lang: core: util: Add hostname mapper function

This adds a new util package with some useful functionality which could
be implemented as pure mcl, but instead we add it here as a good place
to help with code reuse.

examples/lang/hostname_mapper.mcl

@@ -0,0 +1,12 @@
+import "util"
+
+$m = {
+	"00:11:22:33:44:55" => "hostname1",
+	"00:11:22:33:44:56" => "hostname1",
+	"aa:bb:cc:dd:ee:fe" => "hostname2",
+	"aa:bb:cc:dd:ee:ff" => "hostname2",
+}
+
+print "mapper" {
+	msg => util.hostname_mapper($m),
+}

lang/core/core.go

@@ -55,6 +55,7 @@ import (
 	_ "github.com/purpleidea/mgmt/lang/core/strings"
 	_ "github.com/purpleidea/mgmt/lang/core/sys"
 	_ "github.com/purpleidea/mgmt/lang/core/test"
+	_ "github.com/purpleidea/mgmt/lang/core/util"
 	_ "github.com/purpleidea/mgmt/lang/core/value"
 	_ "github.com/purpleidea/mgmt/lang/core/world"
 )

lang/core/util/hostname_mapper.go

@@ -0,0 +1,110 @@
+// Mgmt
+// Copyright (C) 2013-2024+ 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 <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package coreutil
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"strings"
+
+	"github.com/purpleidea/mgmt/lang/funcs/simple"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	simple.ModuleRegister(ModuleName, "hostname_mapper", &simple.Scaffold{
+		T: types.NewType("func(map{str:str}) str"),
+		F: HostnameMapper,
+	})
+}
+
+// HostnameMapper takes a map from mac address to hostname, and finds a hostname
+// that corresponds to one of the mac addresses on this machine. If it cannot
+// find a match, it returns the empty string. If it's ambiguous, it errors. This
+// is useful for bootstrapping the hostname setup on hosts.
+//
+// XXX: This should produce new values if the list of interfaces change.
+func HostnameMapper(ctx context.Context, input []types.Value) (types.Value, error) {
+	ifs, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	v := input[0].Value() // interface{}
+	m, ok := v.(map[string]string)
+	if !ok {
+		// programming error
+		return nil, err
+	}
+
+	hostnamesMap := make(map[string]struct{})
+
+	for _, iface := range ifs {
+		// Check if the interface has the loopback flag set
+		if iface.Flags&net.FlagLoopback != 0 {
+			continue
+		}
+
+		mac := iface.HardwareAddr.String()
+
+		// Check if the MAC address is valid.
+		if len(mac) != len("00:00:00:00:00:00") {
+			continue // maybe something weird we want to ignore...
+		}
+
+		hostname, exists := m[mac]
+		if !exists {
+			continue
+		}
+		if hostname == "" {
+			// TODO: should we error here?
+			continue
+		}
+		hostnamesMap[hostname] = struct{}{} // found
+	}
+	hostnames := []string{}
+	for k := range hostnamesMap {
+		hostnames = append(hostnames, k)
+	}
+
+	if len(hostnames) > 1 {
+		return nil, fmt.Errorf("multiple hostnames found: %s", strings.Join(hostnames, ", "))
+	}
+
+	result := ""
+	if len(hostnames) == 1 {
+		result = hostnames[0]
+	}
+
+	return &types.StrValue{
+		V: result,
+	}, nil
+}

lang/core/util/util.go

@@ -0,0 +1,35 @@
+// Mgmt
+// Copyright (C) 2013-2024+ 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 <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package coreutil
+
+const (
+	// ModuleName is the prefix given to all the functions in this module.
+	ModuleName = "util"
+)