Function to return bundle's cache directory (#109)

Parallel of `project.CacheDir()` introduced in
https://github.com/databricks/bricks/pull/82.

bundle/bundle.go

@@ -1,6 +1,7 @@
 package bundle
 
 import (
+	"os"
 	"path/filepath"
 	"sync"
 
@@ -49,3 +50,22 @@ func (b *Bundle) WorkspaceClient() *databricks.WorkspaceClient {
 	})
 	return b.client
 }
+
+var cacheDirName = filepath.Join(".databricks", "bundle")
+
+// CacheDir returns directory to use for temporary files for this bundle.
+// Scoped to the bundle's environment.
+func (b *Bundle) CacheDir() (string, error) {
+	if b.Config.Bundle.Environment == "" {
+		panic("environment not set")
+	}
+
+	// Make directory if it doesn't exist yet.
+	dir := filepath.Join(b.Config.Path, cacheDirName, b.Config.Bundle.Environment)
+	err := os.MkdirAll(dir, 0700)
+	if err != nil {
+		return "", err
+	}
+
+	return dir, nil
+}

bundle/bundle_test.go

@@ -2,6 +2,8 @@ package bundle
 
 import (
 	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -19,3 +21,21 @@ func TestLoadExists(t *testing.T) {
 	require.Nil(t, err)
 	assert.Equal(t, "basic", b.Config.Bundle.Name)
 }
+
+func TestBundleCacheDir(t *testing.T) {
+	projectDir := t.TempDir()
+	f1, err := os.Create(filepath.Join(projectDir, "bundle.yml"))
+	require.NoError(t, err)
+	f1.Close()
+
+	bundle, err := Load(projectDir)
+	require.NoError(t, err)
+
+	// Artificially set environment.
+	// This is otherwise done by [mutators.SelectEnvironment].
+	bundle.Config.Bundle.Environment = "default"
+
+	cacheDir, err := bundle.CacheDir()
+	assert.NoError(t, err)
+	assert.True(t, strings.HasPrefix(cacheDir, projectDir))
+}