Use external temporary storage for release builds

internal/build/candidate.go

@@ -5,10 +5,9 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"path/filepath"
-	"strings"
 
 	"gitea.maximumdirect.net/eric/ReleaseKit/internal/candidate"
+	"gitea.maximumdirect.net/eric/ReleaseKit/internal/tempdir"
 )
 
 func CandidateCallback(spec *Spec, output io.Writer) func(context.Context, candidate.Candidate) error {
@@ -18,18 +17,11 @@ func CandidateCallback(spec *Spec, output io.Writer) func(context.Context, candi
 	selected := *spec
 	selected.Targets = append([]Target(nil), spec.Targets...)
 	return func(ctx context.Context, captured candidate.Candidate) error {
-		temporaryDir, err := os.MkdirTemp(filepath.Dir(captured.RepositoryDir), ".releasekit-candidate-build-")
+		temporaryDir, err := tempdir.Outside(captured.RepositoryDir, "releasekit-candidate-build-")
 		if err != nil {
 			return fmt.Errorf("create temporary candidate build output: %w", err)
 		}
 		defer os.RemoveAll(temporaryDir)
-		relative, err := filepath.Rel(captured.RepositoryDir, temporaryDir)
-		if err != nil {
-			return fmt.Errorf("verify temporary candidate build output: %w", err)
-		}
-		if relative == "." || relative != ".." && !strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
-			return fmt.Errorf("temporary candidate build output %q is inside the repository", temporaryDir)
-		}
 		_, err = Build(ctx, Request{
 			RepositoryDir: captured.RepositoryDir,
 			Version:       captured.Version,

internal/tempdir/tempdir.go

@@ -0,0 +1,33 @@
+// Package tempdir allocates disposable storage outside a source worktree.
+package tempdir
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// Outside uses the system temporary directory and leaves cleanup to its caller.
+func Outside(repositoryDir, pattern string) (string, error) {
+	directory, err := os.MkdirTemp("", pattern)
+	if err != nil {
+		return "", err
+	}
+	repository, err := filepath.Abs(repositoryDir)
+	if err == nil && !strings.EqualFold(filepath.VolumeName(repository), filepath.VolumeName(directory)) {
+		return directory, nil
+	}
+	if err == nil {
+		var relative string
+		relative, err = filepath.Rel(repository, directory)
+		if err == nil && (relative == "." || relative != ".." && !strings.HasPrefix(relative, ".."+string(filepath.Separator))) {
+			err = fmt.Errorf("temporary storage %q is inside repository %q; select an external temporary directory", directory, repository)
+		}
+	}
+	if err != nil {
+		os.RemoveAll(directory)
+		return "", err
+	}
+	return directory, nil
+}