolodurin <alexbn@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831157}

base/files/file_util.cc

@@ -49,6 +49,35 @@ bool Move(const FilePath& from_path, const FilePath& to_path) {
   return internal::MoveUnsafe(from_path, to_path);
 }
 
+bool CopyFileContents(File& infile, File& outfile) {
+  static constexpr size_t kBufferSize = 32768;
+  std::vector<char> buffer(kBufferSize);
+
+  for (;;) {
+    int bytes_read = infile.ReadAtCurrentPos(buffer.data(), buffer.size());
+    if (bytes_read < 0) {
+      return false;
+    }
+    if (bytes_read == 0) {
+      return true;
+    }
+    // Allow for partial writes
+    int bytes_written_per_read = 0;
+    do {
+      int bytes_written_partial = outfile.WriteAtCurrentPos(
+          &buffer[bytes_written_per_read], bytes_read - bytes_written_per_read);
+      if (bytes_written_partial < 0) {
+        return false;
+      }
+
+      bytes_written_per_read += bytes_written_partial;
+    } while (bytes_written_per_read < bytes_read);
+  }
+
+  NOTREACHED();
+  return false;
+}
+
 bool ContentsEqual(const FilePath& filename1, const FilePath& filename2) {
   // We open the file in binary format even if they are text files because
   // we are just comparing that bytes are exactly same in both files and not

base/files/file_util.h

@@ -138,6 +138,12 @@ BASE_EXPORT bool ReplaceFile(const FilePath& from_path,
 //   read permissions. i.e. Always 0644.
 BASE_EXPORT bool CopyFile(const FilePath& from_path, const FilePath& to_path);
 
+// Copies the contents of one file into another.
+// The files are taken as is: the copy is done starting from the current offset
+// of |infile| until the end of |infile| is reached, into the current offset of
+// |outfile|.
+BASE_EXPORT bool CopyFileContents(File& infile, File& outfile);
+
 // Copies the given path, and optionally all subdirectories and their contents
 // as well.
 //

base/files/file_util_posix.cc

@@ -132,32 +132,6 @@ bool AdvanceEnumeratorWithStat(FileEnumerator* traversal,
   return true;
 }
 
-bool CopyFileContents(File* infile, File* outfile) {
-  static constexpr size_t kBufferSize = 32768;
-  std::vector<char> buffer(kBufferSize);
-
-  for (;;) {
-    ssize_t bytes_read = infile->ReadAtCurrentPos(buffer.data(), buffer.size());
-    if (bytes_read < 0)
-      return false;
-    if (bytes_read == 0)
-      return true;
-    // Allow for partial writes
-    ssize_t bytes_written_per_read = 0;
-    do {
-      ssize_t bytes_written_partial = outfile->WriteAtCurrentPos(
-          &buffer[bytes_written_per_read], bytes_read - bytes_written_per_read);
-      if (bytes_written_partial < 0)
-        return false;
-
-      bytes_written_per_read += bytes_written_partial;
-    } while (bytes_written_per_read < bytes_read);
-  }
-
-  NOTREACHED();
-  return false;
-}
-
 bool DoCopyDirectory(const FilePath& from_path,
                      const FilePath& to_path,
                      bool recursive,
@@ -289,7 +263,7 @@ bool DoCopyDirectory(const FilePath& from_path,
       return false;
     }
 
-    if (!CopyFileContents(&infile, &outfile)) {
+    if (!CopyFileContents(infile, outfile)) {
       DLOG(ERROR) << "CopyDirectory() couldn't copy file: " << current.value();
       return false;
     }
@@ -1160,7 +1134,7 @@ bool CopyFile(const FilePath& from_path, const FilePath& to_path) {
   if (!outfile.IsValid())
     return false;
 
-  return CopyFileContents(&infile, &outfile);
+  return CopyFileContents(infile, outfile);
 }
 #endif  // !defined(OS_APPLE)