launcher: implement launcher wrapper

There is no way to have machinectl pass part of its argv to the child, and formatting the string for a shell is highly error-prone and complex, so the argv slice is encoded and passed to a launcher process launched by machinectl which then calls execve(2) to start the final process.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

launcher.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"bytes"
+	"encoding/base64"
+	"encoding/gob"
+	"fmt"
+	"os"
+	"strings"
+	"syscall"
+)
+
+const (
+	// hidden path for main to act as a launcher
+	egoLauncher = "EGO_LAUNCHER"
+)
+
+// hidden launcher path
+func tryLauncher() {
+	if printVersion {
+		if r, ok := os.LookupEnv(egoLauncher); ok {
+			// egoLauncher variable contains launcher payload
+			dec := base64.NewDecoder(base64.StdEncoding, strings.NewReader(r))
+
+			var argv []string
+			if err := gob.NewDecoder(dec).Decode(&argv); err != nil {
+				fmt.Println("Error decoding launcher payload:", err)
+				os.Exit(1)
+			}
+
+			if err := os.Unsetenv(egoLauncher); err != nil {
+				fmt.Println("Error unsetting launcher payload:", err)
+				// not fatal, do not fail
+			}
+
+			var p string
+
+			if len(argv) > 0 {
+				if p, ok = which(argv[0]); !ok {
+					fmt.Printf("Did not find '%s' in PATH\n", argv[0])
+					os.Exit(1)
+				}
+			} else {
+				if p, ok = os.LookupEnv("SHELL"); !ok {
+					fmt.Println("No command was specified and $SHELL was unset")
+					os.Exit(1)
+				}
+			}
+
+			if err := syscall.Exec(p, argv, os.Environ()); err != nil {
+				fmt.Println("Error executing launcher payload:", err)
+				os.Exit(1)
+			}
+
+			// unreachable
+			os.Exit(1)
+			return
+		}
+	}
+}
+
+func launcherPayloadEnv() string {
+	r := &bytes.Buffer{}
+	enc := base64.NewEncoder(base64.StdEncoding, r)
+
+	if err := gob.NewEncoder(enc).Encode(command); err != nil {
+		fatal("Error encoding launcher payload:", err)
+	}
+
+	_ = enc.Close()
+	return egoLauncher + "=" + r.String()
+}