Fix log files being opened as unicode files (#5545)

python/ray/log_monitor.py

@@ -144,7 +144,7 @@ class LogMonitor(object):
             # file.
             if file_size > file_info.size_when_last_opened:
                 try:
-                    f = open(file_info.filename, "r")
+                    f = open(file_info.filename, "rb")
                 except (IOError, OSError) as e:
                     if e.errno == errno.ENOENT:
                         logger.warning("Warning: The file {} was not "
@@ -179,6 +179,10 @@ class LogMonitor(object):
             for _ in range(max_num_lines_to_read):
                 try:
                     next_line = file_info.file_handle.readline()
+                    # Replace any characters not in UTF-8 with
+                    # a replacement character, see
+                    # https://stackoverflow.com/a/38565489/10891801
+                    next_line = next_line.decode("utf-8", "replace")
                     if next_line == "":
                         break
                     if next_line[-1] == "\n":