proposal: cmd/vet: warn about copying a time.Timer value

[tulir]

The following code works fine on Go 1.22, but breaks on Go 1.23.0 on the http.DefaultClient.Do() line. On Linux, it freezes the entire runtime and never continues. On macOS, it throws a fatal error: ts set in timer

package main

import (
	"fmt"
	"net/http"
	"time"
)

func main() {
	illegalTimerCopy := *time.NewTimer(10 * time.Second)
	go func() {
		illegalTimerCopy.Reset(10 * time.Second)
	}()
	req, err := http.NewRequest(http.MethodGet, "https://example.com", nil)
	fmt.Println("Request created", err)
	_, err = http.DefaultClient.Do(req)
	fmt.Println("Request completed", err)
}

It's presumably somehow caused by the Go 1.23 timer changes, but I'm not sure how exactly, so I don't know if it's a bug or a feature. Assuming it's not a bug, it would be nice if go vet could report that similar to the mutex copy warnings.

[zigo101]

package main

import "time"

func main() {
	illegalTimerCopy := *time.NewTimer(time.Second)
	illegalTimerCopy.Stop() // block for ever
}

[ianlancetaylor]

I'm kind of surprised this ever worked. I don't think it would work in general cases. It might work in cases where you are careful to call Reset after the copy before doing anything else.

[ianlancetaylor]

Unless we get a lot of reports about breakage due to this, we aren't going to change it. So changing this to a proposal for a vet check. Though I don't know how useful a vet check would be--I don't know how often this occurs. At least since Go 1.4 the time.Timer docs have said

// A Timer must be created with NewTimer or AfterFunc.

That doc was added for #8776. So I think this has always been documented as not working.

[timothy-king]

If we do decide to add it to vet, copylock is a natural home for implementing this (if a mildly inappropriate name).

[gnvk]

Just my 2 cents on this: in a real code base this can lead to a crash that doesn't trivially show the root cause. And the exact same code did work in go 1.22, so in my opinion something should be done about this, because technically this is a breaking change. The linked documentation

A Timer must be created with NewTimer or AfterFunc

doesn't mention that after creation you cannot copy the timer's value.

[ianlancetaylor]

Unless we get a lot of reports about breakage due to this, we aren't going to change it.