Prevent a full file read when a file is reopened after truncation

[ouwe-knutselaar]

L.S.

I have an issue with Tail. We have an application that watches a file to send changes to an event bus to generate alerts.
The issue is that when the file is truncated and reopened it re-reads the file. The problem is that in our case also all the lines are send to the event bus again. I was digging in to the docs but I could not find a ways to prevent this.

So is there an option in Tail to go to directly to the end of a file without fully reading it when a file is reopened after a truncate?

[KaczDev]

Hey, just had the same issue but I think I managed to resolve it.

Have you tried setting the Location when creating the Tail with config?
This is my conf and it doesn't read the file from the start. I only want to listen for the new logs.

t, err := tail.TailFile(catalinaPath,
	tail.Config{
		Follow:        true,
		CompleteLines: true,
		Location:      &tail.SeekInfo{Offset: 0, Whence: io.SeekEnd},
	})

[ouwe-knutselaar]

I also found this workaround:

 for line := range tailfile.Lines {
		pos, _ := tailfile.Tell()
		if pos != line.SeekInfo.Offset {
			continue
		}
		sendNewLogLine(line.Text)
	}

It works a little, however the last line in a truncated file is always send this eample.

[ouwe-knutselaar]

What also Could be done is to add an extra field in the config struct:
ToEndAfterTruncate bool // Go to the end of a file after a truncation to prevent double file read

And then add this piece of code to the function waitForChanges()

	case <-tail.changes.Truncated:
		// Always reopen truncated files (Follow is true)
		tail.Logger.Printf("Re-opening truncated file %s ...", tail.Filename)
		if err := tail.reopen(); err != nil {
			return err
		}
		tail.Logger.Printf("Successfully reopened truncated %s", tail.Filename)
		tail.openReader()
		
		// New snippet
		if tail.ToEndAfterTruncate {
			tail.Logger.Printf("Jump to the end of the file after being truncated %s", tail.Filename)
			if err := tail.seekEnd(); err != nil {
				return err
			}
		}
		// End

		return nil

Test it locally and it works really nice.