Proabably should change code to:
func httpDo(ctx context.Context, req \*http.Request, f
func(\*http.Response, error) error) error { // Run the HTTP request in a
goroutine and pass the response to f. c := make(chan error, 1) req =
req.WithContext(ctx) go func() { c <- http.DefaultClient.Do(req) }() select
{ case <-ctx.Done(): <-c // Wait for f to return. return ctx.Err() case err
:= f(<-c): return err }
Then execution of f will be cancelled, if the context is already done. And
we will save computation.
Is my understanding correct?
On Friday, February 15, 2019 at 3:02:40 AM UTC+5:30 Ian Lance Taylor wrote:
> On Thu, Feb 14, 2019 at 2:13 AM Jingguo Yao <yaoji...@gmail.com> wrote:
> >
> > https://blog.golang.org/context uses the following code to cancel the
> request if ctx.Done is closed before the goroutine exits:
> >
> > func httpDo(ctx context.Context, req *http.Request, f
> func(*http.Response, error) error) error {
> > // Run the HTTP request in a goroutine and pass the response to f.
> > c := make(chan error, 1)
> > req = req.WithContext(ctx)
> > go func() { c <- f(http.DefaultClient.Do(req)) }()
> > select {
> > case <-ctx.Done():
> > <-c // Wait for f to return.
> > return ctx.Err()
> > case err := <-c:
> > return err
> > }
> > }
> >
> > I have two questions about this piece of code.
> >
> > First, if neither "http.DefaultClient.Do" nor "f" does a check of
> "ctx.Done()", "f(http.DefaultClient.Do(req))" can't be cancelled. It
> means that if I want to use "ctx.Done()" to cancel some functions, I need
> to check "ctx.Done()" at various points in the functions which I want to
> cancel. Is my understanding correct?
>
> Yes.
>
> > Second, if I don't want to do a cleanup of "http.DefaultClient.Do" and
> "f", I can just remove "<-c // Wait for f to return.". If I do this, there
> will be a dangling running goroutine after httpDo returns. Is this
> approached considered bad?
>
> While there are specific cases where leaving a dangling goroutine is
> harmless, as a general guideline it should be avoided. It's easy to
> imagine a busy server quickly building up many thousands of dangling
> goroutines, and they do have a cost.
>
> Ian
>
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