fix: check the target directory before cutting

internal/slicer/slicer.go

@@ -90,6 +90,18 @@ func Run(options *RunOptions) error {
 		targetDir = filepath.Join(dir, targetDir)
 	}
 
+	// Check the target directory before fetching any packages, so that the
+	// error names it instead of surfacing later while extracting a package.
+	targetInfo, err := os.Stat(targetDir)
+	if os.IsNotExist(err) {
+		return fmt.Errorf("target directory does not exist: %s", targetDir)
+	} else if err != nil {
+		return err
+	}
+	if !targetInfo.IsDir() {
+		return fmt.Errorf("target is not a directory: %s", targetDir)
+	}
+
 	pkgArchive, err := selectPkgArchives(options.Archives, options.Selection)
 	if err != nil {
 		return err

internal/slicer/slicer_test.go

@@ -634,6 +634,40 @@ var slicerTests = []slicerTest{{
 		opts.TargetDir, err = filepath.Rel(dir, opts.TargetDir)
 		c.Assert(err, IsNil)
 	},
+}, {
+	summary: "Target directory must exist",
+	slices:  []setup.SliceKey{{"test-package", "myslice"}},
+	release: map[string]string{
+		"slices/mydir/test-package.yaml": `
+			package: test-package
+			slices:
+				myslice:
+					contents:
+						/dir/file:
+		`,
+	},
+	hackopt: func(c *C, opts *slicer.RunOptions) {
+		opts.TargetDir = filepath.Join(opts.TargetDir, "missing")
+	},
+	error: `target directory does not exist: .*/missing`,
+}, {
+	summary: "Target directory must be a directory",
+	slices:  []setup.SliceKey{{"test-package", "myslice"}},
+	release: map[string]string{
+		"slices/mydir/test-package.yaml": `
+			package: test-package
+			slices:
+				myslice:
+					contents:
+						/dir/file:
+		`,
+	},
+	hackopt: func(c *C, opts *slicer.RunOptions) {
+		opts.TargetDir = filepath.Join(opts.TargetDir, "file")
+		err := os.WriteFile(opts.TargetDir, nil, 0644)
+		c.Assert(err, IsNil)
+	},
+	error: `target is not a directory: .*/file`,
 }, {
 	summary: "Can list parent directories of normal paths",
 	slices:  []setup.SliceKey{{"test-package", "myslice"}},