fix(gateway): send on closed chan - (*Handle).webRequestQ

[Sidddddarth]

Description

Using a waitGroup to wait on all in flight requests before closing gateway workers' webRequestQ. It is an anti-pattern to close a channel if there's multiple concurrent senders. So we make sure there's no more senders using a simple waitgroup(inFlightRequests). A middleware that adds and Dones this waitgroup is then needed.

Eventually during Shutdown we wait on this waitgroup before closing the workers' webRequestQ channels. All the requests would have been handled by this point.

Linear Ticket

Resolves PIPE-1072

Security

• [ ] The code changed/added as part of this pull request won't create any security issues with how the software is being used.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.35%. Comparing base (5ceb1d4) to head (c8f99f7). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4709      +/-   ##
==========================================
- Coverage   74.37%   74.35%   -0.02%     
==========================================
  Files         427      427              
  Lines       49610    49614       +4     
==========================================
- Hits        36895    36890       -5     
- Misses      10274    10282       +8     
- Partials     2441     2442       +1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[lvrach]

If you wait for server.Shutdown to return normally all the open connections will have been completed. Why this approach is not enough and we have to introduce this counter?

[Sidddddarth]

If you wait for server.Shutdown to return normally all the open connections will have been completed.

True.

Why this approach is not enough and we have to introduce this counter?

server.Shutdown and close(webRequestQ) happens in two different goroutines. This counter is a way to synchronise both. Also note that server.Shutdown is a graceful exit, calling it will immediately lead to ListenAndServe to return ErrServerClosed. From the documentation:

When Shutdown is called, [Serve], [ListenAndServe], and [ListenAndServeTLS] immediately return [ErrServerClosed].
Make sure the program doesn't exit and waits instead for Shutdown to return.

The synchronisation introduced in this PR does the second part - waiting for shutdown to return and not exiting already.

[lvrach]

The synchronisation introduced in this PR does the second part - waiting for shutdown to return and not exiting already.

That's my point. We can do this by waiting on shutdown to complete, rather than introducing a counter.

[Sidddddarth]

the gracefulFunc in kithhtputil can return an error other than one on server.Shutdown too, which prompted me to go with a counter.

func ListenAndServe(ctx context.Context, server *http.Server, shutdownTimeout ...time.Duration) error {
	return gracefulFunc(ctx, server, server.ListenAndServe, shutdownTimeout...)
}


func gracefulFunc(ctx context.Context, server *http.Server, fn func() error, shutdownTimeout ...time.Duration) error {
	errCh := make(chan error, 1)
	go func() {
		errCh <- fn()
	}()
	select {
	case err := <-errCh:
		return err
	case <-ctx.Done():
		switch {
		case len(shutdownTimeout) == 0:
			return server.Shutdown(context.Background())
		case shutdownTimeout[0] == 0:
			return server.Close()
		default:
			ctx, cancel := context.WithTimeout(context.Background(), shutdownTimeout[0])
			defer cancel()

			return server.Shutdown(ctx)
		}
	}
}

[fracasula]

@lvrach I think we should determine the behaviour we want first.

Upon cancellation we could:

1. wait for all the in-flight requests to be done and try to persist them. the risk is that they might not be done before the graceful shutdown period so we could end up shutting down ungracefully.
   • this could be done with a simple WaitGroup added in a middleware like @Sidddddarth is doing and then we Wait() on Shutdown.
2. don't wait for all the in-flight requests to persist. we can discard the data and return 503 Unavailable. we'll always be sure that we can gracefully return.
   • we could use a common context for the workers and use it in a select when adding to the channels, if the context is done then propagate that info and return 503 and then terminate the worker asap.

[ktgowtham]

I'm assuming this is not a SIGTERM exit, but for some reason server.ListenAndServe is being errored out. We probably need to know the reason for it first. logs didn't help me. based on my assumption, server.Shutdown(context.Background()) isn't being executed for sure. wdyt?

func ListenAndServe(ctx context.Context, server *http.Server, shutdownTimeout ...time.Duration) error { return gracefulFunc(ctx, server, server.ListenAndServe, shutdownTimeout...) }

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