termio/exec: move subprocess into cgroup

src/os/cgroup.zig

@@ -52,6 +52,18 @@ pub fn create(
     }
 }
 
+/// Move the given PID into the given cgroup.
+pub fn moveInto(
+    cgroup: []const u8,
+    pid: std.os.linux.pid_t,
+) !void {
+    var buf: [std.fs.MAX_PATH_BYTES]u8 = undefined;
+    const path = try std.fmt.bufPrint(&buf, "/sys/fs/cgroup{s}/cgroup.procs", .{cgroup});
+    const file = try std.fs.cwd().openFile(path, .{ .mode = .write_only });
+    defer file.close();
+    try file.writer().print("{}", .{pid});
+}
+
 /// Returns all available cgroup controllers for the given cgroup.
 /// The cgroup should have a '/'-prefix.
 ///

src/termio/Exec.zig

@@ -1346,8 +1346,7 @@ const Subprocess = struct {
         // If we have a cgroup set, then we want to move into that cgroup.
         if (comptime builtin.os.tag == .linux) {
             if (self.linux_cgroup) |cgroup| {
-                // TODO: do it
-                _ = cgroup;
+                try internal_os.cgroup.moveInto(cgroup, 0);
             }
         }
     }