cli: stream both stdout and stderr when following an alloc.

[jrasell]

This update changes the behaviour when following logs from an
allocation, so that both stdout and stderr files streamed when the
operator supplies the follow flag. The previous behaviour is held
when all other flags and situations are provided.

Closes #2123

[lgfa29]

Not part of this PR, but do we need to close(cancel) here?

[jrasell]

Good spot; I can fix this up in another PR.

[lgfa29]

Looking good!

There seems to always be some empty lines first, but I don't know where those are coming from. Left some random comments, but nothing major so feel free to ignore them

[lgfa29]

Suggested change

	if l.follow && !(l.stderr || l.stdout || l.tail || l.numLines > 0 || l.numBytes > 0) {
	if err := l.tailMultipleFiles(client, alloc); err != nil {
	l.Ui.Error(fmt.Sprintf("Failed to tail stdout and stderr files: %v", err))
	return 1
	}
	} else {
	if l.follow && !(l.stderr || l.stdout || l.tail || l.numLines > 0 || l.numBytes > 0) {
	if err := l.tailMultipleFiles(client, alloc); err != nil {
	l.Ui.Error(fmt.Sprintf("Failed to tail stdout and stderr files: %v", err))
	return 1
	}
	return 0
	}

By returning early we can remove the else clause and dedent the rest of the code, which may be nice, but very much a style nit-pick 😄

[lgfa29]

Not part of the PR, but it may be worth flipping the lines to check for readErr != nil earlier since r will be nil

[lgfa29]

Reading the docs I think passing a ctx.Done() instead of creating a cancel channel would work, and it would make the code look more like modern Go.

You can then even pass the context to the function (tailMultipleFiles(ctx, ...)) and have let the parent Run() handle the lifecycle of the request, so that things like signal handling are handled outside of these sub-functions.

func (l *AllocLogsCommand) Run(...) {
	// ...
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	// ...
	signalCh := make(chan os.Signal, 1)
	signal.Notify(signalCh, os.Interrupt, syscall.SIGTERM)
	go func() {
		for {
			select {
			case <-signalCh:
				return 0
			}
		}
	}()
	// ...
	tailMultipleFiles(ctx, ...)
	// ...
	followFile(ctx, ...)
}

func (l *AllocLogsCommand) tailMultipleFiles(ctx ...) error {
	// ...
	stderrFrames, stderrErrCh := client.AllocFS().Logs(
		alloc, true, l.task, api.FSLogNameStderr, api.OriginEnd, 1, ctx.Done(), nil)
	// ...
}