add truncate() method that discards excess log entries

phew/logging.py

@@ -9,6 +9,50 @@ def log_file(file):
   global _log_file
   _log_file = file
 
+# truncates the log file down to a target size while maintaining
+# clean line breaks
+def truncate(target):
+  # get the current size of the log file
+  size = 0
+  try:
+    size = os.stat(_log_file)[6]
+  except OSError:
+    return
+  # calculate how many bytes we're aiming to discard
+  discard = size - target
+
+  if discard < 0:
+    return
+
+  with open(_log_file, "rb") as infile:
+    # skip a bunch of the input file until we've discarded
+    # at least enough
+    while discard > 0:
+      chunk = infile.read(1024)
+      discard -= len(chunk)
+
+    with open(_log_file + ".tmp", "wb") as outfile:
+      # we have a partial chunk to write, try to find a line
+      # break nearby to split it on
+      break_position = chunk.find(b"\n", -discard)
+      if break_position == -1:
+        break_position = chunk.rfind(b"\n", -discard)
+      if break_position == -1:
+        break_position = 0
+      outfile.write(chunk[break_position + 1:])
+
+      # now copy the rest of the file
+      while True:
+        chunk = infile.read(1024)
+        if not chunk:
+          break
+        outfile.write(chunk)
+
+  # delete the old file and replace with the new
+  os.remove(_log_file)
+  os.rename(_log_file + ".tmp", _log_file)
+
+
 def log(level, text):
   datetime = datetime_string()
   log_entry = "{0} [{1:8} /{2:>8}] {3}".format(datetime, level, gc.mem_free(), text)