lang: Improve string interpolation

The original string interpolation was based on hil which didn't allow
proper escaping, since they used a different escape pattern. Secondly,
the golang Unquote function didn't deal with the variable substitution,
which meant it had to be performed in a second step.

Most importantly, because we did this partial job in Unquote (the fact
that is strips the leading and trailing quotes tricked me into thinking
I was done with interpolation!) it was impossible to remedy the
remaining parts in a second pass with hil. Both operations needs to be
done in a single step. This is logical when you aren't tunnel visioned.

This patch replaces both of these so that string interpolation works
properly. This removes the ability to allow inline function calls in a
string, however this was an incidental feature, and it's not clear that
having it is a good idea. It also requires you wrap the var name with
curly braces. (They are not optional.)

This comes with a load of tests, but I think I got some of it wrong,
since I'm quite new at ragel. If you find something, please say so =D In
any case, this is much better than the original hil implementation, and
easy for a new contributor to patch to make the necessary fixes.

lang/util/util.go

@@ -19,8 +19,11 @@ package util
 
 import (
 	"fmt"
+	"regexp"
+	"strings"
 
 	"github.com/purpleidea/mgmt/lang/types"
+	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
 // HasDuplicateTypes returns an error if the list of types is not unique.
@@ -89,3 +92,38 @@ func FnMatch(typ *types.Type, fns []*types.FuncValue) (int, error) {
 
 	return 0, fmt.Errorf("unable to find a compatible function for type: %+v", typ)
 }
+
+// ValidateVarName returns an error if the string pattern does not match the
+// format for a valid variable name. The leading dollar sign must not be passed
+// in.
+func ValidateVarName(name string) error {
+	if name == "" {
+		return fmt.Errorf("got empty var name")
+	}
+
+	// A variable always starts with an lowercase alphabetical char and
+	// contains lowercase alphanumeric characters or numbers, underscores,
+	// and non-consecutive dots. The last char must not be an underscore or
+	// a dot.
+	// TODO: put the variable matching pattern in a const somewhere?
+	pattern := `^[a-z]([a-z0-9_]|(\.|_)[a-z0-9_])*$`
+
+	matched, err := regexp.MatchString(pattern, name)
+	if err != nil {
+		return errwrap.Wrapf(err, "error matching regex")
+	}
+	if !matched {
+		return fmt.Errorf("invalid var name: `%s`", name)
+	}
+
+	// Check that we don't get consecutive underscores or dots!
+	// TODO: build this into the above regexp and into the parse.rl file!
+	if strings.Contains(name, "..") {
+		return fmt.Errorf("var name contains multiple periods: `%s`", name)
+	}
+	if strings.Contains(name, "__") {
+		return fmt.Errorf("var name contains multiple underscores: `%s`", name)
+	}
+
+	return nil
+}