treewide: use internal pipe for nix command

This essentially does the same thing underneath the hood but the API is less painful to use, and it makes more sense in this use case.

build.go

@@ -2,6 +2,7 @@ package nixbuild
 
 import (
 	"context"
+	"io"
 	"iter"
 	"strings"
 )
@@ -24,12 +25,10 @@ func Build(ctx context.Context, installables iter.Seq[string]) error {
 		cmd.Args = append(cmd.Args, FlagVerbose)
 	}
 
-	stdin, err := cmd.StdinPipe()
-	if err != nil {
-		return err
-	}
+	ir, iw := io.Pipe()
+	cmd.Stdin = ir
 
-	if err = cmd.Start(); err != nil {
+	if err := cmd.Start(); err != nil {
 		return err
 	}
 
@@ -38,12 +37,12 @@ func Build(ctx context.Context, installables iter.Seq[string]) error {
 			// this is just what nix requires now :c
 			drv += "^*"
 		}
-		if _, err = stdin.Write([]byte(drv + "\n")); err != nil {
+		if _, err := iw.Write([]byte(drv + "\n")); err != nil {
 			return err
 		}
 	}
 
-	if err = stdin.Close(); err != nil {
+	if err := iw.Close(); err != nil {
 		return err
 	}
 

instantiated.go

@@ -8,10 +8,9 @@ import (
 	"iter"
 	"os/exec"
 	"path"
-	"runtime"
 	"slices"
 	"strings"
-	"syscall"
+	"sync"
 )
 
 const (
@@ -132,16 +131,18 @@ func (d *InstantiatedDecoder) Decode() ([]string, error) {
 type InstantiatedEvaluator struct {
 	// underlying nix program
 	cmd *exec.Cmd
-	// kills the nix command
-	cancel context.CancelFunc
 	// populated by Close
 	waitErr error
+	// synchronises access to waitErr
+	waitMu sync.RWMutex
 
 	*InstantiatedDecoder
 }
 
-// Err returns the first error encountered by the [InstantiatedEvaluator].
+// Err blocks until process exit and returns the first error encountered by the [InstantiatedEvaluator].
 func (e *InstantiatedEvaluator) Err() error {
+	e.waitMu.RLock()
+	defer e.waitMu.RUnlock()
 	return errors.Join(e.waitErr, e.InstantiatedDecoder.Err())
 }
 
@@ -157,41 +158,26 @@ func NewInstantiatedEvaluator(ctx context.Context, installable string) (*Instant
 			// increase verbosity so nix outputs 'instantiated' messages
 			FlagPrintBuildLogs, FlagDebug,
 		),
-		cancel: cancel,
 	}
 
 	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)
-	}
+	er, ew := io.Pipe()
+	e.cmd.Stderr = ew
+	e.InstantiatedDecoder = NewInstantiatedDecoder(er)
 
 	if err := e.cmd.Start(); err != nil {
 		cancel()
 		// have finalizer take care of the pipe
 		return nil, err
 	}
+	e.waitMu.Lock()
+	go func() { e.waitErr = e.cmd.Wait(); cancel(); _ = ew.Close(); e.waitMu.Unlock() }()
 
-	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)
@@ -199,5 +185,5 @@ func EvalInstantiated(ctx context.Context, installable string) ([]string, error)
 		return nil, err
 	}
 	instantiated, _ := evaluator.Decode() // error joined by Close
-	return instantiated, evaluator.Close()
+	return instantiated, evaluator.Err()
 }