instantiated: embed decoder in evaluator

This makes it easier to directly parse nix output via the decoder interface.

instantiated.go

@@ -6,9 +6,12 @@ import (
 	"errors"
 	"io"
 	"iter"
+	"os/exec"
 	"path"
+	"runtime"
 	"slices"
 	"strings"
+	"syscall"
 )
 
 const (
@@ -61,7 +64,7 @@ type InstantiatedDecoder struct {
 	scanner *bufio.Scanner
 }
 
-// Err returns the first error encountered by the InstantiatedDecoder.
+// Err returns the first error encountered by the [InstantiatedDecoder].
 func (d *InstantiatedDecoder) Err() error { return errors.Join(d.err, d.scanner.Err()) }
 
 // NewInstantiatedDecoder returns a [InstantiatedDecoder] struct for collecting derivations from r.
@@ -123,34 +126,78 @@ func (d *InstantiatedDecoder) Decode() ([]string, error) {
 	return slices.Compact(instantiated), d.Err()
 }
 
-// EvalInstantiated evaluates the installable and returns all derivations instantiated during the evaluation.
-// This interprets verbose output of `nix build` and is certainly not the correct way to do it, but so far it works
-// significantly better than `nix-store -qR` and there does not appear to be a better way.
-func EvalInstantiated(ctx context.Context, installable string) ([]string, error) {
+// InstantiatedEvaluator evaluates a nix installable and collects its instantiated derivations via [InstantiatedDecoder].
+// This interprets verbose output of `nix build` and is certainly not the correct way to do it, but so far its results
+// are significantly more complete than `nix-store -qR` and there does not appear to be a better way.
+type InstantiatedEvaluator struct {
+	// underlying nix program
+	cmd *exec.Cmd
+	// kills the nix command
+	cancel context.CancelFunc
+	// populated by Close
+	waitErr error
+
+	*InstantiatedDecoder
+}
+
+// Err returns the first error encountered by the [InstantiatedEvaluator].
+func (e *InstantiatedEvaluator) Err() error {
+	return errors.Join(e.waitErr, e.InstantiatedDecoder.Err())
+}
+
+// NewInstantiatedEvaluator initialises an [InstantiatedEvaluator] struct and its underlying nix process.
+func NewInstantiatedEvaluator(ctx context.Context, installable string) (*InstantiatedEvaluator, error) {
 	c, cancel := context.WithCancel(ctx)
-	defer cancel()
-
-	cmd := Nix(c, "build", installable,
-		// 'instantiated' messages are only emitted when actually evaluating something
-		"--option", "eval-cache", "false",
-		// do not actually build anything
-		"--dry-run",
-		// increase verbosity so nix outputs 'instantiated' messages
-		"-Lvvv",
-	)
-
-	cmd.Stdout = Stdout
-	stderr, err := cmd.StderrPipe()
-	if err != nil {
-		return nil, err
+	e := &InstantiatedEvaluator{
+		cmd: Nix(c, "build", installable,
+			// 'instantiated' messages are only emitted when actually evaluating something
+			"--option", "eval-cache", "false",
+			// do not actually build anything
+			"--dry-run",
+			// increase verbosity so nix outputs 'instantiated' messages
+			"-Lvvv",
+		),
+		cancel: cancel,
 	}
 
-	if err := cmd.Start(); err != nil {
+	e.cmd.Stdout = Stdout
+
+	// verbose output ends up on stderr in the current nix implementation
+	if stderr, err := e.cmd.StderrPipe(); err != nil {
+		cancel()
+		return nil, err
+	} else {
+		e.InstantiatedDecoder = NewInstantiatedDecoder(stderr)
+	}
+
+	if err := e.cmd.Start(); err != nil {
+		cancel()
 		// have finalizer take care of the pipe
 		return nil, err
 	}
 
-	instantiated, decodeErr := NewInstantiatedDecoder(stderr).Decode()
-	waitErr := cmd.Wait()
-	return instantiated, errors.Join(decodeErr, waitErr)
+	runtime.SetFinalizer(e, (*InstantiatedEvaluator).Close)
+	return e, nil
+}
+
+// Close releases system resources held by [InstantiatedEvaluator].
+func (e *InstantiatedEvaluator) Close() error {
+	if e.cmd.ProcessState != nil {
+		return syscall.EBADE
+	}
+
+	e.cancel()
+	e.waitErr = e.cmd.Wait()
+	runtime.SetFinalizer(e, nil)
+	return e.Err()
+}
+
+// EvalInstantiated calls the underlying [InstantiatedDecoder.Decode] of a new [InstantiatedEvaluator].
+func EvalInstantiated(ctx context.Context, installable string) ([]string, error) {
+	evaluator, err := NewInstantiatedEvaluator(ctx, installable)
+	if err != nil {
+		return nil, err
+	}
+	instantiated, _ := evaluator.Decode() // error joined by Close
+	return instantiated, evaluator.Close()
 }