container/absolute: early absolute pathname check

This is less error-prone, and allows pathname to be checked once.

Signed-off-by: Ophestra <cat@gensokyo.uk>

1 files changed, 78 insertions, 0 deletions

diff --git a/container/absolute.go b/container/absolute.go
new file mode 100644
index 00000000..cafb1804
--- /dev/null
+++ b/container/absolute.go
@@ -0,0 +1,78 @@
+package container
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"path"
+	"syscall"
+)
+
+// AbsoluteError is returned by [NewAbsolute] and holds the invalid pathname.
+type AbsoluteError struct {
+	Pathname string
+}
+
+func (e *AbsoluteError) Error() string { return fmt.Sprintf("path %q is not absolute", e.Pathname) }
+func (e *AbsoluteError) Is(target error) bool {
+	var ce *AbsoluteError
+	if !errors.As(target, &ce) {
+		return errors.Is(target, syscall.EINVAL)
+	}
+	return *e == *ce
+}
+
+// Absolute holds a pathname checked to be absolute.
+type Absolute struct {
+	pathname string
+}
+
+// isAbs wraps [path.IsAbs] in case additional checks are added in the future.
+func isAbs(pathname string) bool { return path.IsAbs(pathname) }
+
+func (a *Absolute) String() string {
+	if a.pathname == zeroString {
+		panic("attempted use of zero Absolute")
+	}
+	return a.pathname
+}
+
+// NewAbsolute checks pathname and returns a new [Absolute] if pathname is absolute.
+func NewAbsolute(pathname string) (*Absolute, error) {
+	if !isAbs(pathname) {
+		return nil, &AbsoluteError{pathname}
+	}
+	return &Absolute{pathname}, nil
+}
+
+// MustAbs calls [NewAbsolute] and panics on error.
+func MustAbs(pathname string) *Absolute {
+	if a, err := NewAbsolute(pathname); err != nil {
+		panic(err.Error())
+	} else {
+		return a
+	}
+}
+
+func (a *Absolute) GobEncode() ([]byte, error) { return []byte(a.String()), nil }
+func (a *Absolute) GobDecode(data []byte) error {
+	pathname := string(data)
+	if !isAbs(pathname) {
+		return &AbsoluteError{pathname}
+	}
+	a.pathname = pathname
+	return nil
+}
+
+func (a *Absolute) MarshalJSON() ([]byte, error) { return json.Marshal(a.String()) }
+func (a *Absolute) UnmarshalJSON(data []byte) error {
+	var pathname string
+	if err := json.Unmarshal(data, &pathname); err != nil {
+		return err
+	}
+	if !isAbs(pathname) {
+		return &AbsoluteError{pathname}
+	}
+	a.pathname = pathname
+	return nil
+}