container/check: intern absolute pathnames

This improves performance in heavy users like internal/pkg.

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

container/check/absolute.go

@@ -9,6 +9,7 @@ import (
 	"slices"
 	"strings"
 	"syscall"
+	"unique"
 )
 
 // AbsoluteError is returned by [NewAbs] and holds the invalid pathname.
@@ -24,25 +25,35 @@ func (e *AbsoluteError) Is(target error) bool {
 }
 
 // Absolute holds a pathname checked to be absolute.
-type Absolute struct{ pathname string }
+type Absolute struct{ pathname unique.Handle[string] }
+
+// ok returns whether [Absolute] is not the zero value.
+func (a *Absolute) ok() bool { return a != nil && *a != (Absolute{}) }
 
 // unsafeAbs returns [check.Absolute] on any string value.
-func unsafeAbs(pathname string) *Absolute { return &Absolute{pathname} }
+func unsafeAbs(pathname string) *Absolute {
+	return &Absolute{unique.Make(pathname)}
+}
 
+// String returns the checked pathname.
 func (a *Absolute) String() string {
-	if a.pathname == "" {
+	if !a.ok() {
 		panic("attempted use of zero Absolute")
 	}
+	return a.pathname.Value()
+}
+
+// Handle returns the underlying [unique.Handle].
+func (a *Absolute) Handle() unique.Handle[string] {
 	return a.pathname
 }
 
+// Is efficiently compares the underlying pathname.
 func (a *Absolute) Is(v *Absolute) bool {
 	if a == nil && v == nil {
 		return true
 	}
-	return a != nil && v != nil &&
-		a.pathname != "" && v.pathname != "" &&
-		a.pathname == v.pathname
+	return a.ok() && v.ok() && a.pathname == v.pathname
 }
 
 // NewAbs checks pathname and returns a new [Absolute] if pathname is absolute.
@@ -70,17 +81,27 @@ func (a *Absolute) Append(elem ...string) *Absolute {
 // Dir calls [path.Dir] with [Absolute] as its argument.
 func (a *Absolute) Dir() *Absolute { return unsafeAbs(path.Dir(a.String())) }
 
-func (a *Absolute) GobEncode() ([]byte, error) { return []byte(a.String()), nil }
+// GobEncode returns the checked pathname.
+func (a *Absolute) GobEncode() ([]byte, error) {
+	return []byte(a.String()), nil
+}
+
+// GobDecode stores data if it represents an absolute pathname.
 func (a *Absolute) GobDecode(data []byte) error {
 	pathname := string(data)
 	if !path.IsAbs(pathname) {
 		return &AbsoluteError{pathname}
 	}
-	a.pathname = pathname
+	a.pathname = unique.Make(pathname)
 	return nil
 }
 
-func (a *Absolute) MarshalJSON() ([]byte, error) { return json.Marshal(a.String()) }
+// MarshalJSON returns a JSON representation of the checked pathname.
+func (a *Absolute) MarshalJSON() ([]byte, error) {
+	return json.Marshal(a.String())
+}
+
+// UnmarshalJSON stores data if it represents an absolute pathname.
 func (a *Absolute) UnmarshalJSON(data []byte) error {
 	var pathname string
 	if err := json.Unmarshal(data, &pathname); err != nil {
@@ -89,16 +110,20 @@ func (a *Absolute) UnmarshalJSON(data []byte) error {
 	if !path.IsAbs(pathname) {
 		return &AbsoluteError{pathname}
 	}
-	a.pathname = pathname
+	a.pathname = unique.Make(pathname)
 	return nil
 }
 
 // SortAbs calls [slices.SortFunc] for a slice of [Absolute].
 func SortAbs(x []*Absolute) {
-	slices.SortFunc(x, func(a, b *Absolute) int { return strings.Compare(a.String(), b.String()) })
+	slices.SortFunc(x, func(a, b *Absolute) int {
+		return strings.Compare(a.String(), b.String())
+	})
 }
 
 // CompactAbs calls [slices.CompactFunc] for a slice of [Absolute].
 func CompactAbs(s []*Absolute) []*Absolute {
-	return slices.CompactFunc(s, func(a *Absolute, b *Absolute) bool { return a.String() == b.String() })
+	return slices.CompactFunc(s, func(a *Absolute, b *Absolute) bool {
+		return a.Is(b)
+	})
 }