engine: resources, lang: funcs, parser: Add panic magic

It's valuable to check your runtime values and to shut down the entire
engine in case something doesn't match. This patch adds some magic
plumbing to support a "panic" mechanism.

A new "panic" statement gets transparently converted into a panic
function and panic resource. The former errors if the input is not
empty. The latter must be present to consume the value, but doesn't
actually do anything.

lang/funcs/core/panic_func.go

@@ -0,0 +1,46 @@
+// Mgmt
+// Copyright (C) 2013-2023+ 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 core
+
+import (
+	"fmt"
+
+	"github.com/purpleidea/mgmt/lang/funcs/simple"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	simple.Register("panic", &types.FuncValue{
+		T: types.NewType("func(x str) str"),
+		V: Panic,
+	})
+}
+
+// Panic returns an error when it receives a non-empty string. The error should
+// cause the function engine to shutdown.
+func Panic(input []types.Value) (types.Value, error) {
+	if s := input[0].Str(); s != "" {
+		// This StrValue not really used here, since we error...
+		return &types.StrValue{
+			V: s,
+		}, fmt.Errorf("panic occurred: %s", s)
+	}
+	return &types.StrValue{
+		V: "panic", // name can't be empty
+	}, nil
+}