Add common symlinks in init

This commits adds creation of common symlinks in the /dev directory for
conveniently accessing the usual file descriptors of a process.

Closes #12

integrationtesting/guest/sysinit_test.go

@@ -71,3 +71,20 @@ func TestEnv(t *testing.T) {
 		assert.Equal(t, "/data", envPath, "PATH env var should be correct")
 	}
 }
+
+func TestCommonSymlinks(t *testing.T) {
+	symlinks := map[string]string{
+		"/dev/fd":     "/proc/self/fd/",
+		"/dev/stdin":  "/proc/self/fd/0",
+		"/dev/stdout": "/proc/self/fd/1",
+		"/dev/stderr": "/proc/self/fd/2",
+	}
+
+	for link, expectedTarget := range symlinks {
+		t.Run(link, func(t *testing.T) {
+			target, err := os.Readlink(link)
+			require.NoError(t, err, "link must be readable")
+			assert.Equal(t, expectedTarget, target, "link target should be as expected")
+		})
+	}
+}

sysinit/mount.go → sysinit/fs.go

@@ -50,6 +50,14 @@ var MountPointPresets = map[string]MountPoints{
 	},
 }
 
+// CommonSymlinks defines symbolic links usually set by init systems.
+var CommonSymlinks = map[string]string{
+	"/dev/fd":     "/proc/self/fd/",
+	"/dev/stdin":  "/proc/self/fd/0",
+	"/dev/stdout": "/proc/self/fd/1",
+	"/dev/stderr": "/proc/self/fd/2",
+}
+
 // MountFs mounts the special file system with type FsType at the given path.
 //
 // If path does not exist, it is created. An error is returned if this or the
@@ -85,3 +93,16 @@ func MountAll() error {
 
 	return nil
 }
+
+// CreateCommonSymlinks creates common symbolic links in the file system.
+//
+// This must be run after all file systems have been mounted.
+func CreateCommonSymlinks() error {
+	for link, target := range CommonSymlinks {
+		if err := os.Symlink(target, link); err != nil {
+			return fmt.Errorf("create common symlink %s: %v", link, err)
+		}
+	}
+
+	return nil
+}

sysinit/sysinit.go

@@ -84,6 +84,10 @@ func Run(fn func() (int, error)) error {
 		return err
 	}
 
+	if err = CreateCommonSymlinks(); err != nil {
+		return err
+	}
+
 	rc, err := fn()
 	var eerr *exec.ExitError
 	if err != nil {