watchlogs/rotation.go

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/.
 */
package main

import (
	"fmt"
	"io/fs"
	"os"
	"sync"
)

var FileSysInfoMutex = &sync.Mutex{}

/*
Try to detect whether the file was rotated.

As is the case with logrotate, we assume there are only a
couple ways to perform file rotation: rename then create, or
copy then, usually, truncate.

See `man 8 logrotate` for details.
*/
func detectFileRotation(
	path string,
	file *os.File,
) (rot bool, err error) {
	/*
		The order in which we retrieve this file info matters.

		Suppose an empty file is rotated by copytruncate after
		24 hours. The inode is unchanged. The file size is
		unchanged. Without subscribing to some kernel-level
		stream of filesystem events, all we can do is check for
		metadata modification (ctime) to see whether the new
		file has a ctime that is greater than the ctime of the
		old file.

		But what if the file hasn't been rotated? We test for
		rotation and observe that, again, the inode and size are
		unchanged. However, the ctime could have been updated by
		a write while we were awaiting our syscall. Then we
		would notice a new ctime for the same file, which would
		prompt us to frivolously reopen the file.

		Is this false positive so bad? Well, yes, it would be
		more costly for busier logs.

		Therefore, we propose to retrieve the "new" ctime,
		first, then retrieve the "current" ctime for the
		open file. This way, we never run the risk have a false
		positive.

		That said, if the Go compiler or CPU changes the order
		of these calls, we are properly horked. Therefore we opt
		to use sync.Mutex to guarantee the correct order. See
		https://go.dev/ref/mem#locks for details.
	*/
	var curInfo, newInfo fs.FileInfo
	FileSysInfoMutex.Lock()
	if newInfo, err = os.Stat(path); err != nil {
		err = fmt.Errorf("unable to retrieve info for file %#v: %w", path, err)
		return
	}
	FileSysInfoMutex.Unlock()
	FileSysInfoMutex.Lock()
	if curInfo, err = file.Stat(); err != nil {
		err = fmt.Errorf("unable to retrieve info for file %#v: %w", path, err)
		return
	}
	FileSysInfoMutex.Unlock()

	var curStat, newStat FileSysInfo
	var ok bool
	if curStat, ok = curInfo.Sys().(FileSysInfo); !ok {
		panic(fmt.Sprintf("BUG: expected %s for open file but got type %T instead", FileSysInfoStr, curInfo.Sys()))
	}
	if newStat, ok = newInfo.Sys().(FileSysInfo); !ok {
		panic(fmt.Sprintf("BUG: expected %s for rotated file but got type %T instead", FileSysInfoStr, curInfo.Sys()))
	}
	return fileWasRotated(newStat, curStat), err
}