fix(audit): logs not getting deleted after N days

[ajeetdsouza]

Fixes DGRAPH-3139.

There were two bugs fixed here:

• The wrong parameter was being passed to processOldLogFiles (l.MaxSize instead of l.MaxAge).
• When formatting the closing timestamp of an audit log file, we use the local timezone. However, when parsing it back, we were assuming a UTC timezone. I changed this to assume local timezone.

---

This change is 

[jarifibrahim]

@ajeetdsouza can you please point me to the code that creates the file? I can't find that code.

[jarifibrahim]

https://github.com/dgraph-io/dgraph/blob/4e4842624d26bb24caa08019d7be834bc59e282b/x/log_writer.go#L302-L307

I don't think we're writing the time in Local format.

[ajeetdsouza]

@jarifibrahim time.Now() returns time using the local timezone, but backupTimeFormat does not include timezone information, which is what is causing the problem.

Sample code:

package main

import (
	"fmt"
	"time"
)

func main() {
	const backupTimeFormat = "2006-01-02T15-04-05.000"
	now := time.Now()
	tsLocal := now.Format(backupTimeFormat)
	tsUTC := now.UTC().Format(backupTimeFormat)
	fmt.Printf("tsLocal: %s\ntsUTC: %s\n", tsLocal, tsUTC) // different values
}

There are three alternate approaches here:

• Change backupTimeFormat to include timezone.
• Use UTC when formatting to backupTimeFormat.
• Both of the above.

I think these would be better than the current approach, since there is no ambiguity that way.

[jarifibrahim]

Change backupTimeFormat to include timezone.
Use UTC when formatting to backupTimeFormat.
Both of the above.

This would change the file name and we might not be able to parse already existing files. Is this backupTimeFormat used only in audit logs? @aman-bansal why is it called backupTimeFormat?

If this is used only in audit logs, we can use time.UTC() everywhere for simplicity.

[ajeetdsouza]

@jarifibrahim backupTimeFormat is only used within LogWriter, so I think it's safe to switch over to UTC timestamps. I just changed it to use UTC everywhere.

[aman-bansal]

UTC is fine. We are only using this for audit logs. So this will be fine.