const std = @import("std");

const log = std.log.scoped(.@"linux-cgroup");

/// Returns the path to the cgroup for the given pid.
pub fn current(buf: []u8, pid: u32) ?[]const u8 {
    var path_buf: [std.fs.max_path_bytes]u8 = undefined;

    // Read our cgroup by opening /proc/<pid>/cgroup and reading the first
    // line. The first line will look something like this:
    // 0::/user.slice/user-1000.slice/session-1.scope
    // The cgroup path is the third field.
    const path = std.fmt.bufPrint(&path_buf, "/proc/{}/cgroup", .{pid}) catch return null;
    const file = std.fs.openFileAbsolute(path, .{}) catch return null;
    defer file.close();

    var read_buf: [64]u8 = undefined;
    var file_reader = file.reader(&read_buf);
    const reader = &file_reader.interface;
    const len = reader.readSliceShort(buf) catch return null;
    const contents = buf[0..len];

    // Find the last ':'
    const idx = std.mem.lastIndexOfScalar(u8, contents, ':') orelse return null;
    return std.mem.trimRight(u8, contents[idx + 1 ..], " \r\n");
}