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_test.go

@@ -0,0 +1,82 @@
+// Mgmt
+// Copyright (C) 2013-2021+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package util
+
+import (
+	"fmt"
+	"sort"
+	"testing"
+)
+
+func TestValidateVarName(t *testing.T) {
+	testCases := map[string]error{
+		"":        fmt.Errorf("got empty var name"),
+		"hello":   nil,
+		"NOPE":    fmt.Errorf("invalid var name: `NOPE`"),
+		"$foo":    fmt.Errorf("invalid var name: `$foo`"),
+		".":       fmt.Errorf("invalid var name: `.`"),
+		"..":      fmt.Errorf("invalid var name: `..`"),
+		"_":       fmt.Errorf("invalid var name: `_`"),
+		"__":      fmt.Errorf("invalid var name: `__`"),
+		"0":       fmt.Errorf("invalid var name: `0`"),
+		"1":       fmt.Errorf("invalid var name: `1`"),
+		"42":      fmt.Errorf("invalid var name: `42`"),
+		"X":       fmt.Errorf("invalid var name: `X`"),
+		"x":       nil,
+		"x0":      nil,
+		"x1":      nil,
+		"x42":     nil,
+		"x42.foo": nil,
+		"x42_foo": nil,
+
+		// XXX: fix these test cases
+		//"x.": fmt.Errorf("invalid var name: x."),
+		//"x_": fmt.Errorf("invalid var name: x_"),
+	}
+
+	keys := []string{}
+	for k := range testCases {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, k := range keys {
+		e, ok := testCases[k]
+		if !ok {
+			// programming error
+			t.Errorf("missing test case: %s", k)
+			continue
+		}
+
+		err := ValidateVarName(k)
+		if err == nil && e == nil {
+			continue
+		}
+		if err == nil && e != nil {
+			t.Errorf("key: %s did not error, expected: %s", k, e.Error())
+			continue
+		}
+		if err != nil && e == nil {
+			t.Errorf("key: %s expected no error, got: %s", k, err.Error())
+			continue
+		}
+		if err.Error() != e.Error() {
+			t.Errorf("key: %s did not have correct error, expected: %s", k, err.Error())
+			continue
+		}
+	}
+}