text/template: distinguish write errors from template errors

[robpike]

See #11845

The template package is being blamed (unfairly in my opinion) for issues around SIGPIPE, but there is a fair point to be made: The template package should make it easy to distinguish errors caused by execution from errors caused by I/O. The former is a programming problem that should be fixed; the latter is just bad luck. A conscientious program (like cmd/go) could act differently in the two cases.

[bradfitz]

The template package should make it easy to distinguish execution errors caused by execution from errors caused from I/O.

Not just the template package, though. I needed this distinction in net/http the other day, and I always finding myself writing little helper functions like,

func isNetWriteError(err error) bool {
    switch e := err.(type) {
    case *url.Error:
        return isNetWriteError(e.Err)
        case *net.OpError:
        return e.Op == "write"
        default:
        return false
    }
}

Maybe we put some of those style helpers in a common package?

[gopherbot]

CL https://golang.org/cl/13953 mentions this issue.