Webhook进程在另一个goroutine上运行

Question

I want to run some slow routine in another goroutine, is it safe to do it like this:

func someHandler(w http.ResponseWriter, r *http.Request) {
   go someReallySlowFunction() // sending mail or something slow
   fmt.Fprintf(w,"Mail will be delivered shortly..")
}

func otherHandler(w http.ResponseWriter, r *http.Request) {
   foo := int64(0)
   bar := func() {
      // do slow things with foo
   }
   go bar()
   fmt.Fprintf(w,"Mail will be delivered shortly..")
}

Is there any gotchas by doing this?

Answer 1

Serving each http request runs in its own goroutine (more details on this). You are allowed to start new goroutines from your handler, and they will run concurrently, independently from the goroutine executing the handler.

Some things to look out for:

• The new goroutine runs independently from the handler goroutine. This means it may complete before or after the handler goroutine, you cannot (should not) assume anything regarding to this without explicit synchronization.

• The http.ResponseWriter and http.Request arguments of the handler are only valid and safe to use until the handler returns! These values (or "parts" of them) may be reused - this is an implementation detail of which you should also not assume anything. Once the handler returns, you should not touch (not even read) these values.

• Once the handler returns, the response is committed (or may be committed at any moment). Which means your new goroutine should not attempt to send back any data using the http.ResponseWriter after this. This is true to the extent that even if you don't touch the http.ResponseWriter in your handler, not panicing from the handler is taken as a successful handling of the request and thus HTTP 200 status is sent back (see an example of this).

You are allowed to pass the http.Request and http.ResponseWriter values to other functions and to new goroutines, but care must be taken: you should use explicit synchronization (e.g. locks, channels) if you intend to read / modify these values from multiple goroutines (or you want to send back data from multiple goroutines).

Note that seemingly if both your handler goroutine and your new goroutine just reads / inspects the http.Request, that still may be problematic. Yes, multiple goroutines can read the same variable without synchronization (if nobody modifies it). But calling certain methods of http.Request also modify the http.Request, and without synchronization there is no guarantee what other goroutines would see from this change. For example Request.FormValue() returns a form value associated with the given key. But this method calls ParseMultiPartForm() and ParseForm() if necessary which modify the http.Request (e.g. they set the Request.PostForm and Request.Form struct fields).

So unless you synchronize your goroutines, you should not pass Request and ResponseWriter to the new goroutine, but acquire data needed from the Request in the handler goroutine, and pass only e.g. a struct holding the needed data.

Your second example:

foo := int64(0)
bar := func() {
   // do slow things with foo
}
go bar()

This is perfectly fine. This is a closure, and local variables referred by it will survive as long as they are accessible.

Note that alternatively you could pass the value of the local variable to the anonymous function call as an argument like this:

foo := int64(0)
bar := func(foo int64) {
   // do slow things with param foo (not the local foo var)
}
go bar(foo)

In this example the anonymous function will see and use its parameter foo and not the local variable foo. This may or may not be what you want (depending on whether the handler also uses the foo and whether changes made by any of the goroutines need to be visible to the other - but that would require synchronization anyway, which would be superseded by a channel solution).