context: expose more internals

This makes overriding the Nix method easier.

context.go

@@ -25,10 +25,14 @@ type Cmd interface {
 type Context interface {
 	// Streams returns the stdout and stderr writers held by this [Context].
 	Streams() (stdout, stderr io.Writer)
+	// Extra returns a copy of extra arguments held by this [Context].
+	Extra() []string
+	// StoreEnv returns extra [Store] environment variables.
+	StoreEnv() []string
 
 	// Nix returns an implementation of [Cmd] for running a nix command.
 	Nix(ctx context.Context, arg ...string) Cmd
-	// WriteStdin calls [WriteStdin] for [exec.Cmd]. The function f points to is called if [WriteStdin] succeeds.
+	// WriteStdin calls [WriteStdin] for [Cmd]. The function f points to is called if [WriteStdin] succeeds.
 	WriteStdin(cmd Cmd, installables iter.Seq[string], f func() error) (int, error)
 
 	// Unwrap returns the stored [context.Context]

exec.go

@@ -11,7 +11,7 @@ import (
 )
 
 const (
-	defaultWaitDelay = 30 * time.Second
+	DefaultWaitDelay = 30 * time.Second
 )
 
 // Nix is the name of the nix program.
@@ -28,6 +28,13 @@ type nix struct {
 
 func (n *nix) Unwrap() context.Context             { return n.ctx }
 func (n *nix) Streams() (stdout, stderr io.Writer) { return n.stdout, n.stderr }
+func (n *nix) Extra() (v []string)                 { v = make([]string, len(n.extra)); copy(v, n.extra); return }
+func (n *nix) StoreEnv() (v []string) {
+	if n.store == nil {
+		return nil
+	}
+	return n.store.Environ()
+}
 
 const (
 	ExtraExperimentalFeatures  = "--extra-experimental-features"
@@ -42,6 +49,7 @@ A non-nil stderr implies verbose.
 Streams will not be connected for commands outputting JSON.
 */
 func New(ctx context.Context, store Store, extraArgs []string, stdout, stderr io.Writer) Context {
+	// since flakes are supposedly experimental
 	extra := []string{ExtraExperimentalFeatures, ExperimentalFeaturesFlakes}
 	if store != nil {
 		extra = append(extraArgs, FlagStore, store.String())
@@ -50,7 +58,6 @@ func New(ctx context.Context, store Store, extraArgs []string, stdout, stderr io
 		name:  Nix,
 		store: store,
 		ctx:   ctx,
-		// since flakes are supposedly experimental
 		extra: append(extraArgs, extra...),
 
 		stdout: stdout,
@@ -70,7 +77,7 @@ func (cmd ExecCmd) ReplaceEnv(env []string) { cmd.Cmd.Env = env }
 func (n *nix) Nix(ctx context.Context, arg ...string) Cmd {
 	cmd := exec.CommandContext(ctx, n.name, append(n.extra, arg...)...)
 	cmd.Cancel = func() error { return cmd.Process.Signal(os.Interrupt) }
-	cmd.WaitDelay = defaultWaitDelay
+	cmd.WaitDelay = DefaultWaitDelay
 	if n.store != nil {
 		cmd.Env = append(cmd.Env, n.store.Environ()...)
 	}

exec_test.go

@@ -5,13 +5,62 @@ import (
 	"errors"
 	"os"
 	"os/exec"
+	"reflect"
 	"slices"
 	"syscall"
 	"testing"
+	"unsafe"
 
 	"gensokyo.uk/nix"
 )
 
+func TestContext(t *testing.T) {
+	ctx := nix.New(t.Context(), &nix.BinaryCache{
+		Compression:         "none",
+		ParallelCompression: false,
+		Bucket:              "example",
+		Endpoint:            "s3.example.org",
+		Region:              "us-east-1",
+		Scheme:              "http",
+		CredentialsPath:     "/dev/null",
+		KeyPath:             nonexistent,
+	}, []string{
+		nix.FlagOption, nix.OptionBuildUseSubstitutes, nix.ValueFalse,
+		nix.FlagOption, nix.OptionSubstituters, "",
+		nix.FlagOption, nix.OptionTrustedSubstituters, "",
+		nix.FlagOption, nix.OptionTrustedPublicKeys, "",
+	}, nil, nil)
+
+	extraVal := reflect.ValueOf(ctx).Elem().FieldByName("extra")
+	wantExtra := reflect.NewAt(extraVal.Type(), unsafe.Pointer(extraVal.UnsafeAddr())).Elem().Interface().([]string)
+
+	t.Run("extra", func(t *testing.T) {
+		got := ctx.Extra()
+		if !slices.Equal(got, wantExtra) {
+			t.Errorf("Extra: %#v, want %#v", got, wantExtra)
+		}
+		got[0] = "\x00"
+		if slices.Equal(got, wantExtra) {
+			t.Errorf("Extra did not return a copy")
+		}
+	})
+
+	t.Run("store env", func(t *testing.T) {
+		t.Run("nil", func(t *testing.T) {
+			want := nix.New(t.Context(), nil, nil, nil, nil).StoreEnv()
+			if want != nil {
+				t.Errorf("StoreEnv: %#v", want)
+			}
+		})
+
+		want := []string{"AWS_SHARED_CREDENTIALS_FILE=/dev/null"}
+		got := ctx.StoreEnv()
+		if !slices.Equal(got, want) {
+			t.Errorf("StoreEnv: %#v, want %#v", got, want)
+		}
+	})
+}
+
 func TestNixWriteStdin(t *testing.T) {
 	t.Run("store", func(t *testing.T) {
 		ctx := nix.New(t.Context(), &nix.BinaryCache{