proc: setup payload receive

Generic implementation of setup payload receiver adapted from finit.

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

internal/proc/fd.go

@@ -0,0 +1,32 @@
+package proc
+
+import (
+	"encoding/gob"
+	"errors"
+	"os"
+	"strconv"
+)
+
+var (
+	ErrNotSet  = errors.New("environment variable not set")
+	ErrInvalid = errors.New("bad file descriptor")
+)
+
+func Receive(key string, e any) (func() error, error) {
+	var setup *os.File
+
+	if s, ok := os.LookupEnv(key); !ok {
+		return nil, ErrNotSet
+	} else {
+		if fd, err := strconv.Atoi(s); err != nil {
+			return nil, err
+		} else {
+			setup = os.NewFile(uintptr(fd), "setup")
+			if setup == nil {
+				return nil, ErrInvalid
+			}
+		}
+	}
+
+	return func() error { return setup.Close() }, gob.NewDecoder(setup).Decode(e)
+}