lakebox: skip state file writes when nothing changed

setDefault and clearDefault unconditionally rewrote ~/.databricks/
lakebox.json even when the in-memory state was identical to what was
already on disk: clearing a profile that wasn't in the map, or
re-setting the same value. That created or touched the file for
no-op operations.

Add change-detection guards to both: setDefault is a no-op when the
profile already maps to the requested ID; clearDefault is a no-op
when the profile isn't in the map. Result: a CLI invocation that
doesn't change state can no longer cause a file to spring into
existence on a fresh machine.

Tests:
 - clearDefault on a missing profile must leave the file absent
 - setDefault with an unchanged value must not bump the mtime
 - getDefault on a fresh state must not create the file (regression
   test for the read-only path)

Co-authored-by: Isaac

Author: pietern

cmd/lakebox/state.go

@@ -81,6 +81,9 @@ func setDefault(ctx context.Context, profile, lakeboxID string) error {
 	if err != nil {
 		return err
 	}
+	if state.Defaults[profile] == lakeboxID {
+		return nil
+	}
 	state.Defaults[profile] = lakeboxID
 	return saveState(ctx, state)
 }
@@ -90,6 +93,9 @@ func clearDefault(ctx context.Context, profile string) error {
 	if err != nil {
 		return err
 	}
+	if _, ok := state.Defaults[profile]; !ok {
+		return nil
+	}
 	delete(state.Defaults, profile)
 	return saveState(ctx, state)
 }

cmd/lakebox/state_test.go

@@ -70,9 +70,36 @@ func TestStateClearDefault(t *testing.T) {
 	assert.Equal(t, "lakebox-b", getDefault(ctx, "profile-b"))
 }
 
-func TestStateClearDefaultMissingProfileNoError(t *testing.T) {
-	ctx, _ := stateCtx(t)
-	assert.NoError(t, clearDefault(ctx, "no-such-profile"))
+func TestStateClearDefaultMissingProfileDoesNotCreateFile(t *testing.T) {
+	ctx, path := stateCtx(t)
+
+	require.NoError(t, clearDefault(ctx, "no-such-profile"))
+
+	_, err := os.Stat(path)
+	assert.ErrorIs(t, err, fs.ErrNotExist, "clearDefault must not create the state file when there's nothing to remove")
+}
+
+func TestStateSetDefaultSameValueDoesNotRewriteFile(t *testing.T) {
+	ctx, path := stateCtx(t)
+
+	require.NoError(t, setDefault(ctx, "profile-a", "lakebox-a"))
+	before, err := os.Stat(path)
+	require.NoError(t, err)
+
+	// Re-set with the same value should be a no-op.
+	require.NoError(t, setDefault(ctx, "profile-a", "lakebox-a"))
+	after, err := os.Stat(path)
+	require.NoError(t, err)
+	assert.Equal(t, before.ModTime(), after.ModTime(), "no-op setDefault must not rewrite the file")
+}
+
+func TestStateSetDefaultMissingNoFileBeforeWrite(t *testing.T) {
+	ctx, path := stateCtx(t)
+
+	// Loading state on a fresh tempdir must not create the file.
+	assert.Equal(t, "", getDefault(ctx, "profile-a"))
+	_, err := os.Stat(path)
+	assert.ErrorIs(t, err, fs.ErrNotExist, "getDefault must not create the state file")
 }
 
 // Pre-existing files from earlier CLI versions carry a `last_profile` field