Add LoggingSeekFile

As the whence values are "helpfully" available in windows.h as well
( http://msdn.microsoft.com/en-us/library/windows/desktop/dd757336(v=vs.85).aspx )
don't bother inventing a new enum. Add implementations for POSIX and Win32.

R=mark@chromium.org
BUG=crashpad:1

Review URL: https://codereview.chromium.org/812593005

util/file/file_io.h

@@ -30,10 +30,18 @@ class FilePath;
 namespace crashpad {
 
 #if defined(OS_POSIX) || DOXYGEN
+
 //! \brief Platform-specific alias for a low-level file handle.
 using FileHandle = int;
+
+//! \brief Platform-specific alias for a position in an open file.
+using FileOffset = off_t;
+
 #elif defined(OS_WIN)
+
 using FileHandle = HANDLE;
+using FileOffset = LONGLONG;
+
 #endif
 
 //! \brief Determines the mode that LoggingOpenFileForWrite() uses.
@@ -166,6 +174,17 @@ FileHandle LoggingOpenFileForWrite(const base::FilePath& path,
                                    FileWriteMode write_mode,
                                    bool world_readable);
 
+//! \brief Wraps `lseek()` or `SetFilePointerEx()`. Logs an error if the
+//!     operation fails.
+//!
+//! Repositions the offset of the open \a file to the specified \a offset,
+//! relative to \a whence. \a whence must be one of `SEEK_SET`, `SEEK_CUR`, or
+//! `SEEK_END`, and is interpreted in the usual way.
+//!
+//! \return The resulting offset in bytes from the beginning of the file, or
+//!     `-1` on failure.
+FileOffset LoggingSeekFile(FileHandle file, FileOffset offset, int whence);
+
 //! \brief Wraps `close()` or `CloseHandle()`, logging an error if the operation
 //!     fails.
 //!

util/file/file_io_posix.cc

@@ -98,6 +98,12 @@ FileHandle LoggingOpenFileForWrite(const base::FilePath& path,
   return fd;
 }
 
+FileOffset LoggingSeekFile(FileHandle file, FileOffset offset, int whence) {
+  off_t rv = lseek(file, offset, whence);
+  PLOG_IF(ERROR, rv < 0) << "lseek";
+  return rv;
+}
+
 bool LoggingCloseFile(FileHandle file) {
   int rv = IGNORE_EINTR(close(file));
   PLOG_IF(ERROR, rv != 0) << "close";

util/file/file_io_win.cc

@@ -103,6 +103,34 @@ FileHandle LoggingOpenFileForWrite(const base::FilePath& path,
   return file;
 }
 
+FileOffset LoggingSeekFile(FileHandle file, FileOffset offset, int whence) {
+  DWORD method = 0;
+  switch (whence) {
+    case SEEK_SET:
+      method = FILE_BEGIN;
+      break;
+    case SEEK_CUR:
+      method = FILE_CURRENT;
+      break;
+    case SEEK_END:
+      method = FILE_END;
+      break;
+    default:
+      NOTREACHED();
+      break;
+  }
+
+  LARGE_INTEGER distance_to_move;
+  distance_to_move.QuadPart = offset;
+  LARGE_INTEGER new_offset;
+  BOOL result = SetFilePointerEx(file, distance_to_move, &new_offset, method);
+  if (!result) {
+    PLOG(ERROR) << "SetFilePointerEx";
+    return -1;
+  }
+  return new_offset.QuadPart;
+}
+
 bool LoggingCloseFile(FileHandle file) {
   BOOL rv = CloseHandle(file);
   PLOG_IF(ERROR, !rv) << "CloseHandle";