Paused time and local network I/O

[jorendorff]

The documentation for tokio::time::pause says,

If time is paused and the runtime has no work to do, the clock is auto-advanced to the next pending timer.

It doesn't exactly say what work is, but

• If there are futures tokio should be polling, that counts as work.

• If there are any unfinished blocking tasks in the whole runtime, that counts as work. (I happen to know this because I implemented it.)

Now I have a test that sends some requests to a network server on localhost, using reqwest. I've observed that, with paused time, the clock auto-advances some random amount of time while this network request is in flight, usually half a second or so. In CI, sometimes it's 10+ seconds and then my test fails, even though less than 1 millisecond of actual time has passed.

What is happening? Does network I/O not count as "work"? If not, is there any way to make this test work reliably with paused time? (I guess I could create a blocking task, just during the request, to prevent auto-advancing the clock, but that would be a pretty weird hack.)

[mox692]

Looking at the code, auto advance is checked in the parking logic in the worker thread (code), so I think waiting a network IO could trigger the auto-advance.

[Darksonn]

If you run the client and server on the same Tokio runtime, then the work performed by the server will count as work and prevent auto-advance. Paused time cannot be reliably used with external servers.

[jorendorff]

If you run the client and server on the same Tokio runtime, then the work performed by the server will count as work and prevent auto-advance.

Thank you. Just to double-check my understanding: suppose some OS buffers exist in the network stack. Would it then be possible to get auto-advance happening even with client and server in the same Tokio runtime?

[Darksonn]

No, that should not happen. After performing a write on a socket, Tokio will check the IO driver before auto-advancing time. Since the IO driver check happens after the write on the socket, the IO driver check is guaranteed to see the write.