Write output to a file and rotate the files when limits have been exceeded.

Defines a simple [std::io::Write] object that you can plug into your writers as middleware.

Rotating by Lines

We can rotate log files by using the amount of lines as a limit.

use file_rotate::{FileRotate, RotationMode};
use std::{fs, io::Write};

// Create a directory to store our logs, this is not strictly needed but shows how we can
// arbitrary paths.
fs::create_dir("target/my-log-directory-lines");

// Create a new log writer. The first argument is anything resembling a path. The
// basename is used for naming the log files.
//
// Here we choose to limit logs by 10 lines, and have at most 2 rotated log files. This
// makes the total amount of log files 4, since the original file is present as well as
// file 0.
let mut log = FileRotate::new("target/my-log-directory-lines/my-log-file", RotationMode::Lines(3), 2);

// Write a bunch of lines
writeln!(log, "Line 1: Hello World!");
for idx in 2..11 {
    writeln!(log, "Line {}", idx);
}

assert_eq!("Line 10\n", fs::read_to_string("target/my-log-directory-lines/my-log-file").unwrap());

assert_eq!("Line 1: Hello World!\nLine 2\nLine 3\n", fs::read_to_string("target/my-log-directory-lines/my-log-file.0").unwrap());
assert_eq!("Line 4\nLine 5\nLine 6\n", fs::read_to_string("target/my-log-directory-lines/my-log-file.1").unwrap());
assert_eq!("Line 7\nLine 8\nLine 9\n", fs::read_to_string("target/my-log-directory-lines/my-log-file.2").unwrap());

fs::remove_dir_all("target/my-log-directory-lines");

Rotating by Bytes

Another method of rotation is by bytes instead of lines.

use file_rotate::{FileRotate, RotationMode};
use std::{fs, io::Write};

fs::create_dir("target/my-log-directory-bytes");

let mut log = FileRotate::new("target/my-log-directory-bytes/my-log-file", RotationMode::Bytes(5), 2);

writeln!(log, "Test file");

assert_eq!("Test ", fs::read_to_string("target/my-log-directory-bytes/my-log-file.0").unwrap());
assert_eq!("file\n", fs::read_to_string("target/my-log-directory-bytes/my-log-file").unwrap());

fs::remove_dir_all("target/my-log-directory-bytes");

Rotation Method

The rotation method used is to always write to the base path, and then move the file to a new location when the limit is exceeded. The moving occurs in the sequence 0, 1, 2, n, 0, 1, 2...

Here's an example with 1 byte limits:

use file_rotate::{FileRotate, RotationMode};
use std::{fs, io::Write};

fs::create_dir("target/my-log-directory-small");

let mut log = FileRotate::new("target/my-log-directory-small/my-log-file", RotationMode::Bytes(1), 3);

write!(log, "A");
assert_eq!("A", fs::read_to_string("target/my-log-directory-small/my-log-file").unwrap());

write!(log, "B");
assert_eq!("A", fs::read_to_string("target/my-log-directory-small/my-log-file.0").unwrap());
assert_eq!("B", fs::read_to_string("target/my-log-directory-small/my-log-file").unwrap());

write!(log, "C");
assert_eq!("A", fs::read_to_string("target/my-log-directory-small/my-log-file.0").unwrap());
assert_eq!("B", fs::read_to_string("target/my-log-directory-small/my-log-file.1").unwrap());
assert_eq!("C", fs::read_to_string("target/my-log-directory-small/my-log-file").unwrap());

write!(log, "D");
assert_eq!("A", fs::read_to_string("target/my-log-directory-small/my-log-file.0").unwrap());
assert_eq!("B", fs::read_to_string("target/my-log-directory-small/my-log-file.1").unwrap());
assert_eq!("C", fs::read_to_string("target/my-log-directory-small/my-log-file.2").unwrap());
assert_eq!("D", fs::read_to_string("target/my-log-directory-small/my-log-file").unwrap());

write!(log, "E");
assert_eq!("A", fs::read_to_string("target/my-log-directory-small/my-log-file.0").unwrap());
assert_eq!("B", fs::read_to_string("target/my-log-directory-small/my-log-file.1").unwrap());
assert_eq!("C", fs::read_to_string("target/my-log-directory-small/my-log-file.2").unwrap());
assert_eq!("D", fs::read_to_string("target/my-log-directory-small/my-log-file.3").unwrap());
assert_eq!("E", fs::read_to_string("target/my-log-directory-small/my-log-file").unwrap());


// Here we overwrite the 0 file since we're out of log files, restarting the sequencing
write!(log, "F");
assert_eq!("E", fs::read_to_string("target/my-log-directory-small/my-log-file.0").unwrap());
assert_eq!("B", fs::read_to_string("target/my-log-directory-small/my-log-file.1").unwrap());
assert_eq!("C", fs::read_to_string("target/my-log-directory-small/my-log-file.2").unwrap());
assert_eq!("D", fs::read_to_string("target/my-log-directory-small/my-log-file.3").unwrap());
assert_eq!("F", fs::read_to_string("target/my-log-directory-small/my-log-file").unwrap());

fs::remove_dir_all("target/my-log-directory-small");

Filesystem Errors

If the directory containing the logs is deleted or somehow made inaccessible then the rotator will simply continue operating without fault. When a rotation occurs, it attempts to open a file in the directory. If it can, it will just continue logging. If it can't then the written date is sent to the void.

This logger never panics.