fixed rubout of Q-Register specifications

* This was a regression introduced by 257a0bf128e109442dce91c4aaa1d97fed17ad1a.
* The undo token that frees newly allocated teco_machine_qregspec_t must actually
  reset the pointer as well since any subsequent token, pushed by teco_undo_qregspec_own(),
  will expect a valid pointer.
* Could have been done via
  ctx->expectqreg = NULL;
  teco_undo_qregspec_own(ctx->expectqreg);
  but using a special clear function requires less memory and is easier to understand.
* Added test case. This wouldn't always crash, but should definitely show up in Valgrind.

4 files changed, 12 insertions, 4 deletions

diff --git a/src/qreg-commands.c b/src/qreg-commands.c
index ebf6caa..3da9c46 100644
--- a/src/qreg-commands.c
+++ b/src/qreg-commands.c
@@ -45,7 +45,7 @@ teco_state_expectqreg_initial(teco_machine_main_t *ctx, GError **error)
 	ctx->expectqreg = teco_machine_qregspec_new(current->expectqreg.type, ctx->qreg_table_locals,
 	                                            ctx->parent.must_undo);
 	if (ctx->parent.must_undo)
-		undo__teco_machine_qregspec_free(ctx->expectqreg);
+		undo__teco_machine_qregspec_clear(&ctx->expectqreg);
 	return TRUE;
 }
 
@@ -240,7 +240,7 @@ teco_state_queryqreg_initial(teco_machine_main_t *ctx, GError **error)
 	ctx->expectqreg = teco_machine_qregspec_new(type, ctx->qreg_table_locals,
 	                                            ctx->parent.must_undo);
 	if (ctx->parent.must_undo)
-		undo__teco_machine_qregspec_free(ctx->expectqreg);
+		undo__teco_machine_qregspec_clear(&ctx->expectqreg);
 	return TRUE;
 }
 
diff --git a/src/qreg.c b/src/qreg.c
index 1b97689..39406a2 100644
--- a/src/qreg.c
+++ b/src/qreg.c
@@ -1715,5 +1715,12 @@ teco_machine_qregspec_free(teco_machine_qregspec_t *ctx)
 	g_free(ctx);
 }
 
-TECO_DEFINE_UNDO_CALL(teco_machine_qregspec_free, teco_machine_qregspec_t *);
+static inline void
+teco_machine_qregspec_clear(teco_machine_qregspec_t **ctx)
+{
+	teco_machine_qregspec_free(*ctx);
+	*ctx = NULL;
+}
+
+TECO_DEFINE_UNDO_CALL(teco_machine_qregspec_clear, teco_machine_qregspec_t **);
 TECO_DEFINE_UNDO_OBJECT_OWN(qregspec, teco_machine_qregspec_t *, teco_machine_qregspec_free);
diff --git a/src/qreg.h b/src/qreg.h
index 4260202..7a9e13c 100644
--- a/src/qreg.h
+++ b/src/qreg.h
@@ -252,7 +252,7 @@ void teco_machine_qregspec_free(teco_machine_qregspec_t *ctx);
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(teco_machine_qregspec_t, teco_machine_qregspec_free);
 
 /** @memberof teco_machine_qregspec_t */
-void undo__teco_machine_qregspec_free(teco_machine_qregspec_t *);
+void undo__teco_machine_qregspec_clear(teco_machine_qregspec_t **);
 TECO_DECLARE_UNDO_OBJECT(qregspec, teco_machine_qregspec_t *);
 
 #define teco_undo_qregspec_own(VAR) \
diff --git a/src/undo.h b/src/undo.h
index 6fbb1c1..459fdb0 100644
--- a/src/undo.h
+++ b/src/undo.h
@@ -143,6 +143,7 @@ gpointer teco_undo_push_size(teco_undo_action_t action_cb, gsize size)
  * At undo-time however, it is expected to be valid.
  * Therefore it is crucial to set the variable afterwards or at least to nullify it.
  * The DELETE function may consequently have to deal with NULL pointers.
+ * Also, any other undo action must always leave the variable in a valid state.
  *
  * @ingroup undo_objects
  */