fix(go): Context copies byte-slice parts

languages/golang/stackencrypt/context.go

@@ -1,6 +1,7 @@
 package stackencrypt
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 )
@@ -20,6 +21,9 @@ import (
 // left, so NewContext("users/age").With(uint64(7)) is the context a row
 // sealed with encrypt_into_with_context(row, 7u64) binds for that field.
 // A one-element list is not the bare part, and this type cannot spell one.
+//
+// A Context owns its parts: a byte-slice part is copied in, so a caller's
+// buffer reused once the Context is built does not change it.
 type Context struct {
 	node any
 }
@@ -41,7 +45,7 @@ func NewContext(part any) (Context, error) {
 	if err := checkRootNonEmpty(part); err != nil {
 		return Context{}, err
 	}
-	return Context{node: part}, nil
+	return Context{node: ownPart(part)}, nil
 }
 
 // MustContext is [NewContext] for a part known to be valid; it panics
@@ -63,7 +67,17 @@ func (c Context) With(part any) (Context, error) {
 	if err := checkPart(part); err != nil {
 		return Context{}, err
 	}
-	return Context{node: []any{c.node, part}}, nil
+	return Context{node: []any{c.node, ownPart(part)}}, nil
+}
+
+// ownPart is part as a context stores it: a byte slice is copied, so
+// neither a Context nor an option that extends one ([ExtendContext])
+// aliases a caller's buffer. Every other part type is a value.
+func ownPart(part any) any {
+	if b, ok := part.([]byte); ok {
+		return bytes.Clone(b)
+	}
+	return part
 }
 
 // value renders the context in the guest's grammar: a scalar or nested

languages/golang/stackencrypt/record.go

@@ -1,7 +1,6 @@
 package stackencrypt
 
 import (
-	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -158,10 +157,7 @@ func (e contextExtension) applyTerm(o *termOptions) { e.appendTo(&o.extension) }
 func ExtendContext(parts ...any) Option {
 	owned := make([]any, len(parts))
 	for i, part := range parts {
-		if b, ok := part.([]byte); ok {
-			part = bytes.Clone(b)
-		}
-		owned[i] = part
+		owned[i] = ownPart(part)
 	}
 	return contextExtension{parts: owned}
 }

languages/golang/stackencrypt/unit_test.go

@@ -128,6 +128,32 @@ func TestSelectorsSpellEveryVariant(t *testing.T) {
 	}
 }
 
+// A Context owns its parts, as an option does: NewContext and With copy a
+// byte-slice part in, so a caller's buffer reused once the context is
+// built does not change it.
+func TestContextOwnsItsByteParts(t *testing.T) {
+	root, ext := []byte("users/email"), []byte("eu")
+	c, err := NewContext(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c, err = c.With(ext); err != nil {
+		t.Fatal(err)
+	}
+	copy(root, "users/phone")
+	copy(ext, "us")
+	parts, ok := c.value().([]any)
+	if !ok || len(parts) != 2 {
+		t.Fatalf("context value is %#v, want a two-part list", c.value())
+	}
+	if got := string(parts[0].([]byte)); got != "users/email" {
+		t.Errorf("root part is %q after the caller's buffer changed, want \"users/email\"", got)
+	}
+	if got := string(parts[1].([]byte)); got != "eu" {
+		t.Errorf("extension part is %q after the caller's buffer changed, want \"eu\"", got)
+	}
+}
+
 func TestContextNestsToTheLeft(t *testing.T) {
 	c := MustContext("users/age")
 	if got := c.value(); got != "users/age" {