Add FileReaderInterface::ReadExactly() and FileSeekerInterface::SeekSet()

util/file/file_seeker.h

Index: util/file/file_seeker.h
diff --git a/util/file/file_seeker.h b/util/file/file_seeker.h
index f6d8b46461f0bbbd0c2fcb9ae2bfbf1e544f7cd9..1b32e104588b1bdbe1987fa3da10a24686d08944 100644
--- a/util/file/file_seeker.h
+++ b/util/file/file_seeker.h
@@ -30,6 +30,21 @@ class FileSeekerInterface {
   //!     with an error message logged.
   virtual FileOffset Seek(FileOffset offset, int whence) = 0;
 
+  //! \brief Wraps Seek(), using `SEEK_CUR` to obtain the file’s current
+  //!     position.
+  //!
+  //! \return The file’s current position on success. `-1` on failure, with an
+  //!     error message logged.
+  FileOffset SeekGet();
+
+  //! \brief Wraps Seek(), using `SEEK_SET`, ensuring that the seek succeeded
+  //!     and the file is positioned as desired.
+  //!
+  //! \return `true` if the operation succeeded, `false` if it failed, with an
+  //!     error message logged. A failure to reposition the file as desired is
+  //!     treated as a failure.
+  bool SeekSet(FileOffset offset);
+
  protected:
   ~FileSeekerInterface() {}
 };