Monday, May 03, 2010

small incompatible change for 3.1

I've just commited a small incompatible change for syslog-ng 3.1, even though theoreticaly I shouldn't have.

The change is not big, simply the 'store-legacy-msghdr' flag became default for all sources, whereas earlier you had to specify that explicitly.

In order to understand why I did that, a short description of the flag follows below.

syslog-ng processes all incoming messages into fields (things like $PROGRAM and $DATE) and then reconstructs the message based on this parsed information when it has to write the message to a file.

Before syslog-ng 3.0 a message was split into the macros: "$DATE $HOST $MSG", which expanded to the actual log message. "$MSG" above was expanded to a line like:

"program[pid]: message"

With syslog-ng 3.0 and the integrated handling of RFC5424 and RFC3164 this format was changed and an $MSGHDR macro was created for the "program[pid]: " part and I got rid of this part from $MSG. (of course if you are running syslog-ng in compatibility mode, you get the old behaviour). The reason is simple: RFC5424 has separate fields for program/pid.

The contents of $MSGHDR is constructed programmatically, e.g. the punctuation characters '[' and ']' around the pid and the colon, is added to the format by syslog-ng, based on the available information in $PROGRAM and $PID.

However (and here comes the magic) there are programs that do not adhere to this format and omit the space after the colon character. E.g. if syslog-ng received:


as the syslog message, it added an explicit space character, and you'd get this in your log file:

"program: value"

NOTE the added space. This resulted in the workaround called "store-legacy-msghdr", which made syslog-ng remember the original formatting of the MSGHDR macro. However this proved to be a performance issue, thus it didn't become default, and I let my users discover this problem and add the flag explicitly if they cared about the extra space.

syslog-ng 3.1 however solves the performance issue (with the NVTable refactorization), and more and more people run into the very same issue, who are migrating from 2.1 or earlier.

Therefore I've decided to make 'store-legacy-msghdr' the default, and added a 'dont-store-legacy-msghdr' flag. My hope is that
  • people who cared: they already had the store-legacy-msghdr, for them, nothing is changed
  • people who didn't notice: they don't have the flag, but should be better of with the original formatting
  • people who changed their parsing scripts: well, those are who I address this message to as a HEADS up.
I hope this post makes things clearer.