Additionally, I've run the exporter with a debugger and the CollectFromLogLine function is not reached.

I believe postfix_exporter currently only reads the log once, and stops on first 0-length read.

I co-maintain the Debian package prometheus-postfix-exporter, and have also just recently discovered that systemd journal support inexplicably broke somewhere between v0.2.0 and v0.3.0. This is somewhat disappointing, since v0.2.0 was working quite reliably.

From the minimal debugging that I've done so far, it appears to bail out of the SystemdLogSource.Read() function with io.EOF when s.journal.Next() returns zero, and never actually calls s.journal.GetEntry()

func (s *SystemdLogSource) Read(ctx context.Context) (string, error) {
    c, err := s.journal.Next()
    if err != nil {
        return "", err
    }
    if c == 0 {
        return "", io.EOF
    }

    e, err := s.journal.GetEntry()
...

That subsequently causes the for-loop in PostfixExporter.StartMetricCollection() to bail out, and that's pretty much game over.

By commenting out the "Start at end of journal" seek in logsource_systemd.go, I can get the exporter to "replay" historical systemd journal entries, and it appears to produce the expected metrics. However, when it reaches the end of the events, the Read() function still bails out with io.EOF. This seems to be the main issue - it doesn't wait for further events, and if it is allowed to seek to the end of the journal (i.e., unmodified code from the 0.3.0 tag), it will immediately bail out as there are no events to read.

This might also be an issue with recent versions of go-systemd: coreos/go-systemd#392

I did some digging today because this bothered me a bit:

• We go to the bottom of the journal (https://github.com/kumina/postfix_exporter/blob/master/logsource_systemd.go#L55 via SeekRealtimeUsec and then it runs into an EOF if nothing else is written to the journal in the meantime. Removing the return fixes the problem and seems to work fine for me.
• For some reason the locking of GetEntry from go-systemd hangs, but I'm not sure here what's wrong so far. <- this was just an error by me while debugging.