1 files changed, 81 insertions, 0 deletions
diff --git a/internal/sys/hsu.go b/internal/sys/hsu.go
new file mode 100644
index 00000000..84bb0731
--- /dev/null
+++ b/internal/sys/hsu.go
@@ -0,0 +1,81 @@
+package sys
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"strconv"
+	"sync"
+
+	"hakurei.app/container"
+	"hakurei.app/hst"
+	"hakurei.app/internal"
+)
+
+// Hsu caches responses from cmd/hsu.
+type Hsu struct {
+	uidOnce sync.Once
+	uidCopy map[int]struct {
+		uid int
+		err error
+	}
+	uidMu sync.RWMutex
+}
+
+var ErrHsuAccess = errors.New("current user is not in the hsurc file")
+
+func (h *Hsu) Uid(identity int) (int, error) {
+	h.uidOnce.Do(func() {
+		h.uidCopy = make(map[int]struct {
+			uid int
+			err error
+		})
+	})
+
+	{
+		h.uidMu.RLock()
+		u, ok := h.uidCopy[identity]
+		h.uidMu.RUnlock()
+		if ok {
+			return u.uid, u.err
+		}
+	}
+
+	h.uidMu.Lock()
+	defer h.uidMu.Unlock()
+
+	u := struct {
+		uid int
+		err error
+	}{}
+	defer func() { h.uidCopy[identity] = u }()
+
+	u.uid = -1
+	hsuPath := internal.MustHsuPath()
+
+	cmd := exec.Command(hsuPath)
+	cmd.Path = hsuPath
+	cmd.Stderr = os.Stderr // pass through fatal messages
+	cmd.Env = []string{"HAKUREI_APP_ID=" + strconv.Itoa(identity)}
+	cmd.Dir = container.FHSRoot
+	var (
+		p         []byte
+		exitError *exec.ExitError
+	)
+
+	const step = "obtain uid from hsu"
+	if p, u.err = cmd.Output(); u.err == nil {
+		u.uid, u.err = strconv.Atoi(string(p))
+		if u.err != nil {
+			u.err = &hst.AppError{Step: step, Err: u.err, Msg: "invalid uid string from hsu"}
+		}
+	} else if errors.As(u.err, &exitError) && exitError != nil && exitError.ExitCode() == 1 {
+		// hsu prints an error message in this case
+		u.err = &hst.AppError{Step: step, Err: ErrHsuAccess}
+	} else if os.IsNotExist(u.err) {
+		u.err = &hst.AppError{Step: step, Err: os.ErrNotExist,
+			Msg: fmt.Sprintf("the setuid helper is missing: %s", hsuPath)}
+	}
+	return u.uid, u.err
+}