internal: wrap calls to os standard library functions

This change helps tests stub out and simulate OS behaviour during the sealing process. This also removes dependency on XDG_RUNTIME_DIR as the internal.System implementation provided to App provides a compat directory inside the tmpdir-based share when XDG_RUNTIME_DIR is unavailable.

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

1 files changed, 120 insertions, 0 deletions

diff --git a/internal/system.go b/internal/system.go
new file mode 100644
index 00000000..6467ea05
--- /dev/null
+++ b/internal/system.go
@@ -0,0 +1,120 @@
+package internal
+
+import (
+	"io/fs"
+	"os"
+	"os/exec"
+	"os/user"
+	"path"
+	"strconv"
+	"sync"
+
+	"git.ophivana.moe/security/fortify/internal/fmsg"
+)
+
+// System provides safe access to operating system resources.
+type System interface {
+	// Geteuid provides [os.Geteuid].
+	Geteuid() int
+	// LookupEnv provides [os.LookupEnv].
+	LookupEnv(key string) (string, bool)
+	// TempDir provides [os.TempDir].
+	TempDir() string
+	// LookPath provides [exec.LookPath].
+	LookPath(file string) (string, error)
+	// Executable provides [os.Executable].
+	Executable() (string, error)
+	// Lookup provides [user.Lookup].
+	Lookup(username string) (*user.User, error)
+	// ReadDir provides [os.ReadDir].
+	ReadDir(name string) ([]os.DirEntry, error)
+	// Stat provides [os.Stat].
+	Stat(name string) (fs.FileInfo, error)
+	// Open provides [os.Open]
+	Open(name string) (fs.File, error)
+	// Exit provides [os.Exit].
+	Exit(code int)
+
+	// Paths returns a populated [Paths] struct.
+	Paths() Paths
+}
+
+// Paths contains environment dependent paths used by fortify.
+type Paths struct {
+	// path to shared directory e.g. /tmp/fortify.%d
+	SharePath string `json:"share_path"`
+	// XDG_RUNTIME_DIR value e.g. /run/user/%d
+	RuntimePath string `json:"runtime_path"`
+	// application runtime directory e.g. /run/user/%d/fortify
+	RunDirPath string `json:"run_dir_path"`
+}
+
+// CopyPaths is a generic implementation of [System.Paths].
+func CopyPaths(os System, v *Paths) {
+	v.SharePath = path.Join(os.TempDir(), "fortify."+strconv.Itoa(os.Geteuid()))
+
+	fmsg.VPrintf("process share directory at %q", v.SharePath)
+
+	if r, ok := os.LookupEnv(xdgRuntimeDir); !ok {
+		// fall back to path in share since fortify has no hard XDG dependency
+		v.RunDirPath = path.Join(v.SharePath, "run")
+		v.RuntimePath = path.Join(v.RunDirPath, "compat")
+	} else {
+		v.RuntimePath = r
+		v.RunDirPath = path.Join(v.RuntimePath, "fortify")
+	}
+
+	fmsg.VPrintf("runtime directory at %q", v.RunDirPath)
+}
+
+// Std implements System using the standard library.
+type Std struct {
+	paths     Paths
+	pathsOnce sync.Once
+}
+
+func (s *Std) Geteuid() int {
+	return os.Geteuid()
+}
+
+func (s *Std) LookupEnv(key string) (string, bool) {
+	return os.LookupEnv(key)
+}
+
+func (s *Std) TempDir() string {
+	return os.TempDir()
+}
+
+func (s *Std) LookPath(file string) (string, error) {
+	return exec.LookPath(file)
+}
+
+func (s *Std) Executable() (string, error) {
+	return os.Executable()
+}
+
+func (s *Std) Lookup(username string) (*user.User, error) {
+	return user.Lookup(username)
+}
+
+func (s *Std) ReadDir(name string) ([]os.DirEntry, error) {
+	return os.ReadDir(name)
+}
+
+func (s *Std) Stat(name string) (fs.FileInfo, error) {
+	return os.Stat(name)
+}
+
+func (s *Std) Open(name string) (fs.File, error) {
+	return os.Open(name)
+}
+func (s *Std) Exit(code int) {
+	os.Exit(code)
+}
+
+const xdgRuntimeDir = "XDG_RUNTIME_DIR"
+
+func (s *Std) Paths() Paths {
+	s.pathsOnce.Do(func() { CopyPaths(s, &s.paths) })
+	return s.paths
+}