[PATCH v2 2/4] Introduce frame_info_ptr smart pointer class

[Bruno Larsen]

From: Tom Tromey <tom@tromey.com>

This adds frame_info_ptr, a smart pointer class.  Every instance of
the class is kept on a circular, doubly-linked list.  When
reinit_frame_cache is called, the list is traversed and all the
pointers are invalidated.  This should help catch the typical GDB bug
of keeping a frame_info pointer alive where a frame ID was needed
instead.
---
 gdb/frame-info.h | 185 +++++++++++++++++++++++++++++++++++++++++++++++
 gdb/frame.c      |   8 ++
 gdb/frame.h      |   2 +
 3 files changed, 195 insertions(+)
 create mode 100644 gdb/frame-info.h

diff --git a/gdb/frame-info.h b/gdb/frame-info.h
new file mode 100644
index 00000000000..001e8984c90
--- /dev/null
+++ b/gdb/frame-info.h
@@ -0,0 +1,185 @@
+/* Frame info pointer
+
+   Copyright (C) 2022 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_FRAME_INFO_H
+#define GDB_FRAME_INFO_H
+
+struct frame_info;
+
+extern void reinit_frame_cache ();
+
+/* A wrapper for "frame_info *".  frame_info objects are invalidated
+   whenever reinit_frame_cache is called.  This class arranges to
+   invalidate the pointer when appropriate.  This is done to help
+   detect a GDB bug that was relatively common.
+
+   A small amount of code must still operate on raw pointers, so a
+   "get" method is provided.  However, you should normally not use
+   this in new code.  */
+
+class frame_info_ptr
+{
+public:
+  /* Create a frame_info_ptr from a raw pointer.  */
+  explicit frame_info_ptr (struct frame_info *ptr)
+    : m_ptr (ptr),
+      m_next (&root),
+      m_prev (root.m_prev)
+  {
+    root.m_prev->m_next = this;
+    root.m_prev = this;
+  }
+
+  /* Create a null frame_info_ptr.  */
+  frame_info_ptr ()
+    : frame_info_ptr ((struct frame_info *) nullptr)
+  {
+  }
+
+  frame_info_ptr (std::nullptr_t)
+    : frame_info_ptr ((struct frame_info *) nullptr)
+  {
+  }
+
+  frame_info_ptr (const frame_info_ptr &other)
+    : frame_info_ptr (other.m_ptr)
+  {
+  }
+
+  frame_info_ptr (frame_info_ptr &&other)
+    : frame_info_ptr (other.m_ptr)
+  {
+  }
+
+  ~frame_info_ptr ()
+  {
+    m_next->m_prev = m_prev;
+    m_prev->m_next = m_next;
+  }
+
+  frame_info_ptr &operator= (const frame_info_ptr &other)
+  {
+    m_ptr = other.m_ptr;
+    return *this;
+  }
+
+  frame_info_ptr &operator= (std::nullptr_t)
+  {
+    m_ptr = nullptr;
+    return *this;
+  }
+
+  frame_info_ptr &operator= (frame_info_ptr &&other)
+  {
+    m_ptr = other.m_ptr;
+    return *this;
+  }
+
+  frame_info *operator-> () const
+  {
+    return m_ptr;
+  }
+
+  /* Fetch the underlying pointer.  Note that new code should
+     generally not use this -- avoid it if at all possible.  */
+  frame_info *get () const
+  {
+    return m_ptr;
+  }
+
+  /* This exists for compatibility with pre-existing code that checked
+     a "frame_info *" using "!".  */
+  bool operator! () const
+  {
+    return m_ptr == nullptr;
+  }
+
+  /* This exists for compatibility with pre-existing code that checked
+     a "frame_info *" like "if (ptr)".  */
+  explicit operator bool () const
+  {
+    return m_ptr != nullptr;
+  }
+
+private:
+
+  /* This constructor is used only for the root of the doubly-linked
+     list.  See "root", below.  It is explicit and given a parameter
+     to readily distinguish it from ordinary constructors.  */
+  explicit frame_info_ptr (bool ignored)
+    : m_ptr (nullptr),
+      m_next (this),
+      m_prev (this)
+  {
+  }
+
+  /* The underlying pointer.  */
+  frame_info *m_ptr;
+  /* Point to next and previous items in the circular list.  */
+  frame_info_ptr *m_next;
+  frame_info_ptr *m_prev;
+
+  /* All frame_info_ptr objects are kept on a circular doubly-linked
+     list.  This keeps their construction and destruction costs
+     reasonably small.  To make the implementation a little simpler,
+     we guarantee that there is always at least one object on the list
+     -- this "root".  */
+  static frame_info_ptr root;
+
+  /* A friend so it can invalidate the pointers.  */
+  friend void reinit_frame_cache ();
+};
+
+static inline bool
+operator== (const frame_info *self, const frame_info_ptr &other)
+{
+  return self == other.get ();
+}
+
+static inline bool
+operator== (const frame_info_ptr &self, const frame_info_ptr &other)
+{
+  return self.get () == other.get ();
+}
+
+static inline bool
+operator== (const frame_info_ptr &self, const frame_info *other)
+{
+  return self.get () == other;
+}
+
+static inline bool
+operator!= (const frame_info *self, const frame_info_ptr &other)
+{
+  return self != other.get ();
+}
+
+static inline bool
+operator!= (const frame_info_ptr &self, const frame_info_ptr &other)
+{
+  return self.get () != other.get ();
+}
+
+static inline bool
+operator!= (const frame_info_ptr &self, const frame_info *other)
+{
+  return self.get () != other;
+}
+
+#endif /* GDB_FRAME_INFO_H */
diff --git a/gdb/frame.c b/gdb/frame.c
index c0cf3d585bf..0354cf2dbd7 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -56,6 +56,9 @@ static struct frame_info *sentinel_frame;
 /* Number of calls to reinit_frame_cache.  */
 static unsigned int frame_cache_generation = 0;
 
+/* See frame-info.h.  */
+frame_info_ptr frame_info_ptr::root (true);
+
 /* See frame.h.  */
 
 unsigned int
@@ -2006,6 +2009,11 @@ reinit_frame_cache (void)
   select_frame (NULL);
   frame_stash_invalidate ();
 
+  for (frame_info_ptr *iter = frame_info_ptr::root.m_next;
+       iter != &frame_info_ptr::root;
+       iter = iter->m_next)
+    *iter = nullptr;
+
   frame_debug_printf ("generation=%d", frame_cache_generation);
 }
 
diff --git a/gdb/frame.h b/gdb/frame.h
index 75bb3bd2aa0..9ad2599331f 100644
--- a/gdb/frame.h
+++ b/gdb/frame.h
@@ -20,6 +20,8 @@
 #if !defined (FRAME_H)
 #define FRAME_H 1
 
+#include "frame-info.h"
+
 /* The following is the intended naming schema for frame functions.
    It isn't 100% consistent, but it is approaching that.  Frame naming
    schema:
-- 
2.31.1