[binutils-gdb] Fix scoped_value_mark not working with empty value chain

[binutils-gdb] Fix scoped_value_mark not working with empty value chain

[Tom Tromey]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=3422b26537123bb63240996feea4aeb1a317e507

commit 3422b26537123bb63240996feea4aeb1a317e507
Author: Ciaran Woodward <ciaranwoodward@xmos.com>
Date:   Thu May 25 11:14:15 2023 +0000

    Fix scoped_value_mark not working with empty value chain
    
    The scoped_value_mark helper class was setting its internal
    mark value to NULL to indicate that the value chain had already
    been freed to mark.
    
    However, value_mark() also returns NULL if the value chain is
    empty at the time of call.
    
    This lead to the situation that if the value chain was empty
    at the time the scoped_value_mark was created, the class
    would not correctly clean up the state when it was destroyed,
    because it believed it had already been freed.
    
    I noticed this because I was setting a watchpoint very early
    in my debug session, and it was becoming a software watchpoint
    rather than hardware. Running any command that called evaluate()
    beforehand (such as 'x 0') would mean that a hardware watchpoint
    was correctly used. After some careful examination of the
    differences in execution, I noticed that values were being freed
    later in the 'bad case', which lead me to notice the issue with
    scoped_value_mark.

Diff:
---
 gdb/value.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gdb/value.h b/gdb/value.h
index a9c77a033ab..508367a4159 100644
--- a/gdb/value.h
+++ b/gdb/value.h
@@ -1170,16 +1170,17 @@ class scoped_value_mark
   /* Free the values currently on the value stack.  */
   void free_to_mark ()
   {
-    if (m_value != NULL)
+    if (!m_freed)
       {
 	value_free_to_mark (m_value);
-	m_value = NULL;
+	m_freed = true;
       }
   }
 
  private:
 
   const struct value *m_value;
+  bool m_freed = false;
 };
 
 extern struct value *value_cstring (const char *ptr, ssize_t len,