Include [DEFAULT] section header when writing ~/.databrickscfg (#464)

## Changes

The ini library omits the default section header and in doing so breaks
compatibility with Python's config parser. It raises:

```
Error: MissingSectionHeaderError: File contains no section headers.
```

This commit makes sure the DEFAULT section header is included.

If the config file doesn't include a DEFAULT section itself, we include
a comment describing its purpose.

## Tests

New tests pass. Manually confirmed the DEFAULT section header is
included.

---------

Co-authored-by: PaulCornellDB <paul.cornell@databricks.com>

libs/databrickscfg/ops.go

@@ -13,6 +13,8 @@ import (
 
 const fileMode = 0o600
 
+const defaultComment = "The profile defined in the DEFAULT section is to be used as a fallback when no profile is explicitly specified."
+
 func loadOrCreateConfigFile(filename string) (*config.File, error) {
 	if filename == "" {
 		filename = "~/.databrickscfg"
@@ -104,6 +106,12 @@ func SaveToProfile(ctx context.Context, cfg *config.Config) error {
 		key.SetValue(attr.GetString(cfg))
 	}
 
+	// Add a comment to the default section if it's empty.
+	section = configFile.Section(ini.DefaultSection)
+	if len(section.Keys()) == 0 && section.Comment == "" {
+		section.Comment = defaultComment
+	}
+
 	orig, backupErr := os.ReadFile(configFile.Path())
 	if len(orig) > 0 && backupErr == nil {
 		log.Infof(ctx, "Backing up in %s.bak", configFile.Path())
@@ -120,3 +128,9 @@ func SaveToProfile(ctx context.Context, cfg *config.Config) error {
 	}
 	return configFile.SaveTo(configFile.Path())
 }
+
+func init() {
+	// We document databrickscfg files with a [DEFAULT] header and wish to keep it that way.
+	// This, however, does mean we emit a [DEFAULT] section even if it's empty.
+	ini.DefaultHeader = true
+}

libs/databrickscfg/ops_test.go

@@ -2,11 +2,13 @@ package databrickscfg
 
 import (
 	"context"
+	"os"
 	"path/filepath"
 	"testing"
 
 	"github.com/databricks/databricks-sdk-go/config"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestLoadOrCreate(t *testing.T) {
@@ -129,7 +131,7 @@ func TestSaveToProfile_ErrorOnMatch(t *testing.T) {
 	assert.Error(t, err)
 }
 
-func TestSaveToProfile_NewFile(t *testing.T) {
+func TestSaveToProfile_NewFileWithoutDefault(t *testing.T) {
 	ctx := context.Background()
 	path := filepath.Join(t.TempDir(), "databrickscfg")
 
@@ -141,6 +143,39 @@ func TestSaveToProfile_NewFile(t *testing.T) {
 	})
 	assert.NoError(t, err)
 	assert.NoFileExists(t, path+".bak")
+
+	contents, err := os.ReadFile(path)
+	require.NoError(t, err)
+	assert.Equal(t,
+		`; The profile defined in the DEFAULT section is to be used as a fallback when no profile is explicitly specified.
+[DEFAULT]
+
+[abc]
+host  = https://foo
+token = xyz
+`, string(contents))
+}
+
+func TestSaveToProfile_NewFileWithDefault(t *testing.T) {
+	ctx := context.Background()
+	path := filepath.Join(t.TempDir(), "databrickscfg")
+
+	err := SaveToProfile(ctx, &config.Config{
+		ConfigFile: path,
+		Profile:    "DEFAULT",
+		Host:       "https://foo",
+		Token:      "xyz",
+	})
+	assert.NoError(t, err)
+	assert.NoFileExists(t, path+".bak")
+
+	contents, err := os.ReadFile(path)
+	require.NoError(t, err)
+	assert.Equal(t,
+		`[DEFAULT]
+host  = https://foo
+token = xyz
+`, string(contents))
 }
 
 func TestSaveToProfile_ClearingPreviousProfile(t *testing.T) {