text/template: add break and continue support

[bep]

This is a repost of #20523 with, hopefully, a more clear description.

Go templates currently do not support the break nor the continue keyword -- which surprises people trying a construct similar to the one below.

{{ range $i, $e := .SomeSlice }}
  {{ if gt $i 2 }}
    {{ break }}
  {{ end }}
  {{ $e }}
{{ end }}

/cc @igramnet

[odeke-em]

/cc @rsc @robpike and other text/template folks.

[rsc]

It does seem a little reasonable. I wouldn't want labels (or goto), though. Thoughts, @robpike?

[robpike]

It does seem reasonable but low priority.

[rsc]

CLs welcome for Go 1.10. Thanks.

[gopherbot]

Change https://golang.org/cl/66410 mentions this issue: text/template: add break, continue actions in ranges

[gopherbot]

Change https://golang.org/cl/92155 mentions this issue: text/template: revert CL 66410 "add break, continue actions in ranges"

[ianlancetaylor]

The patch was rolled back, so reopening.

[jimmyfrasche]

This seems like an easy way to create goroutine leaks that weren't possible before since range can iterate over a channel.

[robpike]

@jimmyfrasche There are zillions of ways to create goroutine leaks. Adding one more doesn't seem to be that much of a change to the world.

[jimmyfrasche]

@robpike Certainly true. I originally thought that this is different as, if the templates are user provided, it's a way for the provided template to cause the leak, but then I realized that the same goroutine leak could be caused by the template simply choosing to not range over the channel at all. Either way, it's up to the program running the template to ensure all chans are drained after execution.

[gopherbot]

Change https://golang.org/cl/321491 mentions this issue: html/template, text/template: implement break and continue for range loops

[gopherbot]

Change https://golang.org/cl/373915 mentions this issue: doc/go1.18: document {text,html}/template {break,continue} commands