embedbin: implement fallback for missing memfd

Some Linux platforms (notably WSL1) do not implement memfd.
This adds a fallback at runtime.

(cherry picked from commit 5a9a055)

internal/embedbin/embedbin.go

@@ -2,3 +2,11 @@
 //
 // The Install function creates a temporary file and writes the contents to it.
 package embedbin
+
+// Installed is a handle to an installed binary.
+// Users must call Uninstall to clean it up.
+type Installed interface {
+	Path() string
+	IsRegular() bool
+	Uninstall() error
+}

internal/embedbin/install_linux.go

@@ -3,20 +3,37 @@ package embedbin
 import (
 	"fmt"
 
+	"github.com/spf13/afero"
 	"golang.org/x/sys/unix"
 )
 
 // MemfdInstaller installs embedded binaries.
-type MemfdInstaller struct{}
+type MemfdInstaller struct {
+	fallback *RegularInstaller
+}
 
 // New returns a new installer.
 func New() *MemfdInstaller {
-	return &MemfdInstaller{}
+	return &MemfdInstaller{
+		fallback: &RegularInstaller{fs: afero.NewOsFs()},
+	}
 }
 
 // Install creates a memfd and writes the contents to it.
 // the first argument is ignored on Linux (would be the prefix on other implementations).
-func (*MemfdInstaller) Install(_ string, contents []byte) (*MemfdInstall, error) {
+func (i *MemfdInstaller) Install(_ string, contents []byte) (Installed, error) {
+	// Try to install using memfd.
+	install, err := New().installMemfd(contents)
+	if err == nil {
+		return install, nil
+	}
+
+	// Fallback to regular installer.
+	return i.fallback.Install("", contents)
+}
+
+// installMemfd creates a memfd and writes the contents to it.
+func (*MemfdInstaller) installMemfd(contents []byte) (*MemfdInstall, error) {
 	// Create a memfd.
 	fd, err := unix.MemfdCreate("embedded-binary", 0)
 	if err != nil {

internal/embedbin/regular.go

@@ -14,7 +14,7 @@ type RegularInstaller struct {
 // Install creates a regular file and writes the contents to it.
 // prefix is an optional prefix for the temporary file.
 // If prefix is empty, a temporary directory will be used.
-func (r *RegularInstaller) Install(prefix string, contents []byte) (*RegularInstall, error) {
+func (r *RegularInstaller) Install(prefix string, contents []byte) (Installed, error) {
 	if prefix != "" {
 		if err := r.fs.MkdirAll(prefix, os.ModePerm); err != nil {
 			return nil, err