How does a test wait an hour in a millisecond?

Level: 201 · anyone whose timeout test takes as long as the timeout

Stub — a question and its expected answers, not yet a lesson. No program behind this page has been written or run, so nothing on it has been through the check that backs every finished page. The table is what the finished lesson is expected to find, and every cell is unverified until a program prints it. (Not machine-checked here.)

One line: A test of a retry-with-backoff or a timeout either really waits — minutes per test — or replaces the clock with one the test controls, advancing it by an hour in one call and waking every sleeper that was due; Go's synctest and tokio's paused clock do this for their runtimes, and everywhere else the clock has to be an interface the code under test was written against.

The question

A function retries three times with one-minute backoffs. Test it. Really waiting takes three minutes. Under synctest, time.Sleep(time.Minute) returns when every goroutine in the bubble is blocked and the virtual clock jumps; the test takes a millisecond and is deterministic. The page runs the retry test both ways, then shows what the other languages need: a clock passed in.

What each language is expected to say

	Expected answer, to be verified by the program
Rust	tokio::time::pause() and advance() (link, not run); the std column passes a Fn() -> Instant clock
Go	testing/synctest (Go 1.25): synctest.Test runs the code in a bubble with a fake clock
C	a clock function pointer; the test supplies one that counts calls
C++	std::chrono clocks are types, so the code is templated on the clock and the test supplies a fake
Java	java.time.Clock injected; Clock.fixed and a hand-advanced test clock
Python	unittest.mock.patch('time.sleep') and a fake monotonic

What the programs have to show

• the retry test under real time as Real runs (three seconds, scaled down), and under virtual time as the CI key: three retries, zero wall time
• Go's synctest.Wait replacing a sleep in a test, from the Go library
• the clock interface in each other language, and the test's fake

See also

• Before this: How do you write a test that provokes the race?
• After this: Can every interleaving of a small program be checked?
• How do you write a test that provokes the race?
• What does a sleep promise, and what does a yield?
• The Go library's synctest makes time virtual ↗
• The Go library's synctest.Wait instead of a sleep ↗
• The Rust library's Testing async code ↗
• Concepts: Deterministic scheduling for tests · Timers and tickers · Timeout · Stress testing