feat: Added feature to run rendering in synchronous mode.

[fritzduchardt]

This makes debugging easier and takes load of slow machines. It has proven useful in the past.

[Zebradil]

What do you think about a flag to control the level of parallelism instead of just switching it on and off?

• --async=0 to disable
• --async=1 to enable
• --async=8 to run maximum of 8 parallel threads

For now, we can skip implementing the last one, but I would like to settle with the flag name.

[kbudde]

Error is logged but not returned.
Named result values are your friend :) https://blog.learngoprogramming.com/5-gotchas-of-defer-in-go-golang-part-iii-36a1ab3d6ef1

[fritzduchardt]

Good point, using them now, but got to say, they are NOT my friends looking at the complexity of the resulting code:

saveClose := func(srcFile fs.File) {
		closeErr := srcFile.Close()
		if closeErr != nil {
			if err == nil {
				err = closeErr
			} else {
				err = errors.Join(err, closeErr)
			}
		}
	}

[kbudde]

In this case, what about:

saveClose := func(srcFile fs.File) {
	closeErr := srcFile.Close()
	err = errors.Join(err, closeErr)
}

errors.Join is ignoring nil errors. If all errors are nil it'll return nil.
Downside you always have to unwrap the error.

[kbudde]

Now as async controls the number of parallel routines, I would propose to set some better default.
1 is the same as 0 just using different code.
What about 4 or 8?

[Zebradil]

What about inverting the logic a little bit and doing the following:

• async level is N means that maximum N applications are being processed in parallel,
• async level is 0 means the limit is not set, any number of applications can be processed in parallel.

In this case, the default might be 0. I see it hard to set a viable default value. Moreover, for different operations (sync, render) the "ideal" number would differ.

[kbudde]

Could be replaced by eg.SetLimit(asyncLevel). No semaphore needed in this case :)
https://pkg.go.dev/golang.org/x/sync/errgroup#Group.SetLimit

[kbudde]

@fritzduchardt , @Zebradil I added one commit. I think it can be merged now.