If you write a really big string to an empty file, and that string is > maxBytes, then `RotatingFileHandler` (or even `BaseRotatingHandler`) will roll that empty file into the backup queue.

I think it should instead use that empty file for the mega-string. By not doing so it "wastes" a slot in the backup queue.

It's a very minor issue: I doubt it really happens IRL, but I noticed it when extending RotatingFileHandler in py2.7 to add gzip stuff and my test cases used a really small size (16 bytes!)


Here's a test file:

    #!/usr/bin/env python3
    # coding=utf-8
    
    import logging
    import os
    import tempfile
    
    from logging.handlers import RotatingFileHandler
    
    
    class MockRecord(object):
        def __init__(self, msg):
            self.msg = msg
            self.stack_info = None
            self.exc_info = None
            self.exc_text = None
    
        def getMessage(self):
            return self.msg
    
    
    def test_file_rollover_from_mega_string(temp_dir_path):
        # This is a pretty weird test.
        # It tests that writing a huge string to a blank file causes the blank
        # file to be archived and the huge string written to the next log file.
        #
        # Normally the log files would have a large max bytes so we'd have to
        # be writing a giant string to an empty file for this to happen.
        # But, even if it does, it's what BaseRotatingHandler does, so...
    
        log_path = os.path.join(temp_dir_path, "mylog.log")
    
        handler = RotatingFileHandler(log_path, maxBytes=16, backupCount=5)
        handler.setFormatter(logging.Formatter())
        with open(log_path) as log:
            assert log.read() == ""
    
        # ----------------------------------------------
        handler.emit(MockRecord("There once was a test from bitbucket"))
        with open(log_path) as log:
            log_read = log.read()
            assert log_read == "There once was a test from bitbucket\n"
    
        with open(log_path + ".1") as log:
            log_read = log.read()
            assert log_read == ""
    
        # ----------------------------------------------
        handler.emit(MockRecord("11 chars"))
        with open(log_path) as log:
            log_read = log.read()
            assert log_read == "11 chars\n"
    
        with open(log_path + ".1") as log:
            log_read = log.read()
            assert log_read == "There once was a test from bitbucket\n"
    
        with open(log_path + ".2") as log:
            log_read = log.read()
            assert log_read == ""
    
        handler.close()
    
    
    test_file_rollover_from_mega_string(tempfile.mkdtemp())


and here's a patch that I think will fix it:

~/src/others/cpython (master *%=)$ cat empty_rollover.patch 
diff --git a/Lib/logging/handlers.py b/Lib/logging/handlers.py
index 7d77973..0dabfd7 100644
--- a/Lib/logging/handlers.py
+++ b/Lib/logging/handlers.py
@@ -186,7 +186,11 @@ class RotatingFileHandler(BaseRotatingHandler):
         if self.maxBytes > 0:                   # are we rolling over?
             msg = "%s\n" % self.format(record)
             self.stream.seek(0, 2)  #due to non-posix-compliant Windows feature
-            if self.stream.tell() + len(msg) >= self.maxBytes:
+            size = self.stream.tell()
+            if size == 0:
+                # No point rolling-over an empty file
+                return 0
+            elif size + len(msg) >= self.maxBytes:
                 return 1
         return 0