Make inferior::detaching a bool, and introduce scoped_restore::release()

I left making inferior::detaching a bool to a separate patch, because
doing that makes a make_cleanup_restore_integer call in
infrun.c:prepare_for_detach no longer compile (passing a 'bool *' when
an 'int *' is expected).  Since we want to get rid of cleanups anyway,
I looked at converting that to a scoped_restore.  However,
prepare_for_detach wants to discard the cleanup on success, and
scoped_restore doesn't have an equivalent for that.  So I added one --
I called it "release()" because it seems like a natural fit in the way
standard components call similarly-spirited methods, and, it's also
what the proposal for a generic scope guard calls it too, AFAICS:

  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4189.pdf

I've added some scoped_guard unit tests, while at it.

gdb/ChangeLog:
2017-04-19  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/scoped_restore-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add scoped_restore-selftests.o.
	* common/scoped_restore.h (scoped_restore_base): Make "class".
	(scoped_restore_base::release): New public method.
	(scoped_restore_base::scoped_restore_base): New protected ctor.
	(scoped_restore_base::m_saved_var): New protected field.
	(scoped_restore_tmpl::scoped_restore_tmpl(T*)): Initialize the
	scoped_restore_base base class instead of m_saved_var directly.
	(scoped_restore_tmpl::scoped_restore_tmpl(T*, T2)): Likewise.
	(scoped_restore_tmpl::scoped_restore_tmpl(const
	scoped_restore_tmpl<T>&)): Likewise.
	(scoped_restore_tmpl::~scoped_restore_tmpl): Use the saved_var
	method.
	(scoped_restore_tmpl::saved_var): New method.
	(scoped_restore_tmpl::m_saved_var): Delete.
	* inferior.h (inferior::detaching): Now a bool.
	* infrun.c (prepare_for_detach): Use a scoped_restore instead of a
	cleanup.
	* unittests/scoped_restore-selftests.c: New file.

1 files changed, 26 insertions, 10 deletions

diff --git a/gdb/common/scoped_restore.h b/gdb/common/scoped_restore.h
index ae7a49f..e7dbca9 100644
--- a/gdb/common/scoped_restore.h
+++ b/gdb/common/scoped_restore.h
@@ -21,8 +21,23 @@
 #define SCOPED_RESTORE_H
 
 /* Base class for scoped_restore_tmpl.  */
-struct scoped_restore_base
+class scoped_restore_base
 {
+public:
+  /* This informs the (scoped_restore_tmpl<T>) dtor that you no longer
+     want the original value restored.  */
+  void release () const
+  { m_saved_var = NULL; }
+
+protected:
+  scoped_restore_base (void *saved_var)
+    : m_saved_var (saved_var)
+  {}
+
+  /* The type-erased saved variable.  This is here so that clients can
+     call release() on a "scoped_restore" local, which is a typedef to
+     a scoped_restore_base.  See below.  */
+  mutable void *m_saved_var;
 };
 
 /* A convenience typedef.  Users of make_scoped_restore declare the
@@ -40,7 +55,7 @@ class scoped_restore_tmpl : public scoped_restore_base
      of *VAR.  *VAR will be restored when this scoped_restore object
      is destroyed.  */
   scoped_restore_tmpl (T *var)
-    : m_saved_var (var),
+    : scoped_restore_base (var),
       m_saved_value (*var)
   {
   }
@@ -52,14 +67,14 @@ class scoped_restore_tmpl : public scoped_restore_base
      E.g.: T='base'; T2='derived'.  */
   template <typename T2>
   scoped_restore_tmpl (T *var, T2 value)
-    : m_saved_var (var),
+    : scoped_restore_base (var),
       m_saved_value (*var)
   {
     *var = value;
   }
 
   scoped_restore_tmpl (const scoped_restore_tmpl<T> &other)
-    : m_saved_var (other.m_saved_var),
+    : scoped_restore_base {other.m_saved_var},
       m_saved_value (other.m_saved_value)
   {
     other.m_saved_var = NULL;
@@ -67,18 +82,19 @@ class scoped_restore_tmpl : public scoped_restore_base
 
   ~scoped_restore_tmpl ()
   {
-    if (m_saved_var != NULL)
-      *m_saved_var = m_saved_value;
+    if (saved_var () != NULL)
+      *saved_var () = m_saved_value;
   }
 
- private:
+private:
+  /* Return a pointer to the saved variable with its type
+     restored.  */
+  T *saved_var ()
+  { return static_cast<T *> (m_saved_var); }
 
   /* No need for this.  It is intentionally not defined anywhere.  */
   scoped_restore_tmpl &operator= (const scoped_restore_tmpl &);
 
-  /* The saved variable.  */
-  mutable T *m_saved_var;
-
   /* The saved value.  */
   const T m_saved_value;
 };